SYSTEMATICS OF THE ORDOVICIAN TRILOBITES ISCHYROTOMA AND...

947

J. Paleont., 75(5), 2001, pp. 947–971Copyright q 2001, The Paleontological Society0022-3360/01/0075-947$03.00

SYSTEMATICS OF THE ORDOVICIAN TRILOBITES ISCHYROTOMA ANDDIMEROPYGIELLA, WITH SPECIES FROM THE TYPE IBEXIAN AREA,

WESTERN U.S.A.JONATHAN M. ADRAIN,1 STEPHEN R. WESTROP,2 ED LANDING,3 AND RICHARD A. FORTEY4

1Department of Geoscience, University of Iowa, 121 Trowbridge Hall, Iowa City 52242, ,[email protected]. and Department ofPalaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom,

2Oklahoma Museum of Natural History and School of Geology and Geophysics, University of Oklahoma, Norman 73019, ,[email protected].,3Center for Stratigraphy and Paleontology, New York State Museum, The State Education Department, Albany, New York 12230,

,[email protected]., and4Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom, ,[email protected].

ABSTRACT—Lower Ordovician sections in the type Ibexian area of western Utah contain a considerably more diverse trilobite faunathan has previously been reported. Reinvestigation of these faunas, based on new field sampling, allows a reassessment of the dimer-opygid genera Ischyrotoma Raymond, 1925, and Dimeropygiella Ross, 1951. These taxa have been considered synonyms, but parsimonyanalysis indicates each is a well supported clade, and they are best recognized as sister genera. The number of species known fromIbex has been doubled, from four to eight, and morphological information is now available for most parts of the exoskeleton. Newspecies include Ischyrotoma juabensis (Juab Formation), I. wahwahensis (Wah Wah Formation), Dimeropygiella fillmorensis (FillmoreFormation), and D. mccormicki (Fillmore Formation). The previously named species Dimeropygiella caudanodosa, D. blanda, and D.ovata are fully revised on the basis of abundant new material. Pseudohystricurus is a paraphyletic group, with species distributed as abasal grade of the Ischyrotoma/Dimeropygiella group.

INTRODUCTION

THE IBEX region of western Utah comprises the southern partsof the Confusion and House Ranges and the interlying Tule

Valley and Barn Hills (Fig. 1). It contains some of the most com-plete and well exposed Lower Ordovician sequences in the worldand is the type area for the Ibexian Series (Hintze, 1982; Ross etal., 1993, 1997). The sections through the Pogonip Group areparticularly important because they contain abundant silicified in-vertebrate faunas. The trilobites of these faunas (also Ross, 1951)have formed the basis for a biostratigraphic scheme widely cor-relative across Laurentia and elsewhere (Ross et al., 1997). Witha few minor exceptions, however (see below), the systematics andpaleoecology of the Ibex trilobite faunas have not been dealt within nearly half a century, and Hintze’s original monograph (1953)remains the sole documentation of the majority of the taxa.

The present study represents the first result of a comprehensivesystematic revision of the Lower Ordovician faunas describedoriginally by Hintze and Ross, based on extensive new field sam-pling. Although the project is in its early stages, it is apparentthat trilobite species diversity in the Ibex sections has been greatlyunderestimated, and many new species (and genera) await de-scription. We begin here with a full revision of the dimeropygidgenera Ischyrotoma Ross, 1951, and Dimeropygiella Hintze,1953, including the description of three previously unreportednew species and the first known early growth stages.

HISTORY OF STUDY OF THE IBEX TRILOBITE FAUNAS

Since Hintze’s (1953) original monograph, only a few papershave been published on Ibex trilobites. Hintze (1954) publishedtwo new genus names for preoccupied taxa, and Hintze and Jaan-usson (1956) named three new genera of asaphids with Ibex typespecies. Harrington (1957) named five new genera of pliomerids,two of which had Ibex type species, and a further two of whichhad Garden City Formation type species which also occur at Ibex.No new material was photographically illustrated in any of thesestudies. Subsequent revision of the Lower Ordovician faunas hasincluded Demeter’s (1973) work on the pliomerids, Terrell’s(1973) study of the lower Fillmore Formation zonation, andYoung’s (1973) treatment of a single rich horizon in Section H.The latter group of papers added thirteen new formally named

species and subspecies. They also contained a substantial numberof unnamed yet obviously new taxa reported in open nomencla-ture, supplementing several such undetermined forms in Hintze’soriginal monograph. Fortey and Droser (1996) augmented theIbex faunas with several new species from the earliest Whiter-ockian Juab Formation. Hintze’s monograph concentrated on therich silicified faunas. Trilobites in the Juab Formation are notsilicified and Hintze (1953, p. 19) regarded them as poorly pre-served and largely unidentifiable. Fortey and Droser, however,obtained several good calcareous crackout collections from Sec-tion J, and proposed a new threefold subzonal division of the Juabbased on the newly discovered trilobites. Finally, McCormick andFortey (1999) illustrated some new silicified Ibex material of thetelephinid Carolinites genacinaca and related forms.

As a result, Hintze’s (1953) monograph remains the sole doc-umentation of most of the Ibex trilobites, and comprises much ofthe basis for detailed biostratigraphy published nearly half a cen-tury later (Ross et al., 1997). Hintze’s work is exceptional, es-pecially for the time it was published, but a desire to more fullydocument the taxa, including all sclerites and available ontoge-netic material, coupled with the relatively high number of speciesdescribed only in open nomenclature, led J.M.A. and S.R.W. tobegin a comprehensive, field-based revision of the Ibex trilobitefaunas. The first results are presented herein, and they are rep-resentative of our experience with many resampled horizons inthe area: new collections have revealed exactly double the numberof species of Dimeropygiella and Ischyrotoma as previouslyknown. The new species were not previously reported in opennomenclature; they are newly discovered in the Ibex sections.Similar finds have been made in many other groups at many otherhorizons, and indicate that a significant portion of the trilobitediversity in the type Ibexian area remains to be described.

This situation is likely to be at least partly a result of samplingintensity. Hintze (1953) gave qualitative estimates of relative spe-cies abundances at particular sampling horizons, but did not listtotal sample sizes. Terrell (1973) and Demeter (1973), however,both described their samples as ‘‘large.’’ Neither gave quantitativesampling data, but the average physical dimensions of Demeter’s(1973, p. 40) samples were described as ‘‘about 40 3 40 3 60’’,which would amount less than 1 kg of rock. Terrell’s (1973, p.

948 JOURNAL OF PALEONTOLOGY, V. 75, NO. 5, 2001

FIGURE 1—Geographic position and line of section of sections H and Jof Hintze (1953, 1973), Heckethorn Hills, Ibex area, Millard County,western Utah.

FIGURE 2—New logs of parts of sections H and J, with collection hori-zons of material described herein indicated on right of columns.

69) samples were larger, described as ‘‘5- to 10-pound,’’ or ap-proximately 2–5 kg. This is an order of magnitude less than thelevel of sampling in our current field program, in which 25–40kg of rock are typically collected from individual horizons con-taining silicified trilobites. Experience with silicified faunas ofOrdovician (Adrain and Fortey, 1997), Silurian (Adrain, 1997,and references therein), and Devonian (Adrain and B.D.E. Chat-terton, unpublished data) age has indicated that samples of thesesizes are more likely to yield an accurate faunal list.

Differences in sampling do not account for all of the new taxa,however, as certain new species are by no means rare. Ischyro-toma wahwahensis, for example, is common throughout the mid-dle part of Section J, and is the most abundant species at horizonJ-81, yet it has not previously been reported, and may have beenconflated with I. caudanodosa. Initial sampling of section J hasalso yielded previously unreported species of cheirurids, odonto-pleurids, bathyurids, and the telephinid Opipeuter.

The goal of the present study is to describe all new and revisedIbex species of the dimeropygid genera Dimeropygiella and Is-chyrotoma, including the earliest known growth stages, and toconsider the validity and scope of the genera, which have oftenbeen considered synonyms.

LOCALITIES

Hintze (1951, 1953) originally measured and described sectionsthrough the Pogonip Group in the Ibex area of western Utah (Fig.1). He later (1973) published revised descriptions based on re-measurements made in 1965. We remeasured and redescribed

Hintze’s sections H and J, relating our new measurements to Hin-tze’s via the painted sets of numbers left on the sections by Hintze.Horizons used in the study are identified with a number relatingthem to the original footage of Hintze’s sections, and their posi-tion on our new descriptions (measured in metres) is shown inFigure 2. For example, horizon J-81 is a bed positioned at 81 feetin Hintze’s (1953) scheme, and at about 24.2 m in our remea-surement of section J. The relationship between Hintze’s originalnumbers and the 1965 remeasurements (also in feet) are given byHintze (1973). Complete columnar sections and new descriptionswill be published elsewhere. Here, we report only those parts ofsections H and J that contain the sampling horizons for taxa inthis study (Fig. 2).

DIMEROPYGID CLASSIFICATION

Dimeropygidae was established independently by Hupe (1953)and Whittington and Evitt (1954), who noted Hupe’s work inproof. Hupe’s concept of the family included only Dimeropyge,1937, and Bolbocephalus Whitfield, 1890, a genus firmly estab-lished as a member of Bathyuridae by Whittington (1953). Hupealso erected the family Toernquistiidae to include ToernquistiaReed, 1896, and Pyraustrocranium Ross, 1951. Whittington andEvitt’s (1954) concept of Dimeropygidae included Dimeropyge,

949ADRAIN ET AL.—ORDOVICIAN TRILOBITES ISCHYROTOMA AND DIMEROPYGIELLA

Toernquistia, Dimeropygiella, and their new Chomatopyge andMesotaphraspis. They excluded Pyraustrocranium (a genus re-quiring reinvestigation; it was placed order and family uncertainin the 1959 Treatise) and considered Toernquistiidae a synonymof Dimeropygidae.

This arrangement was followed by Whittington in the Treatise(Moore, 1959), but was emended in 1963. Raymond (1925) haderected Ischyrotoma on the basis of a partial and exfoliated ce-phalon from the early Whiterock of western Newfoundland,which he illustrated only by a line drawing. He listed, but did notfigure, a second specimen from the Mystic Conglomerate of Le-vis, Quebec. Whittington (1963), in describing the rich ‘‘crack-out’’ fauna from the great alpha boulder at Lower Head, nowassigned to the Shallow Bay Formation of the Cow Head Group(James and Stevens, 1986), revised I. twenhofeli on the basis ofthe original holotype and new material. Fully illustrated, it wasevident that I. twenhofeli was very similar to the Utah species ofDimeropygiella described by Ross (1951) and Hintze (1953).Whittington placed Dimeropygiella in synonymy of Ischyrotoma,an opinion followed by all subsequent workers to date.

Whittington (1963) also erected the genus Ischyrophyma as anew member of Dimeropygidae. Earlier, Jaanusson (1956) haderected the family Celmidae for the Baltic genus Celmus Angelin,1854. Jaanusson also erected the dimeropygid subfamily Meso-taphraspidinae to accommodate Mesotaphraspis and Chomato-pyge. Whittington (1963, p. 49–50) compared his Ischyrophymatuberculata directly to Celmus, but noted also its considerablecephalic similarities with Ischyrotoma. Critically, the pygidium ofIschyrophyma tuberculata was unknown; it could not then be es-tablished that it was of the distinctive single-segment Celmustype, and Whittington assigned his new genus tentatively to thedimeropygids. He later erected a second species, I. tumida Whit-tington, 1965, but again the pygidium was not recovered. Bruton(1983) revised Celmus granulata on the basis of good new ma-terial and considered Ischyrophyma a probable synonym of Cel-mus. Recently, J.M.A. and R.A.F. (unpublished data) have col-lected silicified material of Ischyrophyma tumida from the TableCove Formation in the Hare Bay Region of western Newfound-land (equivalent to Whittington’s ‘‘middle Table Head forma-tion’’). This shows unequivocally that Ischyrophyma possessesthoracic segments and a single-segmented, flanged pygidium es-sentially identical to that of Celmus. This was confirmed by a newsilicified species, Celmus michaelmus Adrain and Fortey, 1997,from the Tourmakeady limestone of western Ireland. Adrain andFortey (1997) placed Ischyrophyma in synonymy of Celmus,maintaining the latter in a family Celmidae. Glaphurella Dean,1971, described as a glaphurid, is likely related to Celmus.

Apart from Glaphurella, no new genera were added to the di-meropygids for over thirty years following Whittington’s (1963)work. The only genera to receive extended and detailed treatmentswere Dimeropyge (e.g., Shaw, 1968; Chatterton and Ludvigsen,1976; Chatterton, 1980, 1994; Tripp and Evitt, 1983) and Ischy-rotoma (Fortey, 1979, 1980; Ingham in Ingham et al., 1986). Re-cently, however, Chatterton et al. (1998) resurrected Toernquisti-idae, to which they assigned Toernquistia, Chomatopyge, Meso-taphraspis, and their new Lasarchopyge and Paratoernquistia.Their parsimony analysis included well known species of ‘‘toern-quistiids’’ and dimeropygids, together with 11 ‘‘hystricurids,’’ andsupported the monophyly of the toernquistiid and dimeropygidgroups (although the celmids, represented only by ‘‘Ischyrophy-ma’’ tuberculata Whittington, nested deeply within the dimero-pygids).

At this point, then, there are potentially three families, Dimer-opygidae, Toernquistiidae, and Celmidae, that may be recognizedwithin the broad ‘‘dimeropygoidean’’ group. Whether each ismonophyletic and whether together they comprise a clade is a

question for further study. In the present work we are concernedonly with a subset of the dimeropygid component, the speciesthat have previously been assigned to Ischyrotoma and relatives,which we refer to henceforth as the ‘‘Ischyrotoma group.’’

The primitive members of the Ischyrotoma group have longbeen identified as the poorly known species attributed to Pseu-dohystricurus Ross, 1951 (e.g., Fortey and Owens, 1975). Theaffinities of the group beyond these species are not well estab-lished. Some workers (Ross, 1951, p. 123; Fortey and Owens,1975, p. 228) have regarded the Ischyrotoma group as the ances-tral grade from which Dimeropyge was derived. Chatterton et al.(1998, figs. 4, 5), on the other hand, resolved Dimeropyge as aparaphyletic grade from which the Ischyrotoma group was de-rived (although Dimeropyge was resolved as a clade sister to theIschyrotoma group when they applied successive approximationscharacter weighting, in agreement with the result of Chatterton,1994, fig. 1).

The nature of the relationship will probably require more newdata before it can be definitively assessed. However, there are anumber of reasons to suspect that Dimeropyge is a related butindependent clade. First, Dimeropyge, although broadly similar inmorphology and tuberculate sculpture, has many features notshared with the Ischyrotoma group, including the presence in allspecies for which information is available of a long thoracic axialspine, a highly characteristic ‘‘walled’’ pygidium, and a unique,tiny hypostome and highly modified rostral plate (see Chatterton,1994, for good examples of all). It is conceivable, of course, thatall of these features are modifications of the states seen in theIschyrotoma group, but there is no positive evidence of this. Withthe new information presented herein, we now have a good ideaof what the Ischyrotoma/Dimeropygiella pygidium looks like dur-ing the meraspid and early holaspid period; at no point does itever develop the smooth vertical ‘‘wall’’ of Dimeropyge. Mem-bers of the Ischyrotoma group have a large, natant hypostome.Articulated individuals of I. twenhofeli are known, and lack athoracic axial spine; no such spine has ever been associated withany other species of the group known from disarticulated material.The early cranidial morphology of all members of the Ischyro-toma group shows a distinct median preglabellar furrow, a featureconclusively absent from all known Dimeropyge ontogenies. Fi-nally, there is some evidence that animals with the Dimeropygepygidial morphology were already established by the very earlyOrdovician: Ross (1951, pl. 9, figs. 25, 29, 30) figured a pygidiumfrom the early Ibexian (Zone B; Symphysurina Zone) which ischaracteristically ‘‘walled,’’ has strong fulcral spines, and asidefrom its quite short sagittal length is difficult to distinguish fromDimeropyge (compare, e.g., with D. clintonensis Shaw, 1968(Chatterton, 1994, figs. 7.15, 7.16, 7.19, 7.23)). At present, Di-meropyge seems best construed as an independent clade.

Hence, we assume that the Ischyrotoma group is monophyletic.With the abundant new information presented herein, it is nowpossible to formulate an explicit hypothesis of ingroup structureby means of cladistic parsimony analysis.

STUDY TAXA

We carried out parsimony analysis on all adequately knowningroup species previously assigned to Dimeropygiella, Ischyro-toma, and Pseudohystricurus as well as one previously assignedto Ischyrophyma. Taxa used in the analysis and sources used forcoding are given in Table 1. Species which are definitely part ofthe ingroup but not well enough known for analysis include Is-chyrotoma sp. ind. of Whittington (1965) (Ischyrotoma herein),Ischyrotoma cf. caudata of Dean (1989) (Dimeropygiella herein),and Ischyrotoma sp. A of Brett and Westrop (1996) (?Dimero-pygiella herein). These are discussed under the appropriate genusheading in the systematics section below.


TABLE 1—Coding sources, sclerites known, and type of preservation of species subject to parsimony analysis. Original generic assignment is given, andspecies are listed in alphabetical order.

Original species name Coding sources, sclerites, and preservation

Ischyrotoma anataphra Fortey (1979, pl. 36, figs. 1–13); cephalon, pygidium; calcareous, mostly testiferous.Dimeropygiella blanda Hintze (1953, pl. 19, figs. 6–8), Young (1973, figs. 2, 6, 7 only [see synonymy in text]); cranidium,

librigena, ?hypostome, thoracic segments, pygidium; silicified.Dimeropygiella cf. blanda Figure 16 herein; cranidium, pygidium; calcareous, testiferous.Ischyrophyma? borealis Fortey (1980, pl. 12, figs. 1–7); cranidium, librigena; calcareous crackout, mostly testiferous.Dimeropygiella caudanodosa Ross (1951, pl. 35, figs. 18, 22–28), Hintze (1953, pl. 19, figs. 5, 10), Figures 7.24–7.50, 8, 9 herein;

all sclerites known; silicified.Dimeropygiella fillmorensis Figures 11.9, 12 herein; cranidium, librigena, ?hypostome, pygidium; silicified.Ischyrotoma juabensis Foretey and Droser (1996, fig. 15); cranidia, librigenae, pygidia; calcareous crackout, testiferous.Dimeropygiella mccormicki Figure 15.12–15.33 herein; cephalon, pygidium; silicified.Pseudohystricurus obesus Ross (1951, pl. 16, figs. 25, 30, 34; pl. 19, figs. 34, 38 [see discussion on genus]); cranidium, ?pygidi-

um; silicified.Pseudohystricurus orbus Ross (1953, pl. 63, figs. 10, 11, 15–20, 23); cranidium, librigena; silicified.Dimeropygiella ovata Hintze (1953, pl. 19, figs. 1–4), Young (1973, pl. 2, fig. 1), Figures 10, 11.1–11.8, 11.10–11.12 herein;

cranidium, librigena, ?hypostome, ?thoracic segments, pygidium; silicified.Ischyrotoma parallela Boyce (1989, pl. 34, figs. 7, 8; pl. 35, figs. 1–10); cephala only; calcareous, mainly exfoliated internal

molds.Ischyrotoma stubblefieldi Ingham in Ingham et al. (1986, figs. 8, 9, 15h–15k); all sclerites known; silicified, but quite coarsely

preserved and variably distorted.Ischyrotoma twenhofeli Whittington (1963, pl. 7), Fortey (1980, pl. 11, figs. 9, 10, 12–21); enrolled individual, cephalon, ros-

tral plate, pygidium. Calcareous, crackout, mostly testiferous.Ischyrotoma wahwahensis Figures 5, 6, 7.1–7.23 herein; all sclerites known; silicified.Pseudohystricurus sp. Ross (1951, pl. 16, figs. 26, 27, 31); cranidium, silicified.Ischyrotoma sp. Ross (1967, pl. 7, figs. 1–7); cranidium, librigena, pygidium; silicified but likely immature.

Dimeropygiella eos Kobayashi, 1955, from the McKay Group,British Columbia, was erected on the basis of a single pygidium,illustrated by a small photograph of the unwhitened specimen(Kobayashi, 1955, p. 456, pl. 6, fig. 10). Dean (1989) figuredthree similar pygidia from the Survey Peak Formation, WilcoxPass, Jasper National Park, Alberta, Ibexian (Stairsian), Zone E(Tesselacauda Zone), as ‘‘cf. eos,’’ and discussed the species asa member of Ischyrotoma. Beyond the presence of a pair of ter-minal axial nodes—a feature distributed widely among Ibexian‘‘hystricurids’’—the pygidia bear no compelling similarity withthose of Dimeropygiella or Ischyrotoma, and the species seemsunlikely to be relevant to their ingroup phylogeny. Full evaluationwill only be possible when the remainder of the exoskeleton isdiscovered.

Pseudohystricurus rotundus Ross, 1951, is from the early Ib-exian (Skullrockian; Zone A; Symphysurina Zone) of the GardenCity Formation, Hillyard’s Canyon, southern Idaho. This earliestOrdovician species was based on three crackout cranidia (Ross,1951, pl. 16, figs. 32, 33, 35–37). Although at least superficiallysimilar to the late Ibexian type species of Pseudohystricurus inthe possession of an inflated, tuberculate glabella, rotundus has abroad, flattened anterior border, a very long palpebral lobe bound-ed by a long, deep palpebral furrow, and deep S1 nearly fullyisolating L1. Better assessment of its relationships will requiremore material and knowledge of other sclerites, but its affinitywith the much younger Ischyrotoma/Pseudohystricurus group isat best weakly supported.

Choice of outgroup.Following from the conclusions of Chat-terton (1994), the reweighted cladogram of Chatterton et al.(1998), and the discussion above, Dimeropyge must be consideredas a potential outgroup, as it may represent the sister taxon of theIschyrotoma group. The basal node of Dimeropyge is essentiallyunknown, however, and if the suggestion above that a Zone Bpygidium has affinities to the genus is correct, then it is consid-erably older than previously suspected. The well known Middleand Upper Ordovician species of Dimeropyge are also highly au-tapomorphic, and it is difficult to specify direct homologies forstates relevant to the ingroup structure of the Ischyrotoma group.It is highly likely that a variety of very poorly known earlierOrdovician ‘‘hystricurids’’ need to be considered before a robust

hypothesis of dimeropygid phylogeny can be developed. Thereis, however, one ‘‘hystricurid’’ species that may have bearing onthe origin of the Ischyrotoma group. Parahystricurus smithiaeBoyce, 1989, has an inflated, ovate glabella with strong tuber-culate sculpture and weakly incised glabellar furrows (cf. Dimer-opygiella fillmorensis herein), an obvious median preglabellar fur-row, and a pygidium with tab-shaped fulcral flanges and terminalpaired flanges. Most of its features can be related to characterstates seen in early members of the Ischyrotoma group, and inthe present state of knowledge it appears to be the best candidatefor determining the root of that clade.

CHARACTERS

Cranidium1) Shape of anterior border: 0—broad, shelf-like, nearly flat;

1—medially extended, with inverted ‘‘W’’-shaped bottom (e.g.,Fig. 8.9); 2—tab-shaped, subrectangular, without pronounced me-dian extension (Fig. 5.1); 3—rim-like, subdued.

2) Relation of anterior border furrow to preglabellar furrow:0—separated by preglabellar field with median furrow; 1—sepa-rated by preglabellar field lacking median furrow; 2—in contactmedially, preglabellar field absent, characteristic ‘‘X’’ shape inanterior view (Fig. 5.6).

3) Length of unfurrowed preglabellar field: 0—field absent orfurrowed; 1—very short, nearly with ‘‘X’’ shape, but anteriorborder furrow and preglabellar furrow separated by narrow trans-verse strip of exoskeleton medially (Fig. 8.9); 2—long (Fig. 13.9).

4) Depth and incision of anterior border furrow: 0—relativelylong and shallow; 1—deep, shallowing posteriorly (Fig. 10.2);2—very short (sag.; exsag.) and sharply incised (Fig. 13.1); 3—nearly effaced.

5) Degree of effacement of sculpture on front of cranidium:0—not effaced, anterior of glabella, fixigena, and frontal area withsimilar sculpture to rear of cranidium (Fig. 5.9); 1—effaced (Fig.13.1).

6) Course of anterior sections of facial sutures in front of pal-pebral lobe: 0—subparallel or slightly anteriorly convergent (Fig.5.1); 1—strongly anteriorly divergent, bowed laterally (Fig. 8.1).

7) Dorsal incision of palpebral furrow: 0—not incised, fixigenagrades more or less smoothly into palpebral lobe (Fig. 13.2); 1—


TABLE 2—Characters used in parsimony analysis.

Cranidium Lib. Pygidium

anataphrablandacf. blandaborealiscaudanodosafillmorensis

312301122111221220101111011110

01102100011000?0111?1200112001

010000001010

1011011000?????101111000111002

001000011000110?????0010100101

000110110???010210

juabensismccormickiobesusorbusovata

2201011220011101111011110

0111?10001020000100012001

0000?00010

1001200001?????0000011001

111000011000010?????00101

000110100???210

parallelastubblefielditwenhofeliwahwahensisP. sp.T. sp.smithiae

31230220102201022010020102201000000

0?102021120211201112010000??1?00000

01010100000000

10?11101101011110002?????1000200000

?????101001110010100?????1110000000

???001001001???000000

FIGURE 3—Strict consensus of four most parsimonious cladograms withlength 54, consistency index 0.648, retention index 0.808. See Figure4 for alternative ingroup relationships of Ischyrotoma. Numbering ofnodes corresponds to ACCTRAN and DELTAN optimizations for oneof the most parsimonious cladograms given in Table 2.

definite strong furrow (Fig. 5.1); 2—obvious break in slope, butno incised furrow (Fig. 8.1).

8) Shape of S0 and anterior edge of L0: 0—S0 relatively short,anterior edge of L0 sloping or grading into furrow (Fig. 13.1);1—S0 long, with smooth, flat bottom, anterior edge of L0 sharplydelineated, in some specimens actually overhanging furrow (Fig.5.1, 5.5).

9) Width of posterior fixigena: 0—point at which posterior sec-tion of facial suture crosses posterior border furrow set abaxial tolateral edge of palpebral lobe; 1—point at which posterior sectionof facial suture crosses posterior border furrow aligned with orset adaxial to lateral edge of palpebral lobe.

10) Ventral rostral area: 0—only moderately bowed, with flat

rostral plate; 1—rostral sutures fused, ventral rostral spike-likeprojection; 2—strongly ‘‘V’’ shaped but lacking fusion and ven-tral projection, narrow rostral plate retained.

11) Size of primary fixigenal tubercles in mature holaspides:0—not obviously larger than surrounding tubercles (Figs. 5.1,13.2); 1—large and obviously distinct from surrounding tubercles(Figs. 8.1, 10.1).

12) Width of interocular fixigena: 0—wide, more than one tu-bercle row developed; 1—very narrow, tubercles restricted to sin-gle row.

Librigena

13) Length of holaspid genal spine: 0—long and robust (Fig.15.19); 1—very short, thorn-like (Fig. 12.4).

14) Posterolateral margin of lateral border: 0—laterally convex,more or less evenly arcuate (Fig. 7.25); 1—with laterally concaveindentation in front of genal spine (Fig. 10.10, 10.13).

15) Eye socle: 0—weak, smooth area only; 1—strongly inflatedband.

16) Width of lateral border furrow: 0—narrow (Fig. 7.32); 1—very broad (Whittington, 1963, pl. 7, figs. 3, 5, 13).

17) Lateral border sculpture: 0—prominent raised lines only(Fig. 13.19); 1—tubercles and raised lines (Fig. 7.32); 2—strongtubercles, more or less completely obscuring lines (Fig. 12.4).

Pygidium

18) Spacing of posterior axial tubercles: 0—separated by atleast thin median strip, axis wide at rear (Fig. 7.43); 1—veryclosely crowded medially, axis tapering strongly at rear (Fig.7.12).

19) Fusion of posterior axial tubercles: 0—not fused; 1—par-tially or fully fused medially.

20) Tab-like flanges on fulcra of pleural bands: 0—present(Ross, 1951, pl. 19, figs. 34, 38); 1—absent.

21) Shape of posterior axial tubercles: 0—rounded, node-shaped, subconical (Fig. 7.37, 7.43); 1—tab-shaped, much wider(tr.) than long (exsag.) (Fig. 15.1).

22) Shape of posteriormost axial ring in front of posterior axialtubercles: 0—medially continuous, roughly similar length sagi-tally as exsagitally (Fig. 15.2); 1—subdivided into more inflatedor tuberculate lateral parts and subdued median part (Fig. 12.32).

23) Tubercles on axial rings and pleural bands: 0—present andcontinuously distributed; 1—completely absent; 2—very sub-dued, clustered only on adaxial parts of pleural bands.

24) Pleural bands and furrows of second pygidial segment: 0—anterior pleural band and pleural furrow obvious (Fig. 7.21); 1—anterior band and pleural furrow lost, posterior band inflated intostrong rib (Fig. 7.45).

25) Pleural furrow of first pygidial segment: 0—continuousacross fulcrum; 1—interrupted near fulcrum by inflated band, sep-arated into adaxial and abaxial parts with different course (Fig.7.21).

Characters not used.1. Presence of a juvenile median occipital spine. In contrast to

many other groups of trilobites, no members of the study groupretain an adult median occipital node or spine that can be distin-guished from other tubercles on the occipital ring. The mediannode is visible in early ontogeny, however, and its state is poten-tially significant. The juvenile state in Dimeropygiella is knownfor D. blanda, D. fillmorensis, D. ovata, and D. caudanodosa; allof these species possess a subdued tubercle in meraspid ontogeny,which becomes indistinguishable by the late meraspid or earlyholaspid stage. The condition in Ischyrotoma is known only fromI. wahwahensis, which bears an elongate spine during the mer-aspid period. The spine (as, for example, in the aulacopleuridCyphaspis, see Adrain and Chatterton, 1995) is reduced in relative


FIGURE 4—Topology of Ischyrotoma in each of the four most parsimo-nious cladograms. Node numbers for tree 2 correspond to ACCTRANand DELTRAN character-state optimizations given in Table 2.

TABLE 3—Synapomorphies for nodes in the preferred cladogram of Figure4.2, optimized using accelerated transformation (ACCTRAN) and delayedtransformation (DELTRAN). Equivocal apomorphies indicated by asterisk.Nodes numbered following Figures 3 and 4.2.

NodeACCTRAN

Synapomorphies NodeDELTRAN

Synapomorphies

1234567

89

10

1112

131415

2(1)*, 4(1), 7(1)*3(1)*1(1), 20(1)3(2)*, 13(1)*, 17(1)*6(1), 7(0)*, 10(1), 24(1)3(1)*, 7(2)*, 11(1), 22(1)4(2), 21(1)*, 23(1)*

5(1), 14(1)*, 17(0)*14(1)*, 23(2)*

1(2), 8(1), 10(2), 15(1)*,16(1)*1(3), 4(3), 12(1)*2(2)*, 3(0)*, 9(1), 18(1),

25(1)*—7(2)*, 12(1)*

15(0)*, 17(2)*

1234567

89

10

1112

131415

4(1), 7(1)*2(1)*, 3(1)*1(1)

13(1)*, 17(1)*, 20(1)6(1), 10(1), 24(1)7(2)*, 11(1), 22(1)3(2)*, 4(2), 7(0)*, 21(1)*,

23(1)*5(1)

14(1)*, 23(2)*8(1), 10(2), 16(1)*

1(3), 3(2)*, 4(3), 12(1)*1(2), 2(2)*, 3(0)*, 9(1),

18(1)15(1)*7(2)*, 12(1), 25(1)*

17(2)*

length through ontogeny, but is retained as a distinct node muchlater than in Dimeropygiella. It seems possible that this is a gen-eral condition for at least some species of Ischyrotoma, and if sois likely apomorphic. The only other species of Ischyrotoma forwhich very small cranidia are known, however, is I. stubblefieldi,and although the retention of a node late in ontogeny is evident,none of Ingham’s (in Ingham et al., 1986) smallest specimens bearan occipital spine. The material is more coarsely preserved thanthat of I. wahwahensis, and the absence may be preservational.If presence of a spine is confirmed in more species, it should beincorporated as a character in subsequent analyses.

2. Hypostome. The hypostome of most hystricurids and dimer-opygids is unknown, and that of the Ischyrotoma group has onlyrecently been identified with certainty. Ingham (in Ingham et al.,1986, fig. 15h–15k) assigned specimens with question to I. stub-blefieldi. Chatterton et al. (1998, fig. 11.8) then identified a similarsclerite from Ibex section H-434 as either blanda or ovata. Withthe multiple silicified occurrences documented herein, the hypos-tomal assignment is confirmed. There are certainly morphologicaldifferences among the known examples. The hypostomes as-signed herein to I. wahwahensis, for example, possess a distinctventral bulge on the posterior lobe of the middle body and havenearly parallel sided, versus arcuate, lateral margins of the ante-rior wings. Hypostomes will likely prove to be a source of phy-logenetic information. At present, however, the small number ofspecies for which associations are well supported precludes theiruse. Only I. stubblefieldi, I. wahwahensis, and Dimeropygiellacaudanodosa have definitely assigned hypostomes. The first spe-cies is the only member of the group in the collections in whichit occurs, whereas the latter two occur in collections from sectionJ in which one or the other is much more common. At H-434,three species co-occur and very few large hypostomes have beenrecovered. All have a similar morphology. Hence, general state-ments about the likely condition in all three species are possible,but direct assignment of sclerites is thus far impossible.

3. Number of pygidial axial rings and pleural ribs. Easily count-ed meristic characters are natural candidates in a search for ho-mology. Both of these counts are stable within ingroup species.However, no obvious pattern is discernable between species. In

the case of number of rings, three appears to be the plesiomorphicnumber.

ANALYSIS

The character matrix for 25 characters and 17 ingroup taxainitially coded is given in Table 2. All multistate characters weretreated as nonadditive, with the exception of character 10, whichwas constrained by a state-tree as outlined in the character listabove. Parsimony analysis was performed using PAUP* 4.0b2(Swofford, 1999), employing the exact Branch and Bound algo-rithm. Networks were rooted using the designated outgroup, P.smithiae.

RESULTS

Initial runs used all 17 ingroup species, and resulted in 13equally parsimonious trees of length 55. These trees indicatedsupport for clades encompassing Ischyrotoma and Dimeropygiellaas understood herein. Ingroup relationships of Dimeropygiellawere stable and fully resolved. Ischyrotoma, however, was poorlyresolved, with the only ingroup structure in the strict consensusinvolving the sister species anataphra/parallela and twenhofeli/stubblefieldi. These pairs formed a basal polytomy with the re-maining four species. Most of this lack of resolution was causedby a single labile species, Ross’s (1967) Ischyrotoma sp. Thistaxon is known only from juvenile material, and its codings aretherefore highly ambiguous. Intuitively, it appears very similar toIschyrotoma juabensis n. sp. and to share several obvious apo-morphies. However, other codings either contradict this relation-ship or suggest alternatives, and result in multiple possible posi-tions for the species. We regard many of these contradictory cod-ings as uncertain, because they may reflect juvenile versus adultstates. Because of this uncertainty, Ischyrotoma sp. of Ross wasexcluded from the final analysis. Discussion of its possible rela-tionship to I. juabensis is given under that species below.

Analysis of the remaining 16 ingroup species together with theoutgroup yielded four equally parsimonious cladograms of length54 and consistency index 0.648. The strict consensus cladogramis shown in Figure 3. The cladograms differ only in the structureof part of the Ischyrotoma clade; the alternative topologies areshown in Figure 4. The ambiguity is likely influenced by themissing pygidial codings of I. borealis. On grounds of stratigraph-ic congruence, the topology of Figure 4.2 minimizes ghost lineage


hypotheses and is preferred. Synapomorphies for all nodes of thepreferred cladogram, optimized using the assumptions of accel-erated transformation (ACCTRAN) and delayed transformation(DELTRAN), are shown in Table 3. Relationships are discussedunder each genus in the systematics section below.

SYSTEMATIC PALEONTOLOGY

Repository.Figured material is housed in the Paleontology Re-pository, Department of Geoscience, University of Iowa, Iowa City,with specimen number prefix SUI, and in the Department of Pa-laeontology, Natural History Museum, London, with prefix It.

Terminology.Terms for the trilobite exoskeleton are appliedin standard fashion, as summarized by Whittington and Kelly inWhittington (1997). Number of pygidial pleural rib pairs and ter-minal pieces follows the scheme typically applied to encrinurids(Ramskold, 1986). For example, a species described as having 51 1 pairs of ribs bears 5 pairs and a single, median rib behindthe axial terminal piece, whereas a species with a count of 5 10 bears five pairs but lacks a terminal median rib.

Family DIMEROPYGIDAE Hupe, 1953Genus PSEUDOHYSTRICURUS Ross, 1951

Type species.Pseudohystricurus obesus Ross, 1951, GardenCity Formation, Ibexian (Stairsian), Zone F (Rossaspis superci-liosa Zone), crest of ridge on west side of Hillyard’s Canyon,southern Idaho.

Other species.Pseudohystricurus orbus Ross, 1953, GardenCity Formation, Ibexian (Tulean), Zone G1 (Hintzeia celsaoraZone), crest of ridge on west side of Hillyard’s Canyon, southernIdaho; Pseudohystricurus sp. of Ross (1951), Garden City For-mation, Ibexian (Stairsian), Zone E (Tesselacauda Zone), eastside of Hillyard’s Canyon, southern Idaho.

Discussion.Despite noting that Pseudohystricurus obesuswas ‘‘relatively abundant,’’ Ross (1951, p. 74) illustrated only asingle immature cranidium when he erected this type species. Ter-rell (1973, p. 88, pl. 5, fig. 1) assigned a fragmentary cranidiumfrom either Zone E (Tesselacauda Zone) or low Zone F of sectionEE, near the Ibex area, western Utah. Cranidial characters werecoded herein on the basis of Ross’s illustration, but both occur-rences will require more complete documentation. Ross illustrateda variety of unassigned pygidia from the Rossaspis superciliosaZone (Zone F) of the Hillyard’s Canyon sections, including twotypes (Ross, 1951, pl. 19, figs. 33, 36, 37 and pl. 19, figs. 34, 38)which show some similarity to those of Dimeropygiella and Is-chyrotoma. The wider variety, which occurs in one specimen withan articulated thorax including a thoracic axial spine, has beenshown by Boyce (1989) to belong to Hillyardina. The other(Ross, 1951, pl. 19, figs. 34, 38) is extremely similar to pygidiaof, for example, D. blanda, with the exception that it has prom-inent flanges at the fulcra of the pleural bands, in addition to thepair of pygidial axial terminal nodes. This pygidium is tentativelyassigned to Pseudohystricurus obesus herein, and coded as suchfor the analysis, though the association will obviously requireconfirmation.

Ross (1953) tentatively assigned to Pseudohystricurus orbuspygidia bearing a robust, elongate spine running posteriorly fromthe rear of the axis. It is difficult on present evidence to assesswhether this is correct. The pygidia of all other species that havedefinitely been assigned lack such a spine, but it is possible thatit is a homologue of the pair of terminal axial nodes seen in allother taxa, derived through fusion and extension. However, sucha spine is relatively common among bathyurines. For coding pur-poses, the pygidium of P. orbus was considered unknown, pend-ing new collections.

Pseudohystricurus sp. of Ross (1951) was illustrated by a singlecranidium. This species is actually more fully known than Ross’s

named type species, P. obesus, because it is based on a matureholaspid (more than twice the size of the immature holotype of P.obesus). Terrell (1973, p. 89, pl. 2, figs. 2, 4) assigned a verysimilar cranidium from Zone E (or possibly Zone D) of the Fill-more Formation in western Utah (either section AA or section EE).

Since the analysis indicates that Pseudohystricurus as presentlyconceived is a paraphyletic group, no diagnosis is given. All ofthe species are inadequately known and will require revision onthe basis of new collections before strong hypotheses of theirrelationships can be developed. Only P. orbus is known fromsclerites other than cranidia, and as noted above Ross’s (1953)tentative pygidial assignment remains questionable. Unlike theother species, it shares a characteristic inverted ‘‘W’’-shape of theanterior border with Dimeropygiella. In the present state ofknowledge, this state is optimized under both ACCTRAN andDELTRAN at node 3 (Fig. 3), and transforms to the tab-shapedstate of Ischyrotoma between nodes 4 and 10. Hence, as coded,it cannot be construed as a synapomorphy uniting P. orbus ex-clusively with Dimeropygiella. This coding will need to be re-assessed in light of new information. It depends on the notionthat the specialized inverted ‘‘W’’ shape can plausibly transformto the equally specialized, but very different, tab-shape of Ischy-rotoma. Were this transformation to be disallowed via a character-tree constraint, the inverted ‘‘W’’ shaped anterior border structurewould be liable to uniquely group orbus as a primitive memberof Dimeropygiella.

Genus ISCHYROTOMA Raymond, 1925Type species.Ischyrotoma twenhofeli Raymond, 1925, from

a large early Whiterockian boulder in Bed 14 of the Shallow BayFormation, Cow Head Group, early Whiterockian, Lower Head,western Newfoundland (revised by Whittington, 1963, p. 45, pl.7; Spitsbergen material described by Fortey, 1980, p. 65, pl. 11,figs. 9, 10, 12–21).

Other species.Ischyrotoma anataphra Fortey, 1979, Cato-che Formation, Ibexian, Port au Choix, western Newfoundland;Ischyrophyma? borealis Fortey, 1980, Profilbekken Member,Valhallfonna Formation, Whiterockian (V4b), Profilbekken,Spitsbergen; I. juabensis n. sp., Juab Formation, Ibex Section J,Whiterockian, Zone L (Paralenorthis-Orthidiella Zone), Ibexarea, western Utah, and Shingle Limestone, Shingle Pass, south-ern Egan Range, eastern Nevada; I. parallela Boyce, 1989, low-er limestone sequence of Catoche Formation, Ibexian, Boat Har-bour, western Newfoundland; I. stubblefieldi Ingham in Inghamet al., 1986, Dounans Limstone, probably early Whiterockian,Highland Border Complex, Scotland; I. wahwahensis n. sp.,Wahwah Formation, Ibexian (Blackhillsian), Zone J (Pseudo-cybele nasuta Zone), Section J, Ibex area, western Utah; Ischy-rotoma sp. ind. of Whittington (1965); Ischyrotoma sp. of Ross(1967).

Diagnosis.Diagnosis is presented in the following format, de-rived directly from the parsimony analysis: character-state num-ber, consistency index, whether optimized by ACCTRAN (A),DELTRAN (D), or both, and description/discussion.

1. 8(1), c.i. 5 1.0, A/D; S0 long (sag.; exsag.), front of L0sharply defined, in some species slightly overhanging S0.

2. 10(2), c.i. 5 1.0, A/D; rostral area defining strong inverted‘‘V’’ shape in anterior view, very narrow rostral plate retainedwith functional rostral sutures.

3. ? 1(2), c.i. 5 1.0, A; anterior border tab-shaped (1(2)). Thisstate is optimized at the genus node under ACCTRAN, but thisdepends on the assumption that it can transform into the special-ized rim-like state of the considerably older species pair anata-phra/parallela. In terms of morphocline analysis (e.g., Mickevichand Weller, 1990), the rim-like state seems more plausibly con-sidered an intermediate between the primitive flattened condition


FIGURE 6—Ischyrotoma wahwahensis n. sp., from the Wah Wah Formation, Ibex Section J-81, Ibexian (Blackhillsian; Zone J/Pseudocybele nasutaZone), Millard County, western Utah. All figures are scanning electron micrographs, and are 330 except where noted. 1, 5, cranidium, SUI 94254,dorsal and oblique views. 2, cranidium, SUI 94255, dorsal view. 3, 7, cranidium, SUI 94256, dorsal and oblique views. 4, cranidium, SUI 94257,dorsal view. 6, cranidium, SUI 94258, dorsal view. 8, 13, hypostome, SUI 94259, ventrolateral and ventral views, 320. 9, transitory pygidium,SUI 94260, dorsal view, 340. 10, pygidium, SUI 94261, posterodorsal view. 11, 12, hypostome, SUI 94262, ventral and posteroventral views,325.

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FIGURE 5—Ischyrotoma wahwahensis n. sp., from the Wah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; Zone J/Pseudocybele nasutaZone), Millard County, western Utah. All figures are 3 10. 1, 2, 5, 6, cranidium, holotype, SUI 94246, dorsal, oblique, right lateral, and anteriorviews, J-81. 3, 7, cranidium, SUI 94247, dorsal and right lateral views, J-81. 4, 8, cranidium, SUI 94248, dorsal and right lateral views, J-81. 9,10, 12, cranidium, SUI 94249, dorsal, anterior, and right lateral views, J-46. 11, 14, 22, cranidium, SUI 94250, dorsal, right lateral, and ventralviews, J-46. 13, 18, cranidium, SUI 94251, dorsal and left lateral views, J-81. 15, 19, 23, cranidium, SUI 94252, dorsal, left lateral, and anteriorviews, J-81. 16, 17, 20, 21, cranidium, SUI 94253, dorsal, oblique, right lateral, and anterior views, J-81.

and the derived tab-shape, a hypothesis which matches the oc-currence of the states in time. This constraint was not imposed inthe analysis, but the DELTRAN optimization of state 1(2) as anapomorphy of the more restricted ‘‘twenhofeli group’’ seems pref-erable.

4. 15(1), c.i. 5 0.5, A; eye socle with strongly inflated band.This state is reversed with certainty in I. wahwahensis and I jua-bensis. Some evidence that a genuine reversal is involved is pro-vided by the librigenae of I. juabensis, in which the smaller il-lustrated example (Fortey and Droser, 1996, fig. 15.3) appears toretain a subdued but distinct band (15(1)), whereas the largerspecimen (Fortey and Droser, 1996, fig. 15.2) appears to haveentirely lost the socle (15(0)). The specimens are from different

localities, but in all other proportions and features are nearly iden-tical. Such a loss cannot, however, be demonstrated in the abun-dant silicified librigenae of I. wahwahensis. We have considerableconfidence that this character is a genuine synapomorphy of theall-inclusive Ischyrotoma. It fails to optimize at the basal nodeunder DELTRAN due to missing data—the socle is not preservedin I. parallela and DELTRAN optimizes the missing state as 0.This seems unlikely, given the strong similarity between I. par-allela and I. anataphra in all available details.

5. 16(1), c.i. 5 0.5, A/D; librigenal lateral border furrow verybroad in mature specimens. The character is reversed in I. wah-wahensis, but otherwise seems to be a robust apomorphy.

Discussion.The species borealis was tentatively assigned by


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FIGURE 7—1–23, Ischyrotoma wahwahensis n. sp., from the Wah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; Zone J/Pseudocybele nasutaZone), Millard County, western Utah. All figures are 310. 1, right librigena, SUI 94263, external view, J-81. 2, right librigena, SUI 94264, externalview, J-81. 3, left librigena SUI 94265, external view, J-81. 4, left librigena SUI 94266, external view, J-81. 5, right librigena, SUI 94267, externalview, J-81. 6–8, thoracic segments, SUI 94268, dorsal, oblique, and ventral views, J-81. 9, 10, left librigena, SUI 94269, external and internalviews, J-81. 11–13, 18, pygidium, SUI 94270, dorsal, posterior, ventral, and anteroventral views, J-59. 14–16, thoracic segment, SUI 94271, dorsal,anterior, and ventral views, J-81. 17, 22, thoracic segment, SUI 94272, dorsal and anterior views, J-81. 19, 20, pygidium, SUI 94273, dorsal andposterior views, J-81. 21, 23, pygidium, SUI 94274, dorsal and posterior views, J-59. 24–50, Dimeropygiella caudanodosa Ross, 1951, from theWah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; Zone J/Pseudocybele nasuta Zone), Millard County, western Utah. All figures are310. 24, left librigena, SUI 94275, external view, J-42. 25, left librigena, SUI 94276, external view, J-46. 26, right librigena, SUI 94277, externalview, J-42. 27, left librigena, SUI 94278, external view, J-42. 28, right librigena, SUI 94279, external view, J-42. 29, left librigena, SUI 94280,external view, J-42. 30, 36, right librigena, SUI 94281, external and internal views, J-46. 31, left librigena, SUI 94282, external view, J-42. 32–34, right librigena, SUI 94283, external, ventrolateral, and internal views, J-46. 35, right librigena, SUI 94284, external view, J-46. 37, 43, 47, 48,pygidium, SUI 94285, dorsal, posterior, anteroventral, and ventral views, J-81. 38, 44, pygidium, SUI 94286, dorsal and posterior views, J-81. 39,40, thoracic segment, SUI 94287, dorsal and anterior views, J-42. 41, 42, thoracic segment, SUI 94288, dorsal and posterior views, J-42. 45, 49,pygidium, SUI 94289, dorsal and posterior views, J-81. 46, 50, pygidium, SUI 94290, dorsal and posterior views, J-59. 51–53, Dimeropygiellasp., from the Wah Wah Formation, Ibex Section J-42, upper Ibexian (Blackhillsian; Pseudocybele nasuta Zone), Millard County, western Utah,x10, pygidium, SUI 94291, left lateral, dorsal, and posterior views.

Fortey (1980) to Ischyrophyma Whittington, 1963 (a junior syn-onym of Celmus Angelin, 1854, see above). With the abundantnew information available on both Celmus (Adrain and Fortey,1997 and unpublished) and Ischyrotoma (Ingham in Ingham etal., 1986, and herein), it is now evident that borealis is a memberof the latter.

There are four equally parsimonious resolutions of Ischyrotomaingroup relationships (Fig. 4), of which Figure 4.2 is preferred ongrounds of stratigraphic congruence and minimal ad hoc hypoth-eses of ghost lineages. All trees position the eastern Laurentianspecies pair I. anataphra and I. parallela as sister to the remain-der. Ischyrotoma anataphra is well known, and occurs widely inthe mid-Ibexian of Newfoundland (Fortey, 1979; Boyce, 1989),Greenland (Fortey, 1986), and Ellesmere Island, Arctic Canada(J.M.A. and S.R.W., unpublished data). Ischyrotoma parallela isdifficult to assess, as most of the figured specimens are fairlycoarsely preserved internal molds and the species is known onlyfrom cephala. It does, however, appear to differ from I. anataphrain its subparallel-sided glabella, as indicated by Boyce (1989).The remaining species form a well supported clade characterizedparticularly by narrow posterior fixigenae (character 9(1)) andvery closely crowded pygidial posterior axial tubercles (18(1)).

It is now clear that Ischyrotoma and Dimeropygiella representtwo quite distinct groups whose monophyly is supported by par-simony analysis. Their synonymy was proposed (Whittington,1963) at a time when much less information was available, par-ticularly on the very different rostral structures. With the newmaterial and explicit hypothesis of relationship presented herein,it seems best to resurrect Dimeropygiella and to consider thegroups separate genera. Ischyrotoma and Dimeropygiella differmost obviously in the shape of their anterior border and rostralarea. The rostral plate of Dimeropygiella bears a large, ventrallydirected ‘‘spike.’’ Ross (1951, p. 123) initially misinterpreted therostral plate as absent, but later (1953, p. 637) recognized itspresence, claiming it was separated from the librigena by a ‘‘per-fectly clean-cut suture.’’ Most of the librigenae in our collectionsare isolated from the rostral plate, but there are several examples(e.g., Figs. 9.2, 11.6, 11.8) in which the connective sutures areclearly fused and the librigenae yoked. Among isolated librigenae,many (e.g., Fig. 10.10, 10.20) show what appears to be a smoothand well-defined connective suture, as opposed to breakage froma yoked rostrum. Hence, it appears likely that the connective su-tures were fused at certain times during the life of the animal, butthat they could be made functional when necessary to facilitatemolting (cf. Fortey, 1986).

The situation is very different in all members of Ischyrotomafor which information is available. Librigenae of Ischyrotoma

have never been found yoked and there is no evidence the con-nective sutures were ever fused. The rostral plate is much nar-rower than that of Dimeropygiella (see Whittington, 1963, pl. 7,figs. 12, 13; Ingham in Ingham et al., 1986, fig. 8i) and does notbear any form of large ventral ‘‘spike.’’ These differences arereflected in the shape of the anterior border. That of Ischyrotomais more or less tab-shaped, not markedly longer sagittally, and hasan essentially flat ventral profile (e.g., Fig. 5.1, 5.6, 5.9, 5.10).The anterior border of Dimeropygiella is always much longer me-dially, and the ventral margin describes an inverted ‘‘W’’ shapein anterior profile (e.g., Fig. 8.1, 8.9).

Other differences between the genera include the presence inIschyrotoma of a lower, more elongate glabella which typicallynearly or completely overhangs the anterior border furrow. Di-meropygiella displays a relatively shorter (sag.), broader, andsometimes more inflated glabella, the anterior margin of which isalways set back to expose the anterior border and preglabellarfield (if present). The anterior sections of the facial sutures aresubparallel or anteriorly convergent in Ischyrotoma, but anteriorlydivergent and bowed laterally in Dimeropygiella. Ischyrotomaalso generally features a much broader and deeper librigenal lat-eral border furrow and a more strongly inflated, band-like eyesocle. Finally, separate anterior and posterior pleural bands and apleural furrow are retained on at least the second pygidial segmentin Ischyrotoma, but are lost on all but the first in all species ofDimeropygiella.

ISCHYROTOMA WAHWAHENSIS new speciesFigures 5, 6, 7.1–7.23

Diagnosis.Interocular fixigena broad, with more than oneexsagittal tubercle row; librigena lacking inflated eye socle andwith narrow lateral border furrow; five pygidial axial rings; py-gidial axial tubercles set close together but not fused; 5 1 0 or 61 0 pairs of pygidial pleural ribs; ribs abaxial to fulcrum flat-tened, with fine granular sculpture.

Description.Cranidium with maximum width across poste-rior borders, 92–94 percent sagittal length; width across midpointof palpebral lobes 94–95 percent width across posterior borders;anterior border wide and short, width 26–34 percent of sagittallength; sagittal length 44–46 percent that of L0, width 69–76percent width of L0, anterior margin gently convex, border shorterexsagittally than sagittally, with strong dorsal convexity, nearlytube-like in sagittal profile, and dorsal sculpture of 20–30 smallto medium sized tubercles; anterior border furrow incised later-ally, longer and shallower medially, dorsally bowed in anteriorview (Fig. 5.6, 5.10); anterior sections of facial suture bowedlaterally, anteriorly convergent in dorsal view; preglabellar field


FIGURE 9—Dimeropygiella caudanodosa Ross, 1951, from the Wah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; Zone J/Pseudocybelenasuta Zone), Millard County, western Utah. All figures are scanning electron micrographs, and are 330 except where noted. 1 cranidium, SUI94303, dorsal view, J-59. 2, yolked librigenae/rostral plate, SUI 94304, anteroventral view, J-59. 3, hypostome, SUI 94305, ventral view, J-46. 4,hypostome, SUI 94306, ventral view, J-59. 5, librigena, SUI 94307, external view, J-59. 6, hypostome, SUI 94308, ventral view, J-46. 7, transitorypygidium, SUI 94309, dorsal view, J-59. 8, transitory pygidium, SUI 94310, dorsal view, 345, J-46.

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FIGURE 8—Dimeropygiella caudanodosa Ross, 1951, from the Wah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; Zone J/Pseudocybelenasuta Zone), Millard County, western Utah. All figures are 310. 1, 6, 9, 16, 22, cranidium, SUI 94292, dorsal, left lateral, anterior, ventral, andoblique views, J-59. 2, 7, cranidium, SUI 94293, dorsal and left lateral views, J-46. 3, 8, 11, cranidium, SUI 94294, dorsal, right lateral, andanterior views, J-46. 4, 5, cranidium, SUI 94295, dorsal and left lateral views, J-46. 10, 13, 14, 18, cranidium, SUI 94296, anterior, dorsal, leftlateral, and oblique views, J-42. 12, 15, cranidium, SUI 94297, dorsal and left lateral views, J-46. 17, 20, cranidium, SUI 94298, dorsal and leftlateral views, J-42. 19, 21, cranidium, SUI 94299, dorsal and right lateral views, J-81. 23, 26, cranidium, SUI 94300, dorsal and right lateral views,J-81. 24, 27, 28, cranidium, SUI 94301, dorsal, right lateral, and anterior views, J-81. 25, 29, cranidium, SUI 94302, dorsal and right lateral views,J-81.

absent, anterior border furrow juxtaposed against preglabellar fur-row; frontal areas with moderate dorsal inflation and strong sculp-ture of small, medium, and large sized tubercles; interocular fix-igena slightly wider than palpebral lobes, contiguous with frontalarea and with identical sulpture; eye ridge not expressed dorsally;palpebral furrow narrow and incised, completely isolating palpe-bral lobe; palpebral lobe narrow, length (exsag.) 150–200 percentsagittal length of L0, steeply sloped adaxially, abaxial part hori-zontal, sculpture varying from muted tubercles (Fig. 5.4, 5.16) toentirely smooth (Fig. 5.9); interocular fixigena grades into pos-terior fixigena with similar dorsal convexity and tuberculate sculp-ture; posterior facial sutures slightly bowed laterally and withslight posterior divergence as far as posterior border furrow,bowed strongly across posterior border; posterior border furrowlong and deep, set almost exactly transversely; posterior bordernearly contiguous with abaxial part of L0, axial furrow veryweak, short adaxially, longer abaxially, with strong dorsal con-vexity and sculpture of weak, scattered tubercles; axial furrowsin front of L0 very deep and strongly incised, bordered on eitherside by narrow strip of smooth exoskeleton on the adaxial sideof the fixigena and abaxial side of the glabella; axial furrowsslightly anteriorly divergent in front of L0, reaching maximumdivergence opposite rear third of palpebral lobe, anteriorly con-vergent in front of palpebral lobe, running without break intopreglabellar furrow of similar depth and incision; glabella withmaximum width 80–85 percent sagittal length, maximum widthachieved opposite rear of palpebral lobe; glabella with densesculpture of interspersed moderate and large tubercules, largestequivalent to or slightly larger than those on interocular fixigena;glabellar moderately inflated, sagittal profile describing more orless even arc from L0 to dorsal aspect of anterior border; rear of

glabella sharply circumscribed by steep break in slope to S0; S0long (sag., exsag.), with unsculptured, nearly flat bottom in sag-ittal profile; L0 long medially, shortened behind L1 (Fig. 5.5,5.12), slightly longer abaxially near axial furrow; L0 with verydense sculpture of fine, medium, and large tubercles, slighty morecrowded and smaller than those on glabella.

Librigena with maximum width 60–68 percent exsagittallength; lateral border slightly narrower anteriorly than posteriorly,with dense sculpture of fine tubercles; lateral border tuberclesslightly larger adjacent to lateral border furrow; genal spine re-duced to small knob in mature holaspid (Fig. 7.4, 7.9), indistin-guishable on some specimens; fine raised lines developed on ven-trolateral part of lateral border, running subparallel to lateral mar-gin; lateral border furrow arcuate and deeply incised, similardepth along entire length; librigenal field narrowest near rear ofeye, wider anteriorly, with sculpture of fine and coarse tubercles.larger tubercles developed on anterolateral portion of field; eyelarge and slightly bulbous, separated from field by very narrow,moderately incised furrow; no distinct eye socle present; lateralborder underlain by nearly flat doublure of similar width, lackingsculpture, incised in front of genal angle by prominent Panderiannotch.

Hypostome (Fig. 6.8, 6.11–6.13) with maximum width acrossanterior wings, slightly less than or equal to sagittal length; an-terior margin transversely straight medially, bowing posteriorlyand arcuate laterally; anterior margin gently ‘‘lipped’’ medially,set off from middle body by small transverse furrow; anteriorwings long, broad, and subrectangular, occupying anterior half ofhypostome; lateral border narrow anteriorly on wing, broader pos-teriorly, rounded in section opposite rear of middle body, gradingwithout interruption into posterior border; lateral and posterior


FIGURE 11—1–8, 10–12, Dimeropygiella ovata Hintze, 1953, from the Fillmore Formation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah. All figures are scanning electron micrographs. 1, yolked librigenae and rostral plate,SUI 94323, dorsal view, 350. 2, cranidium, SUI 94324, dorsal view, 350. 3, cranidium, SUI 94325, dorsal view, 350. 4, hypostome (assignmenttentative), SUI 94326, ventral view, 320. 5, cranidium, SUI 94327, dorsal view, 360. 6, yolked librigenae and rostral plate, SUI 94328, anteriorview, 330. 7, right librigena, SUI 94329, external view, 330. 8, yolked librigenae and rostral plate, SUI 94330, anteroventral view, 325. 10,transitory pygidium, SUI 94331, dorsal view, 360. 11, 12, pygidium, SUI 94332, oblique and dorsal views, 330. 9, Dimeropygiella fillmorensisn. sp., from the Fillmore Formation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah,scanning electron micrograph, cranidium, SUI 94333, dorsal view, 350.

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FIGURE 10—Dimeropygiella ovata Hintze, 1953, from the Fillmore Formation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/Trigonocercatypica Zone), Millard County, western Utah. All figures are 310. 1, 5, 11, cranidium, SUI 94311, dorsal, right lateral, and anterior views. 2, 6, 9,12, 17, cranidium, SUI 94312, dorsal, right lateral, anterior, oblique, and ventral views. 3, 7, cranidium, SUI 94313, dorsal and right lateral views.4, 8, cranidium, SUI 94314, dorsal and left lateral views. 10, left librigena, SUI 94315, external view. 13, right librigena, SUI 94316, externalview. 14, 20, right librigena, SUI 94317, internal and external views. 15, 18, 22, 26, pygidium, SUI 94318, dorsal, posterior, ventral, and anter-oventral views. 16, left librigena, SUI 94319, external view. 19, 21, pygidium, SUI 94320, dorsal and posterior views. 23, 27, pygidium, SUI94321, dorsal and posterior views. 24, 25, pygidium, SUI 94322, dorsal and posterior views.

borders with ventral sculpture of very fine subparallel raised lines;lateral border furrow indistinct on wing, slightly deeper posteri-orly; posterior border furrow very fine and incised, deeper thanposterior part of lateral border furrow; middle body sagittally in-flated, keel-like along sagittal line, sloping steeply toward wings;sculpture of a few subdued tubercles on anteromedial part of mid-dle body (Fig. 6.13), otherwise smooth; middle furrow very weak-ly impressed, setting off rounded, knob-like swelling on posteriorpart of middle body.

Thorax with axial lobe occupying 36–38 percent of total width;axial ring short (sag., exsag.), with sculpture of two crowded

transverse rows of medium-sized tubercles; distinct, crescent-shaped preannulus present on some segments (Fig. 7.17); articu-lating half-ring longer than remainder of ring, broad and smooth;axial furrow shallow; fulcrum set 60–70 distance abaxially onpleural lobe; leading edge of anterior pleural band with extremelyshort (exsag.) transverse articulatory ridge between axial furrowand fulcrum, separated from main part of band by very fine fur-row; anterior pleural band lacking tuberculate sculpture except fora single prominent tubercle set just abaxial to fulcrum; pleuralfurrow shallow adaxially, not in contact with axial furrow, deeperabaxially across fulcrum, shallowing and becoming effaced on


FIGURE 12—Dimeropygiella fillmorensis n. sp., from the Fillmore Formation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/Trigonocerca typicaZone), Millard County, western Utah. All figures are 310. 1, 5, 9, 13, cranidium, holotype, SUI 94334, dorsal, right lateral, anterior, and obliqueviews. 2, 6, cranidium, SUI 94335, dorsal and left lateral views. 3, 7, cranidium, SUI 94336, dorsal and right lateral views. 4, right librigena, SUI94337, external view. 8, 12, right librigena, SUI 94338, external and internal views. 10, 14, cranidium, SUI 94339, dorsal and left lateral views.11, 15, 27, cranidium, SUI 94340, dorsal, right lateral, and ventral views. 16, right librigena, SUI 94341, external view. 17, 20, thoracic segment,SUI 94342, dorsal and anterior views. 18, 19, thoracic segment, SUI 94343, anterior and dorsal views. 21, 22, thoracic segment, SUI 94344,


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anterior and dorsal views. 23, 25, thoracic segment, SUI 94345, anterior and dorsal views. 24, 26, thoracic segment, SUI 94346, posterior anddorsal views. 28–31 pygidium, SUI 94347, dorsal, posterior, ventral, and anteroventral views. 32, 33, pygidium, SUI 94348, dorsal and posteriorviews.

articulatory facet; posterior pleural band with transverse row offairly coarse tubercles; posterolateral tip of segment extended intovery small, pointed spine behind facet; tip of segment underlainby narrow doublure; posterior edge of posterior pleural band withventral groove to receive articulatory ridge.

Pygidium with sagittal length 57–62 percent maximum width;axis 34–46 percent of width anteriorly, tapering rapidly posteri-orly; articulating half-ring short (sag., exsag.), relatively smallerthan those of the mid-part of the thorax, set off from axis by deepring furrow; five axial rings, first three with sculpture of subduedtubercles along anterior edge, posterior two very small; terminalpiece with two large, medially crowded but not fused tubercles;axial furrow shallow on first segment, deeper posteriorly; anteriorand posterior pleural bands adaxial to fulcrum similar to those ofthoracic segments; abaxial of fulcrum, pleural bands are flattenedand share a subdued granular sculpture with pygidial border; pleu-ral furrow discontinuous, separated into proximal part adaxial tofulcrum and more posteriorly set distal part abaxial to fulcrum;pleural furrows distinct on anterior three, and rarely fourth (Fig.7.21, 7.23, right side) segments; adaxial part of posterior pleuralband with narrow transverse row of tubercles on first three seg-ments, fourth and fifth segments bearing only single tubercle atfulcrum; border broad, with fine raised lines ventrolaterally, andmoderate median ‘‘notch’’; doublure narrow and tightly incurved,lacking sculpture.

Etymology.After the Wah Wah Formation.Material and occurrence.Holotype, cranidium, SUI 94246

(Fig. 5.1, 5.2, 5.5, 5.6), and paratypes 94247–94274, horizons J-46, J-59, J-81, Wah Wah Formation, Ibex Section J, Ibexian(Blackhillsian; Zone J/Pseudocybele nasuta Zone), Millard Coun-ty, western Utah.

Discussion.Ischyrotoma wahwahensis is common throughthe lower part of the Wahwah Formation in its type section (Sec-tion J) at Ibex, and at some horizons (e.g., J-81) is the mostabundant species of trilobite. Its occurrence has never previouslybeen reported and none of its sclerites have ever been illustrated.

Ischyrotoma wahwahensis is now the best known member ofthe genus, and is only the second species for which any infor-mation on early ontogeny is available (Fig. 6). This shows thatearly transitory pygidia (Fig. 6.9) display an unreleased thoracicsegment with a long pair of axial spines, a feature seen also inthe early ontogeny of some species of Dimeropyge (J.M.A. andR.A.F., unpublished data). Meraspid cranidia have an essentiallyspinose sculpture which is transformed into robust tubercles withmaturity. Juveniles of the species bear a relatively elongate me-dian occipital spine and two or three shorter pairs of spines at therear of L0; all of these are shortened through ontogeny and re-duced to nondescript tubercles in the adult. The cranidium showsmarked shape change from early to late stages, as the glabellabecomes narrower and more elongate. These aspects of meraspidontogeny differ somewhat from their counterparts in I. stubble-fieldi, the only other species for which early stages are known.The material of I. stubblefieldi is more coarsely preserved, butearly cranidia do not display an elongate occipital spine and arein general less spinose than those of I. wahwahensis. Although itcould potentially be a function of the tectonic distortion under-gone by the Dounans material, small cranidia of I. stubblefieldiare also much more consistently narrow and elongate.

According to our analysis, Ischyrotoma wahwahensis is mostclosely related to I. juabensis, with which it is compared under

that taxon below. Collectively, I. wahwahensis differs from theearly species I. anataphra and I. parallela in the possession ofan inflated, tab-shaped, tuberculate, versus low, broad, and ter-race-lined (Fortey, 1979, pl. 36, figs. 6, 8) anterior border; absenceversus presence of a preglabellar field; deep versus extremelyshallow anterior border furrow; possession of narrower posteriorfixigenae and wider interocular fixigenae; possession of weak ver-sus strongly incised eye socle and much narrower librigenal lat-eral border furrow with strong versus absent tuberculate sculpture;more closely spaced posterior pygidial tubercles; and interrupted,versus continuous, first pygidial pleural furrow.

Ischyrotoma wahwahensis differs from I. borealis in the pres-ence of less incised glabellar furrows, a longer (sag.; exsag.) S0;the absence versus presence of small spines on the cranidial pos-terior border; larger and more robust cranidial tuberculate sculp-ture; absence of a strongly inflated eye socle; considerably broad-er band of larger librigenal field tubercles; narrower librigenallateral border furrow; and broader, more inflated librigenal lateralborder with much stronger tuberculation. In addition to the on-togenetic features mentioned above, I. wahwahensis differs fromI. stubblefieldi in the possession of narrower palpebral lobes; alower and less inflated glabella; a broader anterior border; longerS0; more widely divergent anterior sections of the facial suture(cf. Fig. 5.6, 5.10, with Ingham et al., 1986, fig. 8b, 8h); presenceof additional small tubercle rows on the interocular fixigena; ab-sence of an inflated eye socle; librigenal lateral border with strongversus essentially absent tubercles; and pygidia with five versusthree or possibly four axial rings, 6 1 0 versus 5 1 0 moredistally flattened pleural ribs, and a more pronounced posterov-entral ‘‘notch.’’ Finally, I. wahwahensis differs from the type spe-cies, I. twenhofeli, in its less anteriorly inflated glabella (cf. Fig.5.5, 5.12, with Whittington, 1963, pl. 7, figs. 2, 5); librigena lack-ing prominent banded eye socle and with more robust tubercleson the border; pygidium with terminal tubercles crowded but sep-arate versus nearly completely fused; and distal pygidial pleuralribs that are flattened versus convex (cf. Fig. 7.11, 7.21, withFortey, 1980, pl. 11, figs. 17, 21).

ISCHYROTOMA JUABENSIS new speciesNot Figured

Ischryotoma stubblefieldi Ingham; FORTEY AND DROSER, 1996, p. 91, fig.15.1–15.9.

?Ischyrotoma stubblefieldi FORTEY AND DROSER, 1999, p. 191, fig. 5.1–5.4.

Diagnosis.Interocular fixigena with several rows of tuber-cles; glabella relatively short (sag.) and less parallel-sided than istypical for genus; librigenal field very narrow (tr.) with relativelylow number of large, coarse tubercles; eye socle very weak bandin small examples, effaced in larger specimens; librigenal lateralborder with dense tubercles; three and possibly minute fourth py-gidial axial rings; 4 1 1 pygidial pleural ribs; pleural furrowdeveloped only on proximal part of first pygidial segment; pygid-ium very short; terminal axial nodes fused.

Discussion.Fortey and Droser (1996) referred early Whiter-ockian material from the Juab Formation at Ibex Section J andShingle Pass, east-central Nevada, to Ischyrotoma stubblefieldiIngham in Ingham et al., 1986, a late Ibexian species from theHighland Border Complex, Scotland. They later (1999) also re-ferred a cranidium from the basal Whiterockian at MeiklejohnPeak in southern Nevada, and one from the basal Whiterockian


at the type section in Whiterock Canyon, central Nevada. It isnow evident that none of this material figured by Fortey and Dro-ser (1996, 1999) is conspecific with I. stubblefieldi, and that infact more than one species may be represented in basal Whiter-ockian strata of the Great Basin.

The Ibex material figured by Fortey and Droser (1996) certainlyappears to represent a single taxon. Since the cranidium, librigena,and pygidium are well known and the species is obviously dis-tinct, it seems reasonable to formally name the taxon. Ischyrotomajuabensis differs from I. stubblefieldi in the possession of a rel-atively shorter glabella; interocular fixigena with several as op-posed to a single tubercle row; more laterally divergent anteriorsections of the facial suture; librigenal field which is considerablybroader anteriorly than posteriorly opposite the eye, versus similarin length posteriorly and anteriorly (cf. Fortey and Droser, 1996,fig. 15.2, 15.3, with Ingham et al., 1986, fig. 8l, 8n, 8q); moresubdued or absent band-like eye socle; librigenal lateral borderwith dense and robust versus essentially absent tubercles; pygidialterminal axial tubercles that are fused versus clearly separate; rel-atively shorter pygidium; and medial fusion or near-fusion of thefifth pygidial pleural rib pair to form the 4 1 1 condition versusdefinitely separate 5 1 0.

Among species included in the analysis, I. juabensis is sisterto I. wahwahensis, from which it differs in the possession of ashorter glabella; narrower librigenal field with fewer tubercles;four versus five pygidial axial rings; and 4 1 1 versus 6 1 0pygidial pleural rib pairs that are convex versus flattened distally.The species share the presence of several tubercle rows on theinterocular fixigena, a subdued eye socle, and dense tubercles onthe librigenal lateral border.

Ischyrotoma juabensis is similar to, and possibly even conspe-cific with, the material of similar age described from PyramidPeak, southeastern California, by Ross (1967) as Ischyrotoma sp.Ross’s specimens are immature, hampering comparison. Never-theless, Ross’s pygidium in particular is similar to those of I.juabensis in general proportions, rib shape, and particularly in thefully fused terminal axial tubercles. The California pygidiumshows five axial rings, but this could be a function of its smallsize.

Genus DIMEROPYGIELLA Ross, 1951

Type species.Dimeropygiella caudanodosa Ross, 1951, Ib-exian (Blackhillsian), Zone J (Pseudocybele nasuta Zone), GardenCity Formation, Locality 13 of Ross (1951), northeastern Utah,and Wahwah Formation, Ibex area, western Utah (see species dis-cussion below).

Other species.Dimeropygiella blanda Hintze, 1953, FillmoreFormation, section H-434, Ibexian (Blackhillsian), Zone H (Tri-gonocerca typica Zone), Ibex area, western Utah; D. fillmorensisn. sp., Fillmore Formation, section H-434, Ibexian (Blackhillsian),Zone H (Trigonocerca typica Zone), Ibex area, western Utah; D.mccormicki n. sp., Fillmore Formation, section H-573, Ibexian(Blackhillsian), Zone I (Presbynileus ibexensis Zone), Ibex area,western Utah; D. ovata Hintze, 1953, Fillmore Formation, sectionH-434, Ibexian (Blackhillsian), Zone H (Trigonocerca typicaZone), Ibex area, western Utah; Dimeropygiella cf. blanda (here-in), Ninemile Formation, late Ibexian, Little Rawhide Mountain,south end of Hot Creek Range, Nevada; Ischyrotoma cf. caudan-odosa of Dean (1989, pl. 28, figs. 7, 9, 10, 13, 15–17), OutramFormation, Ibexian (Blackhillsian), Zone J (Pseudocybele nasutaZone), Wilcox Pass, Jasper National Park, Alberta, Canada; ?Is-chyrotoma sp. A of Brett and Westrop (1996), Fort Cassin For-mation, Ibexian, correlative with Zone H (Trigonocerca typicaZone), Sciota Road Section, Champlain Valley region, easternNew York State.

Diagnosis.Diagnosis is presented in the same format as forIschyrotoma above.

1. 6(1), c.i. 5 1.0, A/D; anterior sections of facial sutures an-teriorly divergent, and bowed laterally.

2. 10(1), c.i. 5 1.0, A/D; rostral sutures fused, bottom of rostralplate extended ventrally as median spike-like projection.

3. 24(1), c.i. 5 1.0, A/D; anterior band and pleural furrow ofpygidial segments posterior to first lost, posterior band inflatedinto strong rib.

Discussion.Dimeropygiella was compared with Ischyrotomaunder discussion of that genus above. The genus comprises twoclades with fairly distinct morphologies (Fig. 3). A ‘‘caudano-dosa’’ group is densely tuberculate, has a strongly inverted ‘‘W’’-shaped anterior border, and has a short or absent preglabellar field.Monophyly of this group is supported by characters 11(1), reten-tion of large and obviously distinct primary fixigenal tubercles inlarge holaspides, and 22(1), separation of the posteriormost py-gidial axial ring into separate abaxial elements with medial ef-facement. A ‘‘blanda’’ group shows variable effacement of atleast the anterior cephalic tuberculation (on the frontal area, an-terior glabella, and librigenal field), retains a fairly long pregla-bellar field, and has only a moderate or weak inverted ‘‘W’’-shapeto the front of the anterior border. Its monophyly is supported byseveral character-states (Table 3), including the universal devel-opment of a very short (sag., exsag.) and deeply incised anteriorborder furrow.

It is difficult to assess the affinity of the single fragmentarycranidium figured by Brett and Westrop (1996) as Ischyrotomasp. A. It has a ‘‘normal’’ cranidial anterior border but lacks apreglabellar field. It is contemporaneous with, and may be relatedto, Ibex species such as Dimeropygiella blanda, but it also resem-bles older species of Pseudohystricurus. It is certain, however,that it is not a member of Ischyrotoma as understood herein.

DIMEROPYGIELLA CAUDANODOSA Ross, 1951Figures 7.24–7.50, 8, 9

Dimeropygiella caudanodosa ROSS, 1951, p. 124, pl. 35, figs. 18, 22–28;HINTZE, 1953, p. 154, pl. 19, figs. 9, 10; WHITTINGTON AND EVITT,1954, p. 37.

Ischyrotoma caudanodosa (ROSS, 1951). WHITTINGTON, 1963, p. 47;FORTEY, 1979, p. 106; DEAN, 1989, p. 36.

Ischyrotoma caudinodosa [sic] (ROSS, 1951). CHATTERTON, 1994, p. 543,fig. 1, table 1.

Diagnosis.Cranidial axial furrows broad and anterior borderfurrow long (sag., exsag.), both bordered by area of smooth exo-skeleton; anterior border strongly inverted ‘‘W’’-shaped; librigen-al field densely tuberculate; librigenal lateral border furrow quitenarrow; lateral border with more dense tuberculation posteriorly,raised lines ventrolaterally and anteriorly; pygidium with five ax-ial rings, 6 1 0 or faint 7 1 1 rib pairs, and a strong medianindentation in the ventral margin in posterior view.

Material and occurrence.Figured specimens SUI 94275–94290, 94292–94310, from horizons J-42, J-46, J-59, and J-81,Wah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; ZoneJ/Pseudocybele nasuta Zone), Millard County, western Utah.

Discussion.Ross (1951) established this type species on thebasis of three illustrated sclerites from northern Utah (he later(1953) added one juvenile cranidium and one librigena/rostralplate) while Hintze (1953) documented its occurrence at Ibex withsingle dorsal views of one cranidium and one pygidium. We hereextend the treatment of Section J material to include multiplesclerites, including librigenae, thoracic segments, hypostomes,and additional early growth stages. Due to the low number ofillustrations of material from the Utah type locality, it is not pos-sible to be entirely confident that the Ibex section J taxon is con-specific. Ross’s holotype cranidium is small, only about half the


size of our typical examples (Fig. 8.1–8.3). However, it closelyresembles similarly sized specimens in our samples (e.g., Fig.8.23–8.25). The single librigena illustrated by Ross appears tohave a narrow field and a lateral border furrow that broadensposteriorly (Ross, 1951, pl. 35, fig. 27), but this is at least partlya function of photographic orientation. Ross’s pygidium is slightlydistorted. On present evidence, it seems very likely that the sec-tion J species is Dimeropygiella caudanodosa, but this will beconfirmed once the Garden City type locality is recollected.

Some stratigraphic variation in cranidial morphology may oc-cur among the samples here assigned to D. caudanodosa. Cranidiafrom horizon J-42 (the lowest from which samples used in thispaper were obtained) appear to have a slightly more denselycrowded pattern of glabellar tubercles (Fig. 8.13, 8.17), togetherwith a more prominent dorsal expression of the eye ridge, whichis topped by a larger isolated tubercle. No differences are appar-ent, however, in the librigenae or pygidia recovered from thishorizon, and in other respects the cranidia conform closely tothose higher in the section.

The hypostome of D. caudanodosa was previously unknown.It shows that despite the derived rostral morphology, a large,‘‘typical’’ natant hypostome was retained (Fig. 9.3). It differsfrom that of Ischyrotoma wahwahensis, which is common higherin the section, in its less parallel-sided anterior wings, lack of aventral knob-like protrusion on the posterior lobe of the middlebody, and longer posterior border. It is very similar to the olderspecimen recovered from H-434 (Fig. 11.4), which must belongto one of the three species of Dimeropygiella occurring there.

The genal spine of the very smallest specimens is about as longas the remainder of the cheek, and remains distinct until late inmeraspid ontogeny. Small librigenae (Fig. 9.2, 9.5) show that theraised subparallel lines seen on the lateral border of adults beginas rows of fine, closely spaced tubercles which eventually coa-lesce into ridges. Interestingly, the axial furrow and cephalic bor-der furrows of small specimens are also lined with a row of small,fine tubercles (Fig. 9.1, 9.5). Both of these features are seen inthe early ontogeny of Dimeropyge (see Chatterton, 1994), but notin what is known of Ibexian ‘‘hystricurids’’ (Lee and Chatterton,1997).

Dimeropygiella caudanodosa differs from D. ovata in the pos-session of broader cranidial axial furrows; a longer anterior borderfurrow; a relatively smaller and less inflated glabella with lessobvious smooth areas marking S1 and S2; less anteriorly diver-gent anterior sections of the facial sutures; narrower librigenallateral border with stronger and denser tubercles versus moreprominent raised lines; five pygidial axial rings versus three witha minute fourth; 6 1 0 or 7 1 1 versus 4 1 1 or 5 1 0 pygidialpleural ribs; and a pygidium that has a relatively larger proximalpleural area, is not as tall relative to its width, and bears a prom-inent median indentation in the ventral margin versus a simpleinverted ‘‘V’’ shape. It differs from D. fillmorensis, the sister spe-cies of D. ovata, in many of the same ways. In addition, D. cau-danodosa is distinguished from D. fillmorensis in the possessionof a more subtriangular anterior border; much less dense cranidialtuberculation; obscure versus incised glabellar furrows; five ver-sus three pygidial axial rings; 6 1 0 or 7 1 1 versus a faint 5 11 pygidial pleural rib count; and a subtriangular pygidium withmaximum width in front of pygidial sagittal mid-length.

DIMEROPYGIELLA OVATA Hintze, 1953Figures 10, 11.1–11.8, 11.10–11.12

Dimeropygiella ovata HINTZE, 1953, p. 155, pl. 19, figs. 1–4.non Ischyrotoma ovata (Hintze, 1952 [sic]). YOUNG, 1973, p. 102, pl. 2,

figs. 3, 4, 6, 7 (fig. 3 and fig. 4 represent Dimeropygiella fillmorensisn. sp.; figs. 6, 7 are of a pygidium of D. blanda).

Ischyrotoma blanda (Hintze, 1952 [sic]). YOUNG, 1973, p. 102, pl. 2, fig.1 only.

Diagnosis.Glabella relatively large and inflated; S1 and S2defined as smooth, unincised areas of exoskeleton; anterior sec-tions of facial sutures strongly bowed laterally in front of palpe-bral lobe; librigenal lateral border broad and nearly blade-likeanteriorly, with prominent, subparallel raised lines; pygidium withsubdued dorsal sculpture, with three and a very faint fourth axialring and 4 1 0 or 5 1 1 rib pairs; pygidium tall, only slightlytransversely flexed in posterior view.

Material and occurrence.Figured specimens SUI 94311–94332, horizon H-434, Fillmore Formation, Ibex Section H, Ib-exian (Blackhillsian; Zone H/Trigonocerca typica Zone), MillardCounty, western Utah.

Discussion.In Hintze’s (1953) monograph, two species of Di-meropygiella which occurred at horizon H-434, D. ovata and D.blanda, were correctly discriminated. Fieldwork for this projecthas revealed the presence of a third distinct species at this horizon,described as D. fillmorensis below. In Young’s (1973) study, scler-ites belonging to all three species were assigned somewhat hap-hazardly to Hintze’s original two species. Of the five Dimeropy-giella sclerites illustrated by Young, only one, a cranidium of D.blanda, was correctly assigned to species. The librigena Youngassigned to D. blanda belongs in fact to D. ovata, the pygidiumassigned to D. ovata is obviously D. blanda, and the cranidiumand librigena assigned by Young to D. ovata represent our newD. fillmorensis. We have considerable confidence in the assign-ments of sclerites to particular species in the present study, basedon the following criteria:

1) New species similar to D. blanda occur higher in section H(D. mccormicki, see below) and elsewhere (Dimeropygiella n. sp.aff. D. blanda)—the morphology of their librigenae and pygidiaare a good guide to the correct associations for D. blanda.

2) Sculptural correspondence. Mature cranidia of D. blanda arerelatively effaced, and this is reflected also in the librigenae andpygidia. Cranidia of D. fillmorensis have considerably denser tu-berculation than those of D. ovata, and this is matched by thesculpture of the librigenae.

3) Frequency of occurrence. This is the main basis for assigningpygidia to either D. ovata or D. fillmorensis. Cranidia and libri-genae of D. ovata are more than twice as common as those of D.fillmorensis. These relative abundances are reflected also in thetwo pygidial morphotypes, and hence the more common one isassigned to D. ovata.

Dimeropygiella ovata was compared with D. caudanodosaabove. It differs from its sister species, D. fillmorensis, in its lessdense cranidial tubercles; more laterally bowed anterior sectionsof the facial suture; longer preglabellar area; longer anterior bor-der furrow; impression of S1 and S2 as smooth areas versus in-cised furrows; wider librigenal field with less crowded tubercles;wider librigenal lateral border furrow; broader librigenal lateralborder that widens anteriorly, with prominent raised lines versusdense tubercles; pygidia that are longer (sag.) relative to width,taller, and with posterior margin describing even arc in dorsalview versus inverted ‘‘W’’ shape; three strong pygidial axial ringsand very weak fourth versus two strong and weak third; and 4 11 or 5 1 0 pygidial pleural ribs versus weak but definite 5 1 1.

DIMEROPYGIELLA FILLMORENSIS new speciesFigures 11.9, 12

Ischyrotoma ovata (Hintze, 1952 [sic]); YOUNG, 1973, p. 102, pl. 2, figs.3, 4 (non pl. 2, figs. 6, 7 5 D. blanda).

Diagnosis.Cranidial tubercles extremely numerous anddensely crowded; glabellar furrows incised; librigenal field with


FIGURE 14—Dimeropygiella blanda Hintze, 1953, from the Fillmore For-mation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/Trigono-cerca typica Zone), Millard County, western Utah. All figures are scan-ning electron microscopes and 360. 1, cranidium, SUI 94364, dorsalview. 2, cranidium, SUI 94365, dorsal view. 3, cranidium, SUI 94366,dorsal view.

←

FIGURE 13—Dimeropygiella blanda Hintze, 1953, from the Fillmore Formation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/Trigonocercatypica Zone), Millard County, western Utah. All figures are 310. 1, 5, 9, cranidium, SUI 94349, dorsal, right lateral, and anterior views. 2, 6,cranidium, SUI 94350, dorsal and left lateral views. 3, 7, cranidium, SUI 94351, dorsal and left lateral views. 4, 8, 12, cranidium, SUI 94352,dorsal, right lateral, and ventral views. 10, 11, 14, 15, cranidium, SUI 94353, dorsal, oblique, left lateral, and anterior views. 13, 17, cranidium,SUI 94354, dorsal and right lateral views. 16, 21, left librigena, SUI 94355, external and internal views. 18, left librigena, SUI 94356, externalview. 19, 20, left librigena, SUI 94357, external and internal views. 22–24, right librigena and rostral plate, SUI 94358, external, ventrolateral, andanterior views. 25, 27, 28, thoracic segment, SUI 94359, right lateral, dorsal, and anterior views. 26, right librigena, SUI 94360, external view.29, 30, thoracic segment, SUI 94361, dorsal and anterior views. 31, 32, thoracic segment, SUI 94362, dorsal and anterior views. 33, 34, pygidium,SUI 94363, dorsal and posterior views.

dense tubercles; librigenal lateral border furrow narrow and in-cised; lateral border narrow and densely tuberculate; pygidiumshort relative to width, with three axial rings and 5 1 1 pairs ofpleural ribs.

Etymology.After the Fillmore Formation.Material and occurrence.Holotype, cranidium, SUI 94334

(Fig. 12.1, 12.5, 12.9, 12.13), paratypes SUI 94333, 94335–94348, horizon H-434, Fillmore Formation, Ibex Section H, Ib-exian (Blackhillsian; Zone H/Trigonocerca typica Zone), MillