APPROVED:
Steve Wolverton, Major Professor Lisa Nagaoka, Committee Member Charles R. Randklev, Committee Member Paul F. Hudak, Chair of the Department of
Geography Costas Tsatsoulis, Interim Dean of the
Toulouse Graduate School
ZOOARCHAEOLOGY AND BIOGEOGRAPHY OF FRESHWATER MUSSELS IN THE LEON RIVER
DURING THE LATE HOLOCENE
Traci Glyn Popejoy, B.S.
Thesis Prepared for the Degree of
MASTER OF SCIENCE
UNIVERSITY OF NORTH TEXAS
May 2015
Popejoy, Traci Glyn. Zooarchaeology and Biogeography of Freshwater Mussels in the
Leon River during the Late Holocene. Master of Science (Applied Geography), May 2015, 76 pp.,
10 tables, 15 figures, references, 129 titles.
The Leon River, a small-medium river in central Texas, is highly impacted by multiple
impoundments, enrichment from agricultural runoff, and decreased dissolved oxygen levels.
This degraded river contains sixteen unionid species, two of which are both endemic to the
region and candidates for the federal endangered species listing (Quadrula houstonensis and
Truncilla macrodon). While there is a short historical record for this river basin and a recent
modern survey completed in 2011, zooarchaeological data adds evidence for conservation
efforts by increasing the time depth of data available and providing another conservation
baseline. Zooarchaeological data for the Leon River is available from the two Late Holocene
archaeological sites: 41HM61 and the Belton Lake Assemblages. Data generated from these
assemblages describe the prehistoric freshwater mussel community of the Leon River in terms
of taxonomic composition and structure. By comparing this zooarchaeological data to the data
generated by the longitudinal modern survey of the Leon River, long term changes within the
freshwater mussel community can be detected. A conceptual model is constructed to evaluate
how robusticity, identifiability, and life history ecology affect unionid taxonomic abundances in
zooarchaeological data. This conceptual model functions as an interpretive tool for
zooarchaeologists to evaluate forms of equifinality in zooarchaeological assemblages. This
thesis determines differences between the late Holocene and modern freshwater community
of the Leon River, explores how different alternative mechanisms influence zooarchaeological
data, and exemplifies of how zooarchaeological data can be used for conservation biology.
Copyright 2015
by
Traci Glyn Popejoy
ii
ACKNOWLEDGEMENTS
I would like to thank my advisor, Steve Wolverton for guidance on research topics,
writing, and contextualizing my research. I would also like to thank Lisa Nagaoka for instruction
regarding writing and presenting research. I am also grateful to Charles R. Randklev for writing
guidance, freshwater mussel expertise, and direction regarding analytical techniques. Thanks to
all of my committee members for infinite patience and wisdom. I am indebted to my family and
Sean Dubose for their patience when my thesis periodically consumed my life. Costal
Environments, Inc. and Texas Department of Transportation funded the description of the
unionid remains from the 41HM61 assemblage.
iii
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ................................................................................................................... iii
LIST OF TABLES ................................................................................................................................. v
LIST OF FIGURES .............................................................................................................................. vi
CHAPTER 1 INTRODUCTION ............................................................................................................ 1
CHAPTER 2 DESCRIPTION OF THE LATE HOLOCENE FRESHWATER MUSSEL COMMUNITY OF THE LEON RIVER IN CENTRAL TEXAS ............................................................................................... 5
CHAPTER 3 CHANGE IN THE LEON RIVER’S FRESHWATER MUSSEL COMMUNITY STRUCTURE SINCE THE LATE HOLOCENE .......................................................................................................... 18
CHAPTER 4 A CONCEPTUAL MODEL FOR ASSESSING FRESHWATER MUSSEL TAXONOMIC ABUNDANCES IN ZOOARCHAEOLOGICAL FAUNAS ....................................................................... 34
CHAPTER 5 CONCLUSION .............................................................................................................. 54
APPENDIX A SYSTEMATIC PALEONTOLOGY .................................................................................. 57
APPENDIX B DATA USED IN CHAPTER 3INFERENTIAL TESTS ........................................................ 64
REFERENCES .................................................................................................................................. 67
iv
LIST OF TABLES
Page
Table 3.1 Mussels of the Leon River & preferred habitat. ................................................... 22
Table 3.2 Results of all chi-square goodness of fit test and Spearman’s rho correlation.. .. 28
Table 4.1 Taxonomic abundances within the 41HM61 and Belton Lake assemblages........ 36
Table 4.2 Robusticity and life history strategy values for Leon River zooarchaeological freshwater mussel assemblage ............................................................................. 41
Table 4.3 Expectations for abundance filters discussed in this chapter .............................. 44
Table 4.4 Zooarchaeological taxonomic abundance expectations for the Leon River unionid community. .............................................................................................. 47
Table B.1 Data for culled chi-square goodness of fit test ..................................................... 65
Table B.2 Data for culled Spearman’s rho correlation ......................................................... 65
Table B.3 Data for complete chi-square goodness of fit test ............................................... 66
Table B.4 Data for complete Spearman’s rho correlation .................................................... 66
v
LIST OF FIGURES
Page
Fig. 2.1. Map of Leon River in central Texas. ........................................................................ 8
Fig. 2.2. Map of Belton Lake Zooarchaeological assemblages.. ........................................... 8
Fig. 2.3. Example specimen of Q. houstonensis from the 41HM61 assemblage. .............. 10
Fig. 2.4. Graphic representation of the nestedness concept and a nestedness analysis. . 12
Fig. 2.5. Relative abundances of taxa in the 41HM61 and the Belton Lake assemblages. ......................................................................................................... 14
Fig. 2.6. Nestedness matrix generated from the zooarchaeological assemblages from the Leon River. ...................................................................................................... 15
Fig. 3.1. Conceptual diagram of mesohabitats within river systems. ................................ 21
Fig. 3.2. Relative taxonomic abundances of late Holocene and mordern assemblages from the Leon River. ............................................................................................. 24
Fig. 3.3. NMDS ordination of freshwater mussel assemblages based on assemblage species structure. .................................................................................................. 30
Fig. 4.1. Model describing potential filters within zooarchaeological assemblages. ......... 38
Fig. 4.2. Ordination of life history strategies of freshwater mussels in the southeastern United States. ........................................................................................................ 45
Fig. 4.3. Analysis of life history strategy’s influence on the 41HM61 and the Belton Lake assemblages. ................................................................................................. 48
Fig. 4.4. Analysis of sculpture’s influence on the 41HM61 and Belton Lake assemblages. ......................................................................................................... 49
Fig. 4.5. Robusticities’ influence on the 41HM61 and Belton Lake assemblages. ............. 50
Fig. 4.6. Taphonomic analysis of the 41HM61 assemblage and the Belton Lake assemblages .......................................................................................................... 50
vi
CHAPTER 1
INTRODUCTION
Rivers have been subjected to increased anthropogenic change in the past century:
cities have expanded, land use has changed, and urban water use has increased (Humphries
and Winemiller, 2009; Palmer et al., 2008; Parmesan and Matthews, 2006). The Leon River in
central Texas is no exception and has changed since European settlement. A citizen of nearby
Dublin, Texas remembers this stream changing during the early twentieth century:
When I first came to Dublin, we used to go down to the Leon River and fish a lot. It was a nice clear stream most of the time. The river had a deep channel and lots of holes of water 25-30 feet deep. Well, we caught a lot of good fish. After a while fishing began to get poor. The Leon River began to get out of banks almost every time we had a large rain. Well somebody found out that muddy water had been depositing silt all down the river and had filled it up until it hardly had any banks at all… Hallmark in 1938 taken from (Hendrickson Jr., 1981, p. 59)
Since its impoundment in the 1950s, the Leon River has experienced habitat change and an
increase in runoff from urban and agricultural areas (United States Department of Agriculture,
2005; Harmel et al., 2008; Hendrickson Jr., 1981; Rossi et al., 2008). A longitudinal survey of the
freshwater mussel population of the Leon River found decreased abundance and diversity
compared to historic records (which begin in 1931) (Strecker, 1931). There are three unionid
species of conservation concern in this river: false spike (Fusconaia mitchelli), smooth
pimpleback (Quadrula houstonensis) and Texas fawnsfoot (Truncilla macrodon). This thesis
discusses changes to the freshwater mussel assemblage of the Leon River since the late
Holocene in order to assess whether or not there has been change in the mussel community
related to contemporary human impacts.
1
Freshwater mussels (Family: Unionidae) have been highly impacted through
anthropogenic alteration of river systems (Galbraith et al., 2010; Nobles and Zhang, 2011;
Williams et al., 1993) and have experienced population declines in the last 100 years. Of nearly
300 species native to North America, 30 species are now extinct (Haag, 2012 and references
within). As sedentary filter feeders with a parasitic larval stage, these organisms are sensitive to
many of the anthropogenic impacts affecting rivers, such as decreased river conductivity,
increased temperature, and habitat degradation (Haag and Warren Jr., 2008; Shea et al., 2013;
Vaughn and Taylor, 1999). Due to this sensitivity, many freshwater mussels (hereafter unionids)
are indicator species of aquatic ecosystem health and can warn conservation biologists of
declining quality in stream habitat (Cummings and Graf, 2010; Dillon Jr., 2000; Haag, 2012).
Conservation biologists have been diligently studying declining freshwater mussel communities
at a state and national level (Haag and Williams, 2013).
Conservation biology is a scientific approach to “address the biology of species,
communities and ecosystems that are perturbed, either directly or indirectly, by human
activities or other agents” for the purpose of preserving the evolution of biodiversity (Soulé,
1985, p. 727). To conserve remaining ecological processes and biodiversity often requires action
before everything is known about the situation, making it a “crisis discipline” (Soulé, 1985, p.
727). When decisions must be made quickly, evidence of ecological change and past
environments is highly valuable. Conservation biology uses conservation baselines as goals for
wildlife and ecosystem management, but these baselines are culturally influenced and mainly
rely on historical records (Joyce, 2012). Historical records, though an important source of
information, can be incomplete, inaccurate, or represent already degraded environments
2
(Lyman, 2012a, 2012b; Randklev, 2011). When historical records are inadequate,
zooarchaeological data may provide valuable evidence of past ecosystems (Louys et al., 2012;
Lyman, 2012a, 1996; Wolverton and Lyman, 2012). This type of data can then be used to
construct a different conservation baseline at a longer time scale. This thesis examine late
Holocene zooarchaeological data from faunas from the Leon River to describe the past
freshwater mussel community, adding valuable baseline information to the conservation
dialogue regarding this declining freshwater mussel community.
Two zooarchaeological assemblages are reported in this thesis, the 41HM61 and the
Belton Lake assemblages (which are several small archaeological faunas excavated near Belton
Lake that date to the late Holocene). These zooarchaeological assemblages represent the late
Holocene freshwater mussel community of the Leon River in terms of species composition and
taxonomic abundance. These late Holocene unionid assemblages are compared to data from a
modern longitudinal survey of the Leon River unionid community (Randklev et al., 2013a). The
goal of this thesis is two parts: to assess how the unionid community of the Leon River has
changed since the late Holocene and to assess how life history ecology, identifiability, and
robusticity influence taxonomic abundance in zooarchaeological assemblages, which can
influence the ability of applied zooarchaeologists to establish baseline conditions relevant to
conservation. Chapter 2 reports data from the 41HM61 and the Belton Lake zooarchaeological
assemblages to provide a representation of the late Holocene unionid community and
addresses changes in species composition of the Leon River unionid community. Chapter 3 uses
nonparametric inferential tests to address taxonomic structural change in the Leon River
unionid community since the late Holocene. Chapter 3 also uses a nonmetric multidimensional
3
scaling (NMDS) ordination to address the similarity between modern mesohabitat sub-
assemblages and the late Holocene assemblages. Chapter 4 creates a conceptual model to
address how variable life history strategy, identifiability, and robusticity affect taxonomic
abundances in zooarchaeological assemblages. This conceptual model is then used to evaluate
the results of the inferential tests in Chapter 3. These analyses assess ecological change in the
unionid community of the Leon River with implications for wildlife management.
By understanding how the freshwater mussel community of the Leon River has changed
since the late Holocene, conservation biologists can better manage today’s unionid community.
Shifts in the assemblage toward tolerant species might indicate that the river has experienced
anthropogenic impacts. This thesis also helps biologists understand how riverine ecosystems
can change through time. To impact conservation, the results of this thesis need to be
disseminated to the appropriate agencies and wildlife managers (Wolverton and Lyman, 2012).
This is achieved by presenting parts of this thesis at multiple conferences such as the Texas
Freshwater Mussel Symposium and the Freshwater Mollusk Conservation Society Symposium.
The conceptual model created in this thesis helps zooarchaeologists understand how different
mechanisms influence taxonomic abundance data and conclusions based on that data. It is an
interpretive tool designed to help zooarchaeologists understand underlying influences in their
unionid taxonomic abundance data. In total, this thesis uses zooarchaeological data for
conservation purposes in the Leon River and improves the interpretation of this data by using a
conceptual model to evaluate forms of equifinality in zooarchaeological assemblages.
4
CHAPTER 2
DESCRIPTION OF THE LATE HOLOCENE FRESHWATER MUSSEL COMMUNITY OF THE LEON RIVER
IN CENTRAL TEXAS
Introduction
Freshwater mussels (Family: Unionidae) are highly diverse and imperiled across the
North American continent. These sedentary filter-feeders are affected by many anthropogenic
affects, such as impoundments, increased runoff and increased municipal water use. In Texas,
freshwater mussels are experiencing declines in community diversity and abundance (Burlakova
et al., 2011). Though historical records are available for many river basins in Texas, these are
limited as they often represent freshwater mussel communities at a short ecological time scale.
Zooarchaeological data increase knowledge about prehistoric biogeographic distributions of
these organisms, which improves conservation efforts according to the National Strategy for
unionid conservation (Haag and Williams, 2013; Strayer et al., 2004).
The Leon River, located in central Texas, has experienced many of the anthropogenic
effects described above, and as such the freshwater mussel fauna has experienced a decrease
in diversity and abundance (Randklev et al., 2013a). While many historical records exist for the
river, the prehistoric freshwater mussel community of the Leon River has received limited
discussion. This chapter describes freshwater mussel remains collected from late Holocene
archaeological sites located in Hamilton, Bell, and Coryell County and provides information
about the distribution and richness in the Leon River. Data are also reported from site 41HM61
and from multiple small zooarchaeological assemblages surrounding Belton Lake in Bell and
Coryell County. Nestedness is analyzed for both the Belton Lake assemblages and 41HM61 to
5
determine if they represent the same faunal community and if they can be aggregated in future
analyses. By fully describing the prehistoric freshwater mussel community of the Leon River,
conservation biologists can determine if change in the taxonomic composition of this
community has occurred.
Background
Applied zooarchaeology is the use of zooarchaeological data for purposes of
conservation biology (Cannon and Cannon, 2004; Lyman, 2012a, 1996; Peacock, 2005;
Wolverton and Lyman, 2012). It provides information on past biological communities most
often through presence/absence and abundance data for taxa within assemblages.
Presence/absence data are often used to assess the distribution of species of conservation
concern through time and across space (Grayson, 1987; Lyman, 1991). Abundance data can also
be used to quantify the taxonomic abundances in an ecological community (Grayson, 1987;
Randklev and Lundeen, 2012; Wolverton, 2008). Applied zooarchaeological data have also been
used to discuss paleoenvironmental conditions, such as flow regime, substrate, and nutrient
levels through isotopic and morphologic analysis of freshwater mussel remains (Peacock and
Seltzer, 2008; Spurlock, 1981; Warren, 1991). The field has benefited modern freshwater
mussel conservation by providing new information about prehistoric unionid communities, for
example in the Upper Trinity River basin of north Texas (Randklev and Lundeen, 2012) and the
Tombigbee River basin of Mississippi (Peacock, 2012; Peacock et al., 2012). The goal of the
current study is to use zooarchaeological data to analyze the prehistoric freshwater mussel
community composition in the Leon River during the Late Holocene.
6
Study Area
The Leon River is a small to medium size stream in central Texas and a tributary of the
Brazos River (Fig. 2.1). (Anderson et al., 2014; Randklev et al., 2013a). Central Texas rivers
contain 85% of the state’s endemic species, making these rivers important for the conservation
of unionid diversity in Texas (Burlakova et al., 2011). The Leon River basin contains sixteen
unionid species; Fusconaia mitchelli, Quadrula houstonensis, and Truncilla macrodon are
endemic to the region and of conservation concern. Quadrula houstonensis and T. macrodon
are candidates for federal endangered species listing, while F. mitchelli is petitioned for federal
listing (Randklev et al., 2013a; U.S. Fish and Wildlife Service, 2014). The Leon River is impacted
by multiple impoundments, enrichment from agricultural and urban runoff, and subsequent
decreased dissolved oxygen levels (Burlakova et al., 2011; Randklev et al., 2013a). While there
is a short historical record for this river basin and a recent modern survey completed in 2011,
zooarchaeological data can provide a late Holocene baseline for mussel conservation efforts.
The Hamilton County assemblage is from the archaeological site 41HM61 and represents
multiple occupations. The Belton Lake assemblage comprises remains from eighteen separate
cave sites that surround the modern Belton Lake reservoir; the location of these sites can be
seen in Fig. 2.2 (Randklev, 2010). Mussels from these assemblages were identified by Charles
Randklev. These assemblages represent sites that are approximately 60 miles apart, but are
expected to represent the same faunal community in terms of species composition.
7
Fig. 2.1. Map of Leon River in central Texas. This is the map shows the location of Lake Belton on the Leon River and the Leon River’s location within the state of Texas. Taken from Randklev et al. (2013a).
Fig. 2.2. Map of Belton Lake Zooarchaeological assemblages. These assemblages were combined because they represent the same faunal community, and many have low NRE. It is important to note that Belton Lake is a reservoir, and thus an anthropogenically altered landscape, though the assemblages would represent the unaltered Leon River. Map provided by Dr. Charles Randklev (Randklev, 2010).
8
Materials and Methods
The freshwater mussel remains from the 41HM61 assemblage and the Belton Lake
assemblages are identified following guidelines described in Driver (2011, 1992) and Wolverton
(2013). The quantitative unit non-repetitive element (NRE) is used to count each specimen that
represents one unionid shell (Giovas, 2009); to be counted as a NRE, the specimen must include
psuedocardinal teeth or an umbo, which are located on one of the densest areas of the unionid
shell, are present at all life stages, and are often identifiable to a precise taxonomic level. Fig.
4.3 shows the psuedocardinal teeth and umbo of a unionid specimen from 41HM61. While
weighing specimens is a common form of quantification for shell material, counting NRE
provides an accurate ordinal scale tally of unionid abundance (Claassen, 2000; Giovas, 2009;
Glassow, 2000; Mason et al., 1998). Since the 41HM61 assemblage is highly fragmented,
specimens that passed through a twelve millimeter sieve and did not represent a NRE were not
counted. All specimens that are either larger than a twelve millimeter sieve or represent a NRE
are included in the quantification term number of specimens (NSP), which counts the total
number of fragments in an assemblage. Following Driver (2011, 1992), specimens were
identified independently using a comparative collection of shells from the Brazos and Leon
Rivers and multiple identification guides (Howells, 2013; Howells et al., 1996; Williams et al.,
2008). A systematic paleontology can be found in Appendix A to account for data quality
(Lyman, 2011). Specimens were identified to the lowest taxonomic category possible; due to
the morphological plasticity exhibited by unionids, some categories above the species level
were used to categorize specimens from species with similar shell morphology.
9
Fig. 2.3. A specimen identified as Q. houstonensis is provided to show what constituted as a NRE during identification of this assemblage. The umbo/psuedocardinal teeth are circled in yellow. The umbo is the rounded bump on the outer edge of the shell. The psuedocardinal teeth are found as the two nubs in the upper left portion of the yellow circle. Psuedocardinal teeth are often identifiable a precise taxonomic category. The presence of psuedocardinal teeth and/or an umbo constitutes a NRE and one mussel.
Relative abundances are calculated by dividing the number of NRE identified to an
individual taxa by the total NRE in an assemblage. Using relative abundances as ordinal ranks, it
can be determined which unionid taxa are most abundant in the 41HM61 and Belton Lake
assemblages. Using the quantitative data (NRE, NSP, and presence), the fragmentation and
nestedness of the 41HM61 and the Belton Lake assemblages is assessed. For unionid remains,
fragmentation represents the preservation condition of shells in the assemblages. Species with
fragile shell morphology are likely to be under-represented in faunas that are highly
fragmented (Lyman, 1994; Wolverton et al., 2010). Fragmentation can be calculated using a
ratio of NRE:NSP, which measures the proportion of identifiable shells in an assemblage. If
fragmentation is low and most of the specimens in an assemblage are identifiable, then the
ratio will be close to one. Highly fragmented assemblages that consist of many shell fragments
will have a low NRE:NSP value (closer to zero).
10
To ensure that the 41HM61 and Belton Lake assemblages represent the same faunal
community, a nestedness analysis is conducted (Lyman, 2008; Peacock et al., 2012). Nestedness
analysis assesses whether smaller assemblages are composed of the same species as larger
assemblages (see Fig. 2.4) (Wright et al., 1997). Biogeographers use this technique to evaluate
patterns of species presence on island chains; land-bridge islands have nested subsets of the
continental fauna based on the island’s size (and therefore its carrying capacity) (Atmar and
Patterson, 1993). It is also used to analyze how species richness changes among and within
river systems (Rashleigh, 2008; Vaughn and Taylor, 1999). For zooarchaeological studies, it is
expected that assemblages with lower NTAXA (number of taxa represented by the assemblage)
are nested subsets of assemblages with higher NTAXA within a paleoecological community. This
expectation stems from the fact that as a sample size increases (in terms of number of
specimens or area sampled), the number of species captured is expected to increase as well
(Atmar and Patterson, 1993; Lyman and Ames, 2007; Lyman, 2008; Wright et al., 1997).
Nestedness analysis can determine if there are unexpected or rare taxa within a set of samples,
compared to the entire data set for a larger meta-community, such as mussel communities in a
river basin; here it is used to evaluate the assumption that the 41HM61 and Belton Lake
assemblages represent the same late Holocene mussel community within the Leon River in
terms of species composition. If these assemblages represent the same faunal community, they
can be more confidently aggregated in future analyses.
11
Fig. 2.4. Graphic representation of the nestedness concept and a nestedness analysis. A) The rectangular boxes represent the different taxa represented by the different assemblages. The red rectangle has a lower number of NTAXA, of which all are found within the larger, green assemblage. B) This table represents hypothetical nested assemblages possible from the Leon River mussel community. As comparable to part A, the assemblage with smaller NTAXA is represented by a red rectangle. The taxon found within the smaller assemblage is also found within the assemblage with larger NTAXA, the green rectangle. C) This diagram is the visual representation generated by a nestedness analysis of perfectly nested assemblages. Note that the most abundant taxon are in the top row and the site with the largest NTAXA is in the first column.
Nestedness is displayed in a binary matrix that lists the most ubiquitous taxa on the top
rows and the most taxa rich sites in the left column of the table (see Fig. 2.4) (Ulrich et al.,
2009). Though a binary matrix can visually describe the nestedness of a group of assemblages,
quantification of this binary matrix is useful when assemblages are partially nested.
Biogeographers have generated a quantification unit to further describe nestedness: heat of
disorder (Atmar & Patterson, 1993; Lyman, 2008). Heat of disorder quantifies nestedness based
on the species incidence and species composition of different assemblages. At 0 degrees, the
faunas are perfectly nested and exhibit a large amount of similarity, while at 100 degrees the
faunas exhibit no nestedness (Lyman, 2008). A FORTRAN program called Nestedness is used to
calculate the binary matrix and the heat of disorder (Ulrich, 2006). While the 41HM61 and the
Belton Lake assemblages are approximately 60 miles apart, they are expected to represent the
same faunal community in terms of species composition. This expectation is appropriate as
12
both assemblages come from the same tributary of the Brazos River (Haag, 2012; Rashleigh,
2008) and should share similar habitat types and fish community composition, which are two
characteristics that influence mussel community composition. Assemblages comprise shells of
unionids gathered from multi-species beds, and since unionids provide only small amounts of
edible meat, species exclusion is unlikely (Bettinger et al., 1997; Peacock et al., 2012).
Results
Taxonomic abundance data generated from the 41HM61 assemblage and the Belton
Lake assemblage is reported below (Fig. 2.5). The 41HM61 assemblage had a total of 399 NRE
and 1054 NSP. Of the 399 NRE total, 314 were identified; 85 specimens were unidentifiable. In
the 41HM61 assemblage, 14 taxa are represented. Amblema plicata, Quadrula verrucosa, and
Lampsilis sp. dominate the 41HM61 assemblage, with relative abundances of 23.3%, 9.5% and
9.3% respectively. The Belton Lake assemblages totaled 525 NRE, with 487 identified to the
species level. In the Belton Lake assemblages, 11 taxa were identified. Amblema plicata,
Quadrula houstonensis and Fusconaia mitchelli dominate the assemblages, with relative
abundances of 58.5%, 22.5% and 7.2% respectively. The entirety of the abundance data can be
found in Fig. 2.5. From these histograms, it appears that 41HM61 exhibited more species
evenness across the assemblage. A diversity index is not used to quantify species diversities of
the assemblages due to different sample sizes and preservation contexts, which would
inherently affect richness and evenness (Lyman, 2008).
13
Fig. 2.5. Relative abundances of taxa in the 41HM61 and the Belton Lake assemblages.
Fragmentation is used in this chapter as a proxy for preservation of mussel remains from
an assemblage. The 41HM61 assemblage has a NRE:NSP ratio of 0.38 and the Belton Lake
assemblages have a NRE:NSP ratio of 0.93, indicating better shell preservation in the latter
assemblages.
The nestedness of the individual Belton Lake assemblages and the 41HM61 assemblage
is evaluated to determine whether both of these assemblages represent the same faunal
community in terms of species composition. As seen in the binary matrix (Fig. 2.6), the
zooarchaeological assemblages from the Leon River are highly nested, with 1.85 degrees of
heat. Since the zooarchaeological assemblages visually appear nested and have a low matrix
temperature, it can be confidently assumed that they represent the same late Holocene faunal
community from the Leon River, and therefore these assemblages are aggregated in future
analyses.
14
Fig. 2.6. Nestedness matrix generated from the zooarchaeological assemblages from the Leon River. Comparing this binary matrix to the one exampled in Fig. 2.4C, it is evident that the assemblages are well nested. The most diverse sites are generally listed from left to right. The most ubiquitous taxa are listed from the top to the bottom. Almost all assemblages contained A. plicata, other than 41BL780B. The bottom row represents the number of taxa found in each site.
Discussion
This description of the late Holocene freshwater mussel community of the Leon River
provides insight into nominal scale changes in the unionid community. A longitudinal survey
conducted by Randklev et al. (2013) describes the modern freshwater mussel community of the
Leon River. Cyrtonias tampicoensis, Fusconaia mitchelli, and Lampsilis hydiana were not
encountered in the modern survey yet these species were found in prehistoric assemblages and
shell material has been recorded in historic surveys. The importance of the absence of these
species in the modern survey has implications for the conservation of the freshwater mussel
fauna in this stream.
15
Cyrtonaias tampicoensis has never been recorded live in the Leon River basin, but fresh-
dead specimens have been collected in the middle and lower Leon River (Howells, 1997;
Randklev et al., 2013a). As such, it expected that today C. tampicoensis is rare and/or located
only in the lower Leon River. Lampsilis hydiana has not been collected from the Leon River since
the early 1930s and is expected to be locally rare or extirpated within the Leon River basin
(Randklev et al., 2013a; Strecker, 1931). While not considered of conservation concern, C.
tampicoensis and L. hydiana’s absence from the Leon River has implications for unionid
conservation. Both of these species’ presence in zooarcheaolgical assemblages provides
evidence that range constrictions have occurred, which is evidence of population decline. These
zooarchaeological data support that unionid populations are declining in central Texas rivers
(Richardson and Whittaker, 2010; Whittaker et al., 2005).
Both zooarchaeological assemblages contain shell remains of F. mitchelli, which is a
species of conservation concern in Texas. Presumed to be extinct, a small population of F.
mitchelli was found in the Guadalupe River in 2011 (Randklev et al., 2013b, 2012). Historically,
its distribustion included the Guadalupe, San Antonio, Colorado and Brazos River basins
(Randklev et al., 2012). This species has remained listed as state threatened since November
2009 and is pending review as a candidate for federal listing on the Endanged Species Act
(Texas Parks & Wildlife Department 2009, U.S. Fish and Wildlife Service 2014). The presence of
F. mitchelli in these zooarchaeological assemblages and subsequent absences from the survey
in 2011 confirms that the distribution of F. mitchelli in the Brazos River watershed has
decreased or has been eliminated. As more contemporary biogeographic data are gathered
through ecological surveys, the distribution of these species will be better defined and the
16
amount of range constriction and/or extirpation of these species within the Brazos and Leon
Rivers can be more carefully evaluated.
Conclusion
The Leon River of central Texas has likely been impacted through increased agricultural
and urban run off and water use changes (Harmel et al., 2008; Randklev et al., 2013a; Rossi et
al., 2008). These anthropogenic changes to the Leon River have negatively impacted freshwater
mussels causing change in the mussel community since the late Holocene within this tributary
of the Brazos River. The description of the freshwater mussels from the 41HM61 and Belton
Lake assemblages provide evidence for the late Holocene distribution of freshwater mussels
within the Leon River. These descriptions stand as evidence for nominal-scale change within the
Leon River due to negative anthropogenic impacts. The declining richness of the freshwater
mussel community and range constriction of these species indicate that these faunas are likely
experiencing population decline. Though it is evident from the absence of F. mitchelli, L.
hydiana, and C. tampicoensis in the contemporary survey that the Leon River mussel
community has changed since the Late Holocene, a further exploration of this unionid
community change is found in Chapter 3.
17
CHAPTER 3
CHANGE IN THE LEON RIVER’S FRESHWATER MUSSEL COMMUNITY STRUCTURE SINCE
THE LATE HOLOCENE
Introduction
Anthropogenic alteration of streams has negatively affected riverine communities,
causing population declines of many species (Humphries and Winemiller, 2009; Palmer et al.,
2008; Parmesan and Matthews, 2006). Freshwater mussels (Family: Unionidae) are no
exception to this trend (Galbraith et al., 2010; Nobles and Zhang, 2011; Williams et al., 1993).
As sedentary filter feeders with a parasitic larval stage, these organisms are sensitive to many
anthropogenic changes to streams, such as decreased river conductivity, increased water
temperature, and habitat degradation (Haag and Warren Jr., 2008; Shea et al., 2013; Vaughn
and Taylor, 1999). Their sensitivity to ecological change represents impacts experienced by
other riverine fauna, making them indicator species for aquatic environments (Cummings and
Graf, 2010; Haag and Williams, 2013; Strayer, 2008). By understanding how the unionid
community has changed through time, conservation biologists can better understand how river
ecosystems have changed over time.
A longitudinal survey of the Leon River in central Texas revealed decreased freshwater
mussel richness and distribution (Randklev et al., 2013a). Changes to the structure of the
unionid community are concerning as the Leon River harbors many state threatened species
and two candidates for listing on the Endangered Species Act (Fusconaia mitchelli, Quadrula
houstonensis, and Truncilla macrodon) (U.S. Fish and Wildlife Service 2014; TPWD 2009). A
comparison between the late Holocene and modern unionid assemblages of the Leon River
18
potentially reveals ecological shifts in this community. This chapter uses chi square goodness of
fit tests and Spearman’s rho correlation to compare the structure of the unionid community
from the past and present. The habitat represented by the late Holocene freshwater mussel
assemblage will be evaluated using nonmetric multi-dimensional scaling (NMDS) ordination.
These analyses evaluate potential community change in the Leon River with implications for the
management of freshwater mussels.
Background
Zooarchaeological assemblages provide a representation of past environments for
wildlife managers and conservation biologists (Lyman, 2012a; Wolverton and Lyman, 2012).
Changes in community structure have been explored using zooarchaeological data to evaluate
shifts in ecosystem function and animal ecology (Newsome et al., 2007; Peacock and Seltzer,
2008; Randklev and Lundeen, 2012; Wolverton, 2008). The results of applied zooarchaeological
studies benefit freshwater mussel conservation by providing new information about prehistoric
unionid communities (Miller et al., 2014; Peacock, 2012; Peacock et al., 2012; Randklev and
Lundeen, 2012).
An nonmetric multidimensional scaling (NMDS) ordination is used to assess similarity
between the late Holocene and modern freshwater mussel assemblage based on community
structure (Kindt and Coe, 2005; Ricklefs, 2008). This ordination also potentially illuminates what
mesohabitat sub-assemblage is represented by the late Holocene assemblage. In this
ordination, the modern data is separated into sub-assemblages based on mesohabitat, which
should harbor different unionid sub-assemblages based on habitat preferences. This technique
is appropriate for unionid communities as these organisms are sedentary, can only thrive in
19
suitable habitats, and often form discontinuous patches within streams (Dennis, 1984; Haag,
2012; Strayer, 2008; Vaughn and Pyron, 1995). Unionids can be found in a variety of
mesohabitats. Suitable habitats must maintain flow, provide stable sediments, and the
necessary physiological (temperature, dissolved oxygen, etc.) conditions for that species (Allen
and Vaughn, 2010; Baldigo et al., 2004; Haag and Warren Jr., 2010; Haag, 2012; Holland-Bartels,
1990; Strayer, 2008). Within streams there are a variety of mesohabitats available for mussel
colonization, based on different sediment types and stability (Holland-Bartels, 1990; Strayer,
1999). For this chapter, mesohabitats are defined as different locations in a stream based on
sediment type and flow. Three categories define mesohabitats: lotic, lentic, and transitional.
Lotic mesohabitats generally have fast, consistent flow and gravel substrate. Lentic
mesohabitats are usually depositional areas in lotic environments with sand or clay substrate
and slow flow. Transitional mesohabitats are a mixture of both lentic and lotic conditions: deep,
moderate flow with gravel substrate. Fig. 3.1 provides a conceptual diagram of the spatial
distribution of different mesohabitats within a stream.
Definition of species preference for different microhabitat has eluded malacologists, but
mesohabitat preferences are known for different mussel tribes (Haag, 2012; Holland-Bartels,
1990). The Anodontini and Lampsilini tribes prefer lentic mesohabitats and are specialized for
sand/clay substrate and low flow (Haag and Warren Jr., 2010; Haag, 2012). Species of the
Quadrulini tribe are usually found within main channel habitats, in either riffles or deep runs,
but can exhibit variation between species. Other tribes, specifically the Amblemini tribe, are
mesohabitat. Some species exhibit a high correlation with specific mesohabitats. Quadrula
verrucosa is known to inhabit riffles, often with females on the substrate surface (Haag, 2012;
20
Howells et al., 1996). Table 3.1 lists the species found in the Leon River zooarchaeological
assemblages by tribe and its expected preferred mesohabitat.
Fig. 3.1. Conceptual diagram of mesohabitats within river systems. This diagram shows different mesohabitats found within a segment of river. The grey regions are indicative of lentic areas; these areas are protected from strong flows and often have sand substrate. The white regions of the river represent areas of strong, but consistent flow and often have gravel substrate. Taken from Haag (2012).
Study Area
The Leon River is a low-order stream in central Texas. The river was impounded in 1954
and 1963, to create the Belton Lake and Proctor Lake reservoirs (Hendrickson Jr., 1981;
Randklev et al., 2013a). This river experiences a high amount of urban and agricultural runoff
(Harmel et al., 2008; Rossi et al., 2008). A longitudinal survey of the Leon River was conducted
in 2011 and found a unionid community with decreased richness and abundance compared to
historical data (Randklev et al., 2013a). While the nominal differences between the late
Holocene and modern unionid communities are explored in Chapter 2, this chapter evaluates
differences in the community structures and related habitat types at an ordinal scale.
21
Table 3.1 Mussels of the Leon River & preferred habitat. Tribes adapted from Haag (2012), which is adapted from Campbell et al. (2005) and Graf & Cummings (2007). Mesohabitat estimates are found in the following sources: (Haag, 2012; Hammontree, 2013; Howells et al., 1996). * indicates the species is either a candidate or petitioned for federal listing on the Endangered Species Act.
Leon River Taxa Tribe Mesohabitat Amblema plicata Amblemini Lotic Arcidens confragosus Anodontini Lentic Cyrtonaias tampicoensis Lampsilini Fusconaia mitchelli* Pleurobemini Lotic Lampsilis hydiana Lampsilini Lentic Lampsilis sp. Lampsilini Lentic Lampsilis teres Lampsilini Lentic Leptodea fragilis Lampsilini Lentic Megalonaias nervosa Quadrulini Transitional Potamilus purpuratus Lampsilini Transitional Pyganodon gradis Anodontini Lentic Quadrula apiculata Quadrulini Lentic Quadrula houstonensis* Quadrulini Lotic Quadrula sp. Quadrulini Lotic Quadrula verrucosa Quadrulini Transitional Toxolasma sp. Lampsilini Lentic Truncilla macrodon* Lampsilini Uniomerus tetralasmus Quadrulini Utterbackia imbecillis Anodontini Lentic
Methods
The goal of this chapter is to compare the structure of the Leon River’s unionid
community from the late Holocene to today using inferential statistics. These communities are
defined using two different types of data: zooarchaeological assemblages and a modern
longitudinal survey. Since zooarchaeological data are ordinal at best (Grayson, 1984;
Wolverton, 2013; Wolverton et al., 2014), non-parametric inferential statistics are used in this
study. The non-repetitive element (NRE) is used to quantify the number of mussels represented
22
for each taxon in the zooarchaeological assemblages (see Chapter 2 for further discussion of
NRE). Two zooarchaeological assemblages are used to represent the late Holocene unionid
community: the 41HM61 and the Belton Lake assemblages. 41HM61 is a zooarchaeological
assemblage that contains 399 NRE and represents an upstream portion of the Leon River below
the modern Lake Proctor reservoir. The Belton Lake assemblages are an aggregation of small
zooarchaeological assemblages surrounding the modern Lake Belton reservoir; this assemblage
contains 525 NRE. The modern freshwater mussel community is represented by a longitudinal
survey conducted by Randklev et al. (2013). During this survey, mussels were gathered using
qualitative methods that utilized a survey method known for capturing rare and endangered
species (Metcalfe-Smith et al., 2000; Randklev et al., 2013a). The unionid communities
represented by the 41HM61, Belton Lake, and the modern assemblage are described in terms
of taxonomic relative abundance in Fig. 3.2.
23
Fig. 3.2. Relative taxonomic abundances of late Holocene and mordern assemblages from the Leon River. Zooarchaeological assemblages are represented first (41HM61 and the Belton Lake assemblages). Taxa are listed in alphabetic order to facilitate comparison of different community structures.
Before the inferential tests are completed, the zooarchaeological and modern data sets
need to be consolidated and made comparable by eliminating taphonomically relevant and
extirpated taxa. Taphonomically relevant species are those that are thin-shelled and as such
their abundances are potentially altered by taphonomic processes (Wolverton et al., 2010). In
24
this study, taphonomically relevant species include Leptodea fragilis, Pyganodon grandis,
Toxolasma sp., Uniomerus tetralasmus, and Utterbackia imbecillis. Abundance data at the
generic level (e.g. Quadrula sp. and Lampsilis sp.) are excluded as they are not comparable to
the modern data, which were identified to species. Four species have been extirpated from the
Leon River: Cyrtonaias tampicoensis, Fusconaia mitchelli, Lampsilis hydiana and Truncilla
macrodon (Randklev et al., 2013a). These species are excluded from the chi square goodness of
fit test as their absences (or 0% relative abundance) cause undefined terms in the calculations.
Removing the taphonomically relevant and extirpated species makes the data sets comparable
and the inferential tests more conservative. The inferential tests are also conducted including
all taxa identified to the species level to provide clarity and for quality control purposes
(Simmons et al., 2011). The results of the inferential tests using the complete data can be found
in Table 3.2. Since some species are excluded from the analysis, relative abundances and rank-
ordered abundances for the late Holocene and modern data are recalculated (Appendix B).
A comparison between the late Holocene and today’s unionid assemblages is conducted
using chi-square goodness of fit tests and Spearman’s rho correlations. Chi-square goodness of
fit tests have been used before to compare prehistoric and modern unionid assemblages in
Kansas (Miller et al., 2014). A chi-squared goodness of fit test is a categorical test that compares
observed values to expected values in terms of relative abundance. Using the paleo and
modern data as observed and expected values respectively, this test determines if the relative
abundances differ between these two communities. The null hypothesis of a chi-square
goodness of fit test is that the observed values match the expected values, or that the late
Holocene species’ relative abundances match the modern species’ relative abundances.
25
A Spearman’s rho correlation compares the rank order of two data sets to determine if
there is a correlation. This has been used in applied zooarchaeological studies before to assess
differences between the age structure of modern and prehistoric assemblages of Missouri
bears (Wolverton, 2006). The null hypothesis of the Spearman’s rho correlation is that the
assemblages are not correlated, indicating there is a difference between the assemblages in
terms of rank order abundance. A strong correlation would indicate that the community
structures match in terms of ranked taxonomic abundance (most abundant species then are
still the most abundant now). A weak correlation would indicate that the community structures
are different (most abundant species then are lower ranked now). Both of these inferential
tests determine if the prehistoric unionid community matches the modern unionid community
in terms of taxonomic structure. Each inferential test is completed by comparing the individual
zooarchaeological assemblages (the 41HM61 and the Belton Lake assemblages) to the modern
data and then the aggregated late Holocene data to the modern data.
An unconstrained NMDS ordination of all Leon River unionid communities is used to
evaluate similarity between the taxonomic structure and therefore mesohabitat preferences of
the modern and zooarchaeological sub-assemblages. NMDS ordination is a nonmetric form of
multi-dimensional scaling, which identifies variables that cause objects to be similar or different
(Kachigan, 1991; Kindt and Coe, 2005). The variables are then condensed onto dimensions,
which relate the similarity or differences among objects based on those variables, and a
perception map is generated. Stress is a statistical measure of the loss of information due to
fewer dimensions. The fewer dimensions used in a perception map, the more stress an
ordination has. Stress is considered acceptable when it is below 0.15 (Kachigan, 1991). The
26
variables used in this NMDS ordination are relative abundances of unionid taxa; the objects are
the mesohabitat sub-assemblages and zooarchaeological assemblages from the Leon River. By
arranging the data sets this way, differences in the taxonomic relative abundances of the
assemblages will be related on the two dimensions of the perception map. Unconstrained
NMDS ordinations are completed in R using the package vegan (R Core Team, 2014). Bray-Curtis
dissimilarity calculation was used to determine the distance matrix that is used to complete the
NMDS ordination. An ordination was conducted among unionid assemblages that had more
than 30 NRE or mussels represented. Another NMDS ordination was conducted on samples
aggregated by time period and mesohabitat type for conceptual purposes. Prehistoric
assemblages are aggregated by zooarchaeological assemblage and modern assemblages are
aggregated based upon mesohabitat type.
Results
Results of the chi-square tests generally indicate that modern and late Holocene
communities in the Leon River differ. The test comparing the Belton Lake assemblage and the
modern assemblage determined that there was a significant difference between the two
assemblages, with strong effect size (Table 3.2). The residuals indicate that the difference
between the Belton Lake assemblage and the modern community is driven by the relative
abundances of Amblema plicata and Quadrula verrucosa. The test comparing the 41HM61
assemblage and the modern assemblage appears to have been driven by differences between
the relative abundances of A. plicata and Quadrula houstonensis. When the late Holocene
assemblages are aggregated, differences are driven by relative abundances of A. plicata and Q.
verrucosa. In total, the chi-square goodness of fit tests indicate that the structure of the late
27
Holocene unionid community is different than today’s unionid community, primarily between
the abundances of A. plicata, Q. houstonensis, and Q. verrucosa.
The Spearman’s rho correlation between the Belton Lake and modern freshwater
mussel assemblage resulted in rho = 0.44, a moderate correlation between the data sets,
indicating there are some abundance rank switches between these two assemblages. The
results of the Spearman’s rho correlation between the 41HM61 assemblages and the modern
freshwater mussel assemblage indicate that rho = 0.74, a strong correlation between the data
sets, meaning the rank-ordered abundances of the 41HM61 assemblage are similar to those of
the modern assemblage. When the late Holocene assemblages are aggregated, rho = 0.63, a
moderate correlation. In each of these correlations, the largest abundance rank shifts occur
among Fusconaia mitchelli, Lampsilis hydiana, and Potamilus purpuratus, which are species
either missing in the modern or late Holocene assemblages.
Table 3.2 Results of all chi-square goodness of fit test and Spearman’s rho correlation. For the chi-square goodness of fit tests, p<0.05. The degrees of freedom on the culled data are 7; the critical value is 14.1. For the complete data, degrees of freedom are 10 and the critical value is 18.3. In the chi-squared goodness of fit tests, extirpated species are excluded because of their expected abundance of 0. For the culled data, the Spearman’s rho correlations included 13 taxa. For the complete data, 17 taxa are included in the spearman rho correlations. See Appendix B for data used in these inferential tests.
Inferential Test
Culled Data Complete Data Belton
Lake vs Modern
41HM61 vs Modern
Late Holocene
vs. Modern
Belton Lake vs
Modern 41HM61 vs
Modern
Late Holocene
vs. Modern Chi-Square Goodness of Fit Test
χ2 678.9 169.9 721.5 841.1 222.4 919.1
Cohen’s W 1.21 0.89 1.03 1.34 1.02 1.16
Spearman’s rho
Rho 0.44 0.74 0.63 0.34 0.45 0.38 Correlation
strength Moderate Strong Strong Weak Moderate Weak
28
An unconstrained NMDS ordination assesses similarity between mesohabitat sub-
assemblages of the modern Leon River community and the zooarchaeological assemblages. To
do this, a Bray-Curtis dissimilarity calculation is completed to create a distance matrix that
describes the assemblages’ similarities in terms of taxonomic abundance (Kindt and Coe, 2005;
R Core Team, 2014). This analytical technique finds both prehistoric assemblages are most
similar to each other and lotic mesohabitats (Fig. 3.3). The NMDS ordination of the
unaggregated data (Fig. 3.3A) had a stress of 0.066. This indicates that the ordination is a good
representation of differences among the sites. The first dimension (x-axis) used in the NMDS
perception map quantified community differences between lotic and lentic mesohabitat sub-
assemblages. The NMDS ordination of the aggregated data did not contain enough objects to
accurately calculate a stress value for the ordination. It is included in this chapter to
conceptualize the similarities seen in the unaggregated NMDS ordination. From these
ordinations, it is understood that the late Holocene assemblages likely reflect lotic
mesohabitats since they more closely resemble the modern lotic sub-assemblages. This NMDS
ordination and the inferential tests are used to understand differences between the modern
and late Holocene freshwater mussel assemblages of the Leon River.
29
Fig. 3.3. NMDS ordination of freshwater mussel assemblages based on assemblage species structure. Part A is an NMDS ordination of individual sites on the Leon River that had more than 30 mussels represented. Sites starting with M are from the longitudinal survey conducted by Randklev et al. (2013). Sites starting with P are from late Holocene zooarchaeological assemblages, either Belton Lake (BL) or Hamilton County (41HM61). The red area includes modern unionid sub-assemblages from lotic mesohabitats (white areas in Fig. 3.1). The green area includes modern unionid sub-assemblages from lentic mesohabitats (grey areas in Fig. 3.1). Mesohabitat categorization defined in Randklev et al. (2013). Part B is an NMDS ordination of sites aggregated by time period or mesohabitat type.
Discussion
The structure of the freshwater mussel assemblage has changed since the late Holocene
in terms of species composition (see Chapter 2) and relative abundances. The residuals of the
chi-square goodness of fit tests indicate that differences in the relative abundances of A.
plicata, Q. verrucosa, and Q. houstonensis are greatest. Amblema plicata constitute a higher
proportion of the late Holocene assemblages than in the modern assemblages. Quadrula
verrucosa constitute a lower proportion of the late Holocene assemblages than in the modern
assemblages. The difference in the proportion of Q. verrucosa is greatest in the Belton Lake
assemblage. Quadrula houstonensis constitute a smaller proportion of the 41HM61 assemblage
30
than in the modern assemblage. These shifts in community structure could be attributed to
preservation potential, identifiability or ecological change. These possible forms of equifinality
are discussed further in Chapter 4, but this comparison provides evidence of a change in
abundance of Q. houstonensis, a species of conservation concern (TPWD, 2009; U.S. Fish and
Wildlife Service, 2014). Since Q. houstonensis is considered sensitive to anthropogenic impacts,
evidence of abundance shifts since the Late Holocene could be indicative of habitat change in
the Leon River (Howells, 2010a; Howells et al., 1996).
In the Spearman’s rho correlations, the greatest rank switches occur among species that
have been extirpated from the river basin, mainly F. mitchelli and L. hydiana. Since these
species were omitted from the goodness of fit test, which cannot subsume categories with
frequencies of zero, their large rank abundance shifts evident in the Spearman’s rho correlation
are important. The extirpations of F. mitchelli and L. hydiana are discussed in Chapter 2. While
C. tampicoensis was not found in the modern longitudinal survey, its rank abundance has not
shifted as much as other extirpated species. Cyrtonaias tampicoensis is more abundant (ranked
7) in the Belton Lake assemblages compared to all other unionid assemblages of the Leon River,
which aligns with expectations that this species is generally rare in this tributary but more
abundant near the confluence of the Leon and Little Rivers. Potamilus purpuratus also exhibited
a sizeable shift in rank since it is missing from the late Holocene assemblages. This could
potentially be a preservation issue in the zooarchaeological assemblages, or could be related to
a historical range extension into the Leon River. While some rank-order abundances have
switched since the late Holocene, the more abundant species in the past remain higher ranked
in the modern assemblages.
31
The NMDS ordination of the modern mesohabitat communities and the late Holocene
assemblages is informative if problematic. The NMDS ordination relies on the fact that the
zooarchaeological assemblages represent the prehistoric freshwater mussel community’s
structure accurately. As Chapter 4 describes, representation of unionid communities by
zooarchaeological assemblages are affected by multiple mechanisms (Wolverton et al., 2010).
Both late Holocene assemblages are more similar in taxonomic structure to lotic communities
than lentic communities. If the zooarchaeological assemblages are assumed to accurately
represent community structure, this similarity could be the result of two different processes:
either the Leon River was more lotic in the past or the prehistoric humans collected unionids
preferentially from lotic mesohabitats. In either case, this chapter stands as evidence to
preserve remaining lotic mesohabitat in the Leon River since many unionid species, especially
those of conservation concern (F. mitchelli and Q. houstonensis), prefer lotic mesohabitats
(Howells, 2010a, 2010b; Howells et al., 1996).
The results of this comparison between the modern and late Holocene freshwater
mussel fauna suggests ecological change in the Leon River unionid community since the late
Holocene, potentially with an increase in lentic mesohabitat over time. Impoundments of the
Leon River have altered the hydrologic regime of the Leon River, which alters habitat structure
and the resulting modern freshwater mussel community (Allen et al., 2013; Baldigo et al., 2004;
Vaughn and Taylor, 1999). As habitat is intrinsically entwined with life history strategies of
unionids, habitat change directly impacts conservation efforts (Haag, 2013, 2012; Southwood,
1988, 1977). This chapter shows that the Leon River unionid community has changed since the
late Holocene and provides a new conservation baseline for wildlife managers. By conserving
32
the remaining lotic habitat and managing the hydrologic regime, results of this analysis indicate
that mangers would be promoting habitat preferred by native freshwater mussels.
Conclusion
The Leon River has experienced many anthropogenic impacts through multiple
impoundments and an influx of urban and agricultural runoff (United States Department of
Agriculture, 2005; Harmel et al., 2008; Hendrickson Jr., 1981; Randklev et al., 2013; Rossi et al.,
2008), which are likely detrimental to riverine fauna abundance and diversity. A comparison
between the modern and late Holocene freshwater mussel community structure of the Leon
River found that the unionid community structure has changed since the late Holocene and
potentially preferred lotic mesohabitat assemblages. This comparison revealed that Q.
houstonensis, a federal candidate for listing on the Endangered Species Act that is sensitive to
anthropogenic impacts, has increased in abundance since the late Holocene (Texas Parks and
Wildlife Department, 2009; Howells, 2010a; Howells et al., 1996; U.S. Fish and Wildlife Service,
2014). The late Holocene zooarchaeological assemblages also are most similar to lotic
mesohabitat sub-assemblages from the modern Leon River. This could indicate that
impoundments have increased the lentic mesohabitats in this system. This chapter stands as
evidence of the late Holocene freshwater mussel community of the Leon River and promotes
the conservation of this unique community by providing another baseline for conservation.
33
CHAPTER 4
A CONCEPTUAL MODEL FOR ASSESSING FRESHWATER MUSSEL TAXONOMIC ABUNDANCES IN
ZOOARCHAEOLOGICAL FAUNAS
Introduction
Zooarchaeology is the study of faunal remains from archaeological contexts to study
human behavior, past environments, and past animal ecology (Lyman, 2005; Reitz and Wing,
2008; Wolverton and Lyman, 2012). Freshwater mussel (hereafter unionid) remains are
prevalent in many zooarchaeological assemblages and have been used to answer many
archaeological questions (Peacock, 2005). Paleozoological and zooarchaeological unionid
presence/absence data are most often used as evidence of shifts in human subsistence or
biogeographic distributions of taxa (Parmalee and Klippel, 1974; Peacock and Chapman, 2001;
Peacock, 2012). Abundance data from zooarchaeological assemblages can provide additional
data with which to approach these types of questions. For example, abundance data can be
used to study human behavior through foraging theory to study shifts in diet breadth (Byers
and Broughton, 2004; Nagaoka, 2005, 2002). Such data are also used in paleoenvironmental
studies, for conservation purposes, or to assess environmental changes in species abundance as
an alternative hypothesis to change in human subsistence (Braje et al., 2012; Casey, 1986;
Klippel et al., 1978; Peacock and Seltzer, 2008; Peacock et al., 2005). Abundance data can also
be used to quantify ecological community structures, which has implications for conservation
and paleoecology (Grayson, 1987; Miller et al., 2014; Randklev and Lundeen, 2012). Because
taxonomic abundance data are used to address such a wide variety of research questions by
zooarchaeologists and paleozoologists, it is important to understand that the structure of such
34
data. Abundance data produced from zooarchaeological assemblages can be influenced by
many different forces, such as population abundance on the prehistoric landscape, differential
preservation of shells, preferences of prehistoric people who incorporated unionids in their
diets, and/or differential identifiability of some remains over others (Kidwell and Flessa, 1995;
Lyman, 1994; Peacock et al., 2012). These influences then function as alternative mechanisms
that might drive patterns in taxonomic abundance.
Three distinctive mechanisms are addressed in this chapter: variable life history
strategies and their influence on population abundances, identifiability of shells, and
preservation potential of shells from different unionid taxa. Life history strategies play an
integral role in constructing ecological communities (Pianka, 1970; Southwood, 1988, 1977)
and, thus, potentially influence taxonomic abundances in zooarchaeological data (Kidwell and
Rothfus, 2010; Kidwell, 2013). Differential life history strategies could directly impact the
abundance of different taxa within life assemblages, death assemblages and resulting
excavated assemblages. In addition, differences in accuracy and precision of taxonomic
identification could directly affect zooarchaeological mussel abundance. Identifiability is
positively correlated with the presence of sculpture; shells with sculpture are identified to
species more often than those without sculpture (Shea et al., 2011). In addition to identifiability
and abundance related to life history ecology, quantitative taxonomic zooarchaeological data
may be affected by differential preservation of shells from one taxon over another. Different
unionid taxa have distinctive shell preservation potential based on the species’ shell sphericity
and shell density (Wolverton et al., 2010). Together these factors have complex but predictable
influences on zooarchaeological freshwater mussel taxonomic abundance data; thus, a
35
conceptual predictive model that provides general expectations about which species ought to
and ought not to be abundant can aid research in zooarchaeology and paleozoology that
focuses on unionids.
Rank order continua of life history strategy, identifiability (based on sculpture), and
preservation potential are developed for taxa encountered in the Leon River assemblages to
produce a multivariate conceptual model that allows zooarchaeologists to more easily interpret
the meaning of taxonomic abundance data in terms of taphonomy, ecology, and human
behavior. Specifically, this conceptual model will help interpret the comparison between the
modern and late Holocene freshwater mussel communities of the Leon River completed in
Chapter 3 (Table 4.1).
Table 4.1 Taxonomic abundances within the 41HM61 and Belton Lake assemblages. Missing values listed in the Belton Lake Assemblages indicate no specimens of that taxon were identified. Non-repetitive element (NRE) is a quantification unit used to count the total number of mussels represented in an assemblage; it is more fully described in Chapter 2. See Appendix 1 for identification criteria for the 41HM61 assemblage.
Taxa 41HM61 Belton Lake Assemblages
NRE Relative Abundance NRE Relative Abundance
Amblema plicata 93 23.3% 307 54.2% Arcidens confragosus 1 0.3% 1 0.2% Cyrtonaias tampicoensis 2 0.5% 6 1.1% Fusconaia mitchelli 9 2.3% 38 6.7% Lampsilis sp. 37 9.3% Lampsilis hydiana 10 2.5% 14 2.5% Lampsilis teres 34 8.5% 5 0.9% Megalonaias nervosa 16 4.0% 2 0.4% Quadrula sp. 34 8.5% 1 0.2% Quadrula apiculata 13 3.3% 20 3.5% Quadrula houstonensis 20 5.0% 118 21.0% Quadrula verrucosa 38 9.5% 13 2.3% Toxolasma sp. 2 0.5% Truncilla macrodon 5 1.3% Unidentifiable umbo 85 21.3% 41 7.1% Total NRE 399 566
36
Background
Representativeness should be assessed to understand potential explanations for the
structure and composition of zooarchaeological taxonomic abundance data (Grayson, 1987;
Lyman, 2012a, 1994). The excavated assemblage from which zooarchaeological data are
produced has passed through many filters prior to analysis; these filters include aggregation
into a deposited assemblage, time in the lithosphere, potential sampling biases, and analysis by
zooarchaeologists, each potentially resulting in various forms of data loss (as seen in Fig. 4.1).
Some of these filters have been analyzed before for freshwater mussels, such as the ‘cultural
filter’ and potential for differential preservation of shell between the time of human
consumption and archaeological recovery (Peacock et al., 2012; Wolverton et al., 2010). For
example, as mussels were collected from streams by humans, there is potential for preferential
selection of particular species and individuals, which could lead to unrepresentative sampling of
the past ecological community. Peacock et al. (2012) addressed the problem of this ‘cultural
filter’ and provided methods, such as nestedness and isotopic profiling, for analyzing an
assemblage’s representativeness of past local ecological communities. Nestedness analysis was
used in Chapter 2 to evaluate whether or not 41HM61 and the Belton Lake assemblages
represent the same ecological community in terms of species composition.
37
Fig. 4.1. Model describing potential filters within zooarchaeological assemblages. Certain steps (light gray) of the life-cycle of zooarchaeological assemblages are shown to demonstrate different factors that can affect final zooarchaeological data. Though many processes (smaller words between steps) act on each of these steps, only processes relevant to this study were included to explain where different factors affect zooarchaeological remains. Note that at each step the assemblage gets smaller, indicating the potential for data loss as remains transition from the biological community through the taphonomic history to zooarchaeological data. Based on figures from Lyman (2010), Clark and Kietzke (1967), and Behrensmeyer and Kidwell (1985).
In terms of taphonomy, shells from unionid species exhibit different shapes and
structures (Wolverton et al., 2010), which can affect potential for preservation and thus the
quantity or presence of a species in an assemblage. Wolverton et al. (2010) addressed this
preservation potential of shells from different species of unionids in zooarchaeological
assemblages based on two different physical characteristics of shell: shell density and shell
sphericity. Using modern specimens from the Brazos and Trinity River, they determined that as
a shell’s density increases, its preservation potential increases as well. As shell’s sphericity
increases, its preservation potential also increases. A taphonomic analysis is conducted for the
Biological Community
Deposited Assemblage
Excavated Assemblage
Zooarchaeological Data
Human Collection
Sampling Bias
Zooarchaeologist Identification
Zooarchaeological Assemblage
Differential Preservation TIME
38
41HM61 and the Belton Lake assemblages to assess whether or not differential preservation
has affected taxonomic abundance.
There are other filters beyond preservation and foraging preference that can affect
remains and influence zooarchaeological abundance data, for example, identifiability and/or
experience of the faunal analyst. Differences in identifiability of shells between species relates
to two factors: distinctiveness of shell morphology and preservation potential related to
whether or not shells are prone to fragmentation. One way that zooarchaeologists account for
differences in identifiability and also bolster confidence in data quality is to fully describe
identification criteria (Driver, 2011, 1992; Wolverton, 2013). The problem remains that
specimens are often fragmented and eroded, making identifications difficult. This problem is
exacerbated by the fact that identification of live unionids in ecological surveys is difficult due
to morphological plasticity within and between species. Shea et al. (2011) addressed the
identification accuracy and precision of malacologists, and found that increased sculpture, size,
and conservation status (listed as threatened by a government agency) lead to higher
identification rates of individual mussel taxa. Those species with sculptured shells tend to be
more recognizable, those with conservation status tend to be better studied and thus more
identifiable, and those of larger size also tend to be recognizable and easier to identify. In this
study, since sculpture preserves well on fragments of shell from zooarchaeological contexts and
is often diagnostic, it will be used as a proxy for identifiability. Therefore in zooarchaeological
assemblages, species that have sculpture should be identified with more taxonomic precision
(e.g., to finer taxonomic levels, such as species) and more accurately (to the correct taxon) than
those without sculpture if identifiability is driving taxonomic abundance.
39
The life history strategy of a mussel species also has the potential to affect its taxonomic
abundance in a deposited assemblage. Life history strategies describe a species’ differential
allocation of energy based on the rate of population growth and reproductive ecology (Fisher,
1930; Macarthur and Wilson, 1967; Pianka, 1970). As different life history strategies among
species are expected to impact the abundances in a community, they should indirectly affect
the abundances in a zooarchaeological assemblage (Kidwell and Rothfus, 2010; Kidwell, 2001;
Southwood, 1988, 1977). Typically, Pianka’s r versus K selection gradient is used to define life
history strategies among different species (Olden et al., 2006; Pianka, 1970; Southwood, 1977,
1988). While unionids are generally categorized as long lived and slow growing, they exhibit a
wide range of variability of life history characteristics (Haag, 2012; Vaughn, 2012). Unionid life
history strategies are categorized into three types that are based on Winemiller and Rose
(1992)’s life history strategy three endpoint continuum: opportunistic, periodic, and equilibrium
strategies (Dillon Jr., 2000; Grime, 2001; Haag, 2012; Winemiller and Rose, 1992). The
opportunistic life history strategy is similar to Pianka’s r-selection; such mussel species are
characterized by a short life span, early maturity, and high fecundity. Equilibrium selected
mussels live long and mature later in their lives, similar to Pianka’s K-selection. Periodic-
selected mussels are “characterized by moderate to high growth rate, low to intermediate life
span, [low] age at maturity, and [low] fecundity” (Haag, 2012, p. 211). Periodic species are
generally adapted to habitats that experience cyclical environmental variation, which create
beneficial conditions during which they reproduce (Haag, 2012; Winemiller and Rose, 1992).
Species from the Leon River zooarchaeological assemblages are categorized at the taxonomic
tribe level, which is an appropriate scale for categorizing unionids by life history strategy. Table
40
4.2 illustrates the life history strategies common to tribes within central Texas and lists the
species represented in the Leon River zooarchaeological assemblages in their respective tribes.
Table 4.2 Robusticity and life history strategy values for Leon River zooarchaeological freshwater mussel assemblage. Highlighted cells indicate higher expected abundances if variable life history strategy, identifiability or robusticity influences taxonomic abundances. Robusticity mean values are found in Wolverton et al. (2010), which were calculated from a modern Brazos River assemblage. The sphericity and density values are summed to determine the ordinal rank of different species. Some species’ robusticity values were not calculated (due to small sample size); robusticity rank is uncalculated for these species. Life history strategy is determined at a nominal scale using the figure from Haag (2012), seen below as Fig. 4.2. Data Sources: 1Wolverton et al., 2010; 2 Haag, 2012; 3Tribe rank - Howells, 2013 and Haag, 2012; 4Obliquaria reflexa as a proxy - Haag, 2012 and Campbell et al., 2005. Abbreviations that are used by the authors are listed below as well and are used throughout this chapter.
Species Abbrev-iation
Robusticity Tribe2 Life History
Strategy Sculpture Category Sphericity1 Density1 Summed Rank
Amblema plicata AP 0.56 2.02 2.58 2 Amblemini Equilibrium2 High Arcidens confragosus AC Anodontini Opportunistic3 High Cyrtonaias tampicoensis CT 0.54 1.92 2.46 4 Lampsilini Periodic4 None Fusconaia mitchelli FM Pleurobemini Equilibrium3 Low Lampsilis teres LT 0.44 1.27 1.71 7 Lampsilini Opportunistic2 None Lampsilis hydiana LH 0.51 1.13 1.64 8 Lampsilini Opportunistic3 None Megalonaias nervosa MN 0.48 1.52 2.00 5 Quadrulini Equilibrium2 High Quadrula apiculata QA 0.61 1.90 2.51 3 Quadrulini Equilibrium3 High Quadrula houstonensis QH 0.63 1.24 1.87 6 Quadrulini Equilibrium3 Low Quadrula verrucosa QV 0.46 2.46 2.92 1 Quadrulini Equilibrium3 High Toxolasma sp. Ts 0.60 0.34 0.94 9 Lampsilini Periodic3 None Truncilla macrodon TM Lampsilini Opportunistic3 None
Study Area
Two zooarchaeological assemblages are studied to evaluate how the three processes
described above affect abundance data; both assemblages comprise late Holocene freshwater
mussel remains from the Leon River. The Belton Lake faunas come from 18 separate cave sites
that surround the modern Belton Lake reservoir; these sites represent the Leon River close to
its confluence with the Little River (Randklev, 2010). Those assemblages were identified by
41
Charles Randklev, and the fragmentation rate of shell remains from these rock shelter deposits
is low. The 41HM61 assemblage represents the Leon River approximately 60 miles upstream of
the Belton Lake faunas. The 41HM61 assemblage was identified by Traci Popejoy;
fragmentation is high in this assemblage. Nestedness analysis in Chapter 2 determined that
these assemblages appear to represent the same faunal community in terms of species
composition (Peacock et al., 2012), but represent two distinct preservation contexts.
A comparison between the structure of the Leon River freshwater mussel assemblages
from the late Holocene and a modern longitudinal survey is fully described in Chapter 3. From
this comparison, it is evident that Amblema plicata, Quadrula verrucosa, and Quadrula
houstonensis do not constitute the same proportion of the late Holocene assemblage as they
do in the modern assemblage. Amblema plicata is over represented in both zooarchaeological
assemblages. Quadrula verrucosa is underrepresented in the zooarchaeological assemblages,
though the difference is largest in the Belton Lake faunas. Quadrula houstonensis is
underrepresented in the 41HM61 fauna. The conceptual model constructed in this chapter is
used to assess how these patterns in the 41HM61 and Belton Lake assemblages conform to
expectations about taxonomic abundances.
Limitations
Auto-correlation among life history strategy, sculpture, and robusticity possibly disrupts
the interpretive value of this conceptual model. Since equilibrium species allocate energy
toward increased interspecific competitive ability, longer life span, and higher overall growth,
they are likely to produce a more dense shell than opportunistic species (Haag and Rypel, 2011;
Haag, 2013, 2012). Life history strategies are also possibly correlated with shell ornamentation
42
through two different processes. For many organisms, K-selection includes increased
interspecific competition which increases the likelihood for ornamentation (Mccullough, 1999;
Pianka, 1972, 1970). For unionids, correlation between shell ornamentation/sculpture and life
history strategies has not been assessed. Sculpture is a convergent evolutionary trait and
relates to flow variability and predation pressure (Haag, 2012; Hornbach et al., 2010). Since
sculpture often reflects the habitat of unionid species and life history strategies are influenced
by habitat conditions, it is possible that these two factors are correlated (Pianka, 1970;
Southwood, 1988, 1977). There may be aspects of life history, preservation potential of shells,
and sculpture that co-occur, but each also potentially influences abundance independently.
Developing conceptual expectations for each variable allows for clearer interpretation of
zooarchaeological abundance data. In addition, since species’ shell robusticity, shell sculpture
and life history strategy can vary between mussel communities, consideration of these
variables and the expectations should be limited to use within the river basin represented by
the zooarchaeological assemblage.
Materials and Methods
Unionid species can be categorized along an ordinal continuum that indicates their
potential for either being high or low in abundance according to their life history, identifiability,
and preservation potential, which is summarized in Table 4.3. Taken together, an opportunistic
species that produces a robust, sculptured shell has a higher probability of occurrence and high
abundance in zooarchaeological assemblages than an equilibrium species with fragile shell
morphology and a lack of sculpturing.
43
Table 4.3 Expectations for abundance filters discussed in this chapter. High abundance potential refers to the expectation that species with this trait will have higher abundances relative to the other species without this trait.
Filter High Abundance Potential Low Abundance Potential
Life History Strategy Species produce many offspring – Opportunistic Strategy
Species produce few offspring – Equilibrium Strategy
Identifiability Shells with sculpture Shells with no sculpture Preservation Potential Shells are spherical and dense Shells are non-spherical and are
less dense
To define the life history strategy of mussels, data from Haag’s chapter (“These are very
different animals: Life history variation in mussels”) are used to delineate mussels into the
three categories: opportunistic, periodic, and equilibrium (2012). Using Haag’s ordination figure
as a guide (Fig. 4.2), species present in the figure were assigned their expected life history
strategy categorization. When a species’ life history strategy is not found in Haag’s data,
phylogenetic and taxonomic relationships were used to make assignments (see Table 4.2 and
Campbell et al., 2005). Haag’s description of unionid life history strategies is used because it is
the most current data and provides categories to rank the species along a gradient.
44
Fig. 4.2. Ordination of life history strategies of freshwater mussels in the southeastern United States. Unionids were ordinated into life history strategy categories based on fecundity, body size, age at maturity and life span. This figure is found in Haag (2012), Chapter 6. The abbreviations can be found in that chapter. The species used in this study are boxed in red and include Apli- A. plicata, Oref- Obliquaria reflexa, Lter- Lampsilis teres, Mner- Megalonaias nervosa, Pohi- Potamilus ohiensis. Also note that all Quadrulid species (Qpus, Qasp, Qrum) are found within the equilibrium strategy.
Shell sculpture, defined as the presence of plications, wrinkles, pustules, bumps, or
ridges on a shell margin (Hornbach et al., 2010; Watters, 1994), improves identification
accuracy and precision of modern unionids (Shea et al., 2011). Shell sculpture has been
quantified by calculating the number of pustules in a squared centimeter area (Peacock and
Seltzer, 2008). In this chapter, sculpture is evaluated at a nominal scale since fragmentation and
erosion could affect the presence of sculpture on unionid specimens. Sculpture is categorized
into three groups: shells with a high density of sculpture on its outer disk (>50%), shells with
less dense sculpture on their outer margin (<50%), and shells without sculpture; identifiability
categorizations are based on sculpture on modern unionid shells from the Brazos River and its
tributaries.
45
For the robusticity analysis, two physical characteristics of shells are quantified using
data from Wolverton et al. (2010). As the Leon River is a tributary of the Brazos River, use of
Wolverton et al. (2010)’s calculated mean density and sphericity values for taxa is appropriate.
Shell sphericity was calculated by a ratio of shell length, width, and height. Shell density
describes the structural strength of the shell and was measured through volume displacement.
The mean sphericity and density values from Wolverton et al. (2010) are summed to create a
robusticity value, which is then ranked for the Leon River freshwater mussel community (see
Table 4.2). Ranking robusticity quantifies a taxon’s preservation potential within a given
zooarchaeological assemblage at an ordinal scale. A further analysis of the differential
preservation evident in both assemblages is conducted by constructing a 3D scatterplot to
evaluate preservation based on density and sphericity independently following Wolverton et al.
(2010). If life history characteristics, shell robusticity, and shell identifiability are influencing the
zooarchaeological taxonomic abundance data, it can be expected that opportunistic species
with robust, sculptured shells will be higher in abundance than equilibrium species with fragile,
unsculptured shells.
Using this multivariate conceptual model, it can be predicted which unionid species will
be most and least abundant in the Leon River zooarchaeological assemblages (Table 4.4). If
there is auto-correlation in this model, A. plicata, A. confragosus, Q. apiculata, and Q. verrucosa
have high expected abundances since these species exhibit two characteristics likely to increase
abundance. Fusconaia mitchelli, Q. houstonensis, and Toxolasma sp. are expected to be least
abundant as they lack characteristics expected to produce high abundances.
46
Table 4.4 Zooarchaeological taxonomic abundance expectations for the Leon River unionid community. Taxa are listed by alphabetical order, not by likelihood of abundance. Robusticity ranks included the three most abundant and three least abundant species.
Alternative Mechanism
High Abundance Low/Rare Abundance
Life History Strategy
Lampsilis hydiana Lampsilis teres Truncilla macrodon
Amblema plicata Quadrula apiculata Quadrula houstonensis Quadrula verrucosa
Identifiability
Amblema plicata Arcidens confragosus Megalonaias nervosa Quadrula apiculata Quadrula verrucosa
Cyrtonaias tampicoensis Lampsilis hydiana Lampsilis teres
Robusticity Amblema plicata Quadrula apiculata Quadrula verrucosa
Lampsilis hydiana Lampsilis teres Toxolasma sp.
Histograms are used to evaluate how well abundance expectations match the relative
abundance patterns in the zooarchaeological assemblages. The x-axis in each histogram is
arranged from most to least expected from left to right according to each of the three variables
(life history strategy (Fig. 4.3), sculpture and identifiability (Fig. 4.4), and robusticity (Fig. 4.5).
Results
As seen in Fig. 4.3, life history strategy does not relate to taxonomic abundance within
these zooarchaeological assemblages, as opportunistic species, such as Lampsilis teres and
Lampsilis hydiana, are expected to have high abundances. The 41HM61 assemblage is
dominated by equilibrium species, but does have a moderate proportion of opportunistic
strategists as well. Both assemblages have a low abundance of taxa that exhibit periodic life
history strategies. This is expected since many periodic species have low abundance in modern
rivers (Howells, 2013; Howells et al., 1996). The Belton Lake assemblages were dominated by
equilibrium strategists, with only a small proportion of opportunistic taxa. Since equilibrium
47
taxa dominate both assemblages, it can be stated that life history strategy does not appear to
have influenced taxonomic abundance.
Fig. 4.3. Analysis of life history strategy’s influence on the 41HM61 and the Belton Lake assemblages.
As evident in Fig. 4.4, identifiability can influence abundance data generated from
zooarchaeological assemblages. It is expected that species with highly sculptured shells, such as
Q. verrucosa and A. plicata, should exhibit higher relative abundances than species with
unsculptured shells. Both the 41HM61 and the Belton Lake assemblages are dominated by
species with sculptured shells, primarily A. plicata. Quadrula houstonensis has a high relative
abundance in both assemblages, but typically has less sculpture than Quadrula apiculata and
can even be apustulose (without sculpture). While the abundance of some taxa appears to
relate to the presence of sculpture (i.e. A. plicata), other taxa were identified based on other
physical features, such as psuedocardinal teeth (i.e. Q. houstonensis). In general, identifiability,
whether from sculpture or other morphological criteria, seems to have a stronger influence on
the abundance data from the 41HM61 assemblage than does life history.
48
Fig. 4.4. Analysis of sculpture’s influence on the 41HM61 and Belton Lake assemblages.
Generally, species with more robust shells have a higher relative abundance, as seen in
Fig. 4.5. A more in-depth analysis of how physical preservation influenced the abundance data
from these zooarchaeological assemblages is found in Fig. 4.6. The 41HM61 assemblage
includes taxa that are categorized as robust and fragile; species in both categories have high
ranked relative abundances. The Belton Lake assemblages are also affected by differential
preservation; the shells with the highest sphericity have the highest relative abundance. Both
zooarchaeological assemblages are dominated by robust taxa, but the presence of fragile taxa
with moderate abundance indicates the taxonomic abundances in these assemblages are not
solely explained by differential preservation and thus may be representative of the late
Holocene freshwater mussel community of the Leon River at a nominal or ordinal scale.
49
Fig. 4.5. Robusticities’ influence on the 41HM61 and Belton Lake assemblages.
Fig. 4.6. Taphonomic analysis of the 41HM61 assemblage and the Belton Lake assemblages. Sphericity and robusticity values for each species were ordinally ranked from Wolverton et al (2010). The darker blue represents species with high preservation potential; these species have high shell density and high shell sphericity. The lighter blue represents species with low preservation potential; these species are less dense and less spherical.
50
Discussion
This chapter contributes to the zooarchaeological literature by exploring alternative
mechanisms for the structure of unionid abundance data, which are used in many ways to
answer archaeological questions: to understand human diet, paleoenvironmental conditions,
and the paleoecology of mussel taxa. By understanding alternative mechanisms for taxonomic
abundance, zooarchaeologists can determine when forms of equifinality influence the results of
zooarchaeological studies. The conceptual framework presented in this chapter allows
zooarchaeologists to evaluate the influence of life history strategies, identifiability, and
robusticity on taxonomic abundance data. Counter to what was expected, equilibrium life-
history strategists were commonly represented in the zooarchaeological assemblages. This may
relate to sculpture and robusticity auto-correlating with life history traits. Equilibrium life
history strategists are likely to invest more energy into phenotypic expression (Geist, 1998;
Pianka, 1972, 1970), such as shell mass and complexity. In addition, robusticity to some extent
may mediate identifiability as larger fragments tend to have better preserved shell morphology.
Variable life history strategy’s influence on the structure of deposited assemblages cannot be
completely ruled out, however. If more detailed data on the fecundity of different freshwater
mussels in Texas becomes available, comparisons within life history strategy categories might
better elucidate whether or not reproductive ecology influences zooarchaeological assemblage
abundance. As a result, how variable life history strategies’ influence zooarchaeological data
remains an avenue for future research.
A comparison between the late Holocene and modern freshwater mussel community of
the Leon River is used as an example of the utility of this conceptual model. Without
51
understanding how alternative mechanisms influence taxonomic abundances, differences
between the two assemblages might be attributed to ecological change. For example, the
expectations presented here include that A. plicata should have high abundance in
zooarchaeological assemblages while Q. houstonensis is expected to have low abundance. Thus,
the differences in abundance between the Late Holocene and modern unionid community
could be the result of differential preservation, identifiability, or ecological change. Quadrula
verrucosa shells are robust and very identifiable; as such this species is expected to be high in
abundance. The comparison finds Q. verrucosa to be rarer in the late Holocene than today.
Since Q. verrucosa’s shift in abundance is opposite what is predicted by the conceptual model,
it can be more confidently attributed to ecological change in stream condition between the late
Holocene and today.
Conclusion
By assessing variable life history strategies, identifiability, and robusticity’s influence on
zooarchaeological taxonomic abundance data, whether patterns of abundance in
zooarchaeological assemblages are representative changes in human foraging behavior,
paleoenvironmental conditions, or community structure are analyzed. This evaluation revealed
that differential robusticity and identifiability influence the 41HM61 and Belton Lake
assemblages. There are alternative mechanisms for explaining the abundance of different taxa
in zooarchaeological assemblages and the implications of those data for the comparison
between the modern and late Holocene unionid assemblages of the Leon River (found in
Chapter 3). While robusticity and identifiability could not be distinguished from ecological
change as driving factors for the abundance of some taxa, Q. verrucosa is more abundant today
52
than in the late Holocene, which suggests that important environmental changes have occurred
in the Leon River, perhaps related to modern human impacts.
53
CHAPTER 5
CONCLUSION
The comparison between the late Holocene and modern freshwater mussel community
of the Leon River revealed multiple changes in this unionid community. In terms of species
composition, multiple species have been extirpated from the Leon River since the late
Holocene. These species include C. tampicoensis, F. mitchelli, L. hydiana, and T. macrodon. Two
of these species (F. mitchelli and T. macrodon) are of conservation concern, meaning they are
listed as threatened by the state of Texas and are either petitioned or a candidate for federal
listing on the Endangered Species Act (Texas Parks and Wildlife Department, 2009; U.S. Fish and
Wildlife Service, 2014). All of these species absences from the Leon River stand as evidence of
range decline, which is evidence of population decline and contributes to a potential extinction
vortex. The multiple analytical techniques used in Chapter3 to assess ecological change
revealed multiple shifts in community structure. While most rank-ordered abundances
remained the same between the late Holocene and modern assemblages (with the exception of
extirpated taxa), many of the taxonomic relative abundances have significantly changed since
the late Holocene. The chi-square goodness of fit test revealed that relative abundances of A.
plicata, Q. houstonensis, and Q. verrucosa have changed. The nonmetric multidimensional
scaling (NMDS) ordination revealed that the late Holocene freshwater mussel assemblages are
most similar in taxonomic structure to lotic mesohabitat sub-assemblages. Since freshwater
mussels are indicator species, this evidence of differences between the late Holocene and
modern unionid communities show ecological change in the Leon River.
54
A multivariate conceptual model is used to assess the influence of life history ecology,
identifiability, and robusticity on zooarchaeological taxonomic abundances data. This model
contributes to the zooarchaeological literature by improving interpretations of
zooarchaeological taxonomic abundance data. This utility is of this model is exampled by using
it to interpret the results of the chi square goodness of fit test found in Chapter 3. By comparing
the taxonomic abundances of 41HM61 and the Belton Lake assemblages to the model, it is
evident that identifiability and robusticity has influenced taxonomic abundances in these
zooarchaeological assemblages. Thus, identifiability, robusticity, and ecological change could be
driving the differences in the abundances of A. plicata and Q. houstonensis. This model
improves interpretation of the abundance shift of Q. verrucosa. Since Q. verrucosa is expected
to be highly abundant in zooarchaeological assemblages, but is actually less abundant in the
late Holocene, its abundance change is attributable to ecological change.
This thesis is an example of how zooarchaeological data can be used for conservation
purposes. It provides a late Holocene conservation baseline for the Leon River unionid
community, which is appropriate as this stream experienced anthropogenic impacts beginning
in the middle of the twentieth century, which is before most historical records were created for
the unionid community of this river (Hendrickson Jr., 1981; Randklev et al., 2013a). This thesis
also explores unionid community change since the late Holocene and supports the conclusions
of the modern survey conducted by Randklev et al. (2013) that the unionid community has
decreased in species richness since the late Holocene and historical times. Evidence of
ecological change has implications of habitat change in the Leon River, which harbors
threatened mussels (F. mitchelli, Q. houstonensis, and T. macrodon) and a federally endangered
55
fish species (Smalleye Shiner - Notropis buccula). The results of this thesis hopefully influence
conservation of this unique community and the riverine ecosystem of the Leon River.
56
APPENDIX A
SYSTEMATIC PALEONTOLOGY
57
In a typical paleontological study there is a section entitled “Descriptive Paleontology” or “Systematic Paleontology.” There, all identified specimens are listed under each taxon, each specimen is described, and the anatomical and morphometric criteria used to make the identification are described verbally and exemplary specimens are illustrated.
R. Lee Lyman
Zooarchaeology often deals with unreliable data in the form of fragmented specimen
(Driver, 2011, 1992; Gobalet, 2001; Wolverton, 2013). As a way to make data more accessible
and verifiable, a systematic paleontology is included with this thesis. This work includes
descriptions of all criteria used to identify individual specimens to taxa. Some parts of the
unionid valve were unused for identification and are not described in all cases (such as
posterior ridge and lateral teeth). In the case of 41HM61, these parts were often absent on
highly fragmented shell, and therefore unavailable for identification purposes. Along with
personal experience, the following identification guides were utilized in this study: Field Guide
to Texas Freshwater Mussels (Howells, 2013), Freshwater Mussels of Texas (Howells et al.,
1996) and Freshwater Mussels of Alabama and the Mobile Basin in Georgia, Mississippi, and
Tennessee (Williams et al., 2008).
Order: Bivalve
Family: Unionidae
Genus: Amblema
Species: Amblema plicata
Description: Amblema plicata have oval to rectangular shells. The umbo is above the hinge line
and often appears bulbous (very round and inflated). The beak sculpture consists of single
looped ridges. Shells often have three to seven diagonal ridges on the outer margins, but can be
unsculptured when small. The posterior ridge is very obscure. The pseudocardinal teeth are
58
divergent, triangular and massive. The left psuedocardinal teeth tend to be slightly oblique. The
right psuedocardinal tooth points toward the post-ventral margin, which is useful for
differentiating it from Megalonaias nervosa and Quadrula verrucosa.
Genus: Arcidens
Species: Arcidens confragosus
Description: Shell shape is subquadrate to oval and thin. Beak sculpture is nodular. Shells have
intricate exterior sculpture that includes curving ridges, cross hatches, and pustules.
Psuedocardinal teeth are very compressed and flat. The right valve has one psuedocardinal
tooth, while the left valve has two teeth.
Genus: Cyrtonaias
Species: Cyrtonaias tampicoensis
Description: Shell shape is oval, where the umbo shape often looks like the top of a hoe where
it joins the central disk. The beak sculpture consists of ultra-fine single looped ridges that are
rarely visible, making it non-diagnostic. The beak cavity is moderately deep. The psuedocardinal
teeth look like chewed bubble gum, meaning they often are round with many grooves. The
right valve has one psuedocardinal tooth that is triangular with rounded edges. The left valve
has two psuedocardinal teeth that are strong, almost massive. The posterior ridge is almost
non-existent, with a broadly rounded central disk.
Genus: Lampsilis
Description: Specimen identified as Lampsilis sp. exhibit compressed psuedocardinal teeth. The
umbo is often low, with the psuedocardinal teeth close to the beak with no interdentum. The
59
beak sculpture consists of fine broken lines that can be weekly double looped. Lampsilis sp. also
exhibit an oval shape, with a broadly rounded posterior ridge.
Species: Lampsilis hydiana
Description: L. hydiana shells are oval and inflated. The umbo is often low and deep, with a
larger area under the umbo than L. teres. The beak has double loop sculpturing. The
psuedocardinal teeth are generally slightly thicker than L. teres and ‘look cheeky’. The right
valve has one psuedocardinal tooth that is often approximately parallel to the hinge line. The
left valve has two psuedocardinal teeth.
Species: Lampsilis teres
Description: L. teres shells are shaped like very long ovals. The umbo is very low and shallow.
The beak has thin double loop sculpturing. The psuedocardinal teeth are thin and compressed.
The right valve has one psuedocardinal tooth that is often parallel to the hinge line. The left
valve has two psuedocardinal teeth that are thin, compressed, and parallel.
Genus: Megalonaias
Species: Megalonaias nervosa
Description: M. nervosa shells are rectangular shaped. They often contain ridges and
sculpturing in the middle margin. The umbo is low on the shell and shallow. Beak sculpture is
double looped to nodular. The pseudocardinal teeth are molar like. The right valve has one
psuedocardinal tooth that points left of the center of the ventral margin. The left valve has two
psuedocardinal teeth; the posterior tooth is often larger than the anterior tooth.
60
Genus: Quadrula
Description: Quadrulids have diagnostic psuedocardinal teeth. The right valve has a triangular
psuedocardinal tooth that is often torn. The left valve has two psuedocardinal teeth. Of these
two left teeth, the anterior psuedocardinal tooth is noticeably larger than the posterior
psuedocardinal tooth. Quadrulids are quadrate, subquadrate, to rectangularly shaped.
Species: Quadrula apiculata
Description: Quadrula apiculata is quadrate to triangular in shape. The psuedocardinal teeth
are typical of the genus. The right valve has one psuedocardinal tooth that is shaped like an
isosceles triangle, and are often torn. The left valve is typical of other quadrulids (see above). It
often has diagnostic pustules covering a majority of the outer shell surface, including
throughout the sulcus. These pustules are regular and small.
Species: Quadrula houstonensis
Description: Quadula houstonensis is quadrate to triangular in shape. The pseudocardinal teeth
are typical of the genus. The left valve’s psuedocardinal teeth include the typical size
differentiation; the anterior psuedocardinal teeth is usually much larger than the posterior
psuedocardinal teeth. Pustules can occur on the outer shell surface of this species, but are at
lower densities than pustules on both Q. apiculata and Q. verrucosa.
Species: Quadrula verrucosa
Description: Oval to oblong shape. Beak is pointed and low over the hinge line. Shells have
many pustules on them with fluting on the entire post-ventral margin. Pustules can continue
until the ventral margin. The posterior ridge is very strong, which is useful for differentiating it
61
from Megalonaias nervosa. The pseudocardinal teeth are strong and divergent. The right
psuedocardinal tooth is very triangular, with a flat edge on the anterior side.
Genus: Toxolasma
Description: Specimen’s designated Toxolasma sp. exhibited compressed psuedocardinal teeth
with diagnostic beak sculpture. Toxolasma sp. are subeliptical in shape and very small (hence
the common name of Lilliput). The beak is low, just above the hinge line, with a relatively
shallow beak cavity. The beak sculpture is diagnostic as it consists of large, single looped ridges.
Howells describes them as “curved bars… [that] produce a comma shape”. The right valve has
one psuedocardinal tooth; the left valve has two psuedocardinal teeth. All psuedocardinal teeth
are small, and peg-like. Specimen were not identified to the species level as differentiation is
difficult without the entire outer shell, which was not available often in this assemblage.
Genus: Truncilla
Species: Truncilla macrodon
Description: Truncilla macrodon has an elongated oval shape. The umbo is just above the hinge
line, with a very small beak cavity. The beak sculpture consists of very fine ridges. The
psuedocardinal teeth are compressed, very triangular, and close to the umbo. The right valve
has one psuedocardinal tooth that is triangular with the smallest side pointed toward the
anterior margin. The left valve has two psuedocardinal teeth. The lateral teeth are thin and
slightly curved.
Mixed Taxa
These designations were used to determine what shell they represented when the taxa couldn’t
be determined to species level. Often, comments within the data will explain why the
62
designation was given due to individual parts of the element. Preservation within the sample
often limited the confidence with which a species-level label would have been appropriate.
Species: Amblema plicata or Megalonaias nervosa
Description: This categorization was used for shells that exhibited characteristics similar to both
A. plicata and M. nervosa. These characteristics included similarities between the
psuedocardinal teeth. If the left psuedocardinal teeth were massive and angled, they looked
like both of these species. Often, the beak/umbo is the first to erode. As the beak above the
hinge line is useful for differentiating these two species, this erosion causes problems. The
presence of hatchmark sculpture is diagnostic of M. nervosa, but within this sample this
sculpture was eroded away. Without these distinguishing characteristics, these elements were
placed within this broader category for more confident and precise data.
Species: Quadrula apiculata or Quadrula verrucosa
Description: This categorization was used for shells that exhibited Quadrulid teeth and also had
a high density of pustules or fluting on the margins.
63
APPENDIX B
DATA USED IN CHAPTER 3INFERENTIAL TESTS
64
Culled Data
Table B.1 Data for culled chi-square goodness of fit test. Taphonomically relevant and extirpated species are excluded from this test. Numbers from the late Holocene sites are non-repetitive elements (NRE) identified to that taxon. Late Holocene data are used as observed values while modern relative abundances are used as the expected values.
Taxa 41HM61 Belton Lake Late Holocene
Modern Relative
Abundances Amblema plicata 93 307 400 20.1%
Arcidens confragosus 1 1 2 0.2% Lampsilis teres 34 5 39 6.3%
Megalonaias nervosa 16 2 18 4.9% Potamilus purpuratus 0 0 0 0.4%
Quadrula apiculata 13 20 33 1.7% Quadrula houstonensis 20 118 138 35.0%
Quadrula verrucosa 38 13 51 31.3% Total Mussels 215 466 681 1814
Table B.2 Data for culled Spearman’s rho correlation. Taphonomically relevant species are excluded from this test. Numbers represent taxonomic abundance rank.
Taxa Hamilton Belton Late
Holocene Modern Amblema plicata 1 1 1 3
Arcidens confragosus 12 10 11.5 8 Cyrtonaias tampicoensis 10.5 7 9 11
Fusconaia mitchelli 8 3 4 11 Lampsilis hydiana 7 5 7 11
Lampsilis teres 3 8 5 4 Megalonaias nervosa 5 9 8 5 Potamilus purpuratus 13 12 13 7
Quadrula apiculata 6 4 6 6 Quadrula houstonensis 4 2 2 1
Quadrula verrucosa 2 6 3 2 Truncilla macrodon 9 12 10 11
Toxolasma sp. 10.5 12 11.5 11
65
Complete Data Table B.3 Data for complete chi-square goodness of fit test. Numbers from the late Holocene sites are NRE identified to that taxon. Late Holocene data are used as observed values while modern relative abundances are used as the expected values.
Taxa 41HM61 Belton Lake Late Holocene
Modern Relative
Abundances Amblema plicata 93 307 400 17.6%
Arcidens confragosus 1 1 2 0.2% Lampsilis teres 34 5 39 5.6%
Leptodea fragilis 0 0 0 11.1% Megalonaias nervosa 16 2 18 4.3% Potamilus purpuratus 0 0 0 0.3%
Pyganodon grandis 0 0 0 1.0% Quadrula apiculata 13 20 33 1.5%
Quadrula houstonensis 20 118 138 30.6% Quadrula verrucosa 38 13 51 27.4%
Utterbackia imbecillis 0 0 0 0.3% Total Mussels 215 466 681 2071
Table B.4 Data for complete Spearman’s rho correlation. Taphonomically relevant species are excluded from this test. Numbers represent taxonomic abundance rank.
Taxa Hamilton Belton Late Holocene Modern Amblema plicata 1 1 1 3
Arcidens confragosus 12 10 11.5 11 Cyrtonaias tampicoensis 10.5 7 9 15
Fusconaia mitchelli 8 3 4 15 Lampsilis hydiana 7 5 7 15
Lampsilis teres 3 8 5 5 Leptodea fragilis 15 14 15 4
Megalonaias nervosa 5 9 8 6 Pyganodon grandis 15 14 15 8
Potamilus purpuratus 15 14 15 9 Quadrula apiculata 6 4 6 7
Quadrula houstonensis 4 2 2 1 Quadrula verrucosa 2 6 3 2 Truncilla macrodon 9 14 10 15
Toxolasma sp. 10.5 14 11.5 15 Utterbackia imbecillis 15 14 15 10
Uniomerus tetralasmus 15 14 15 12
66
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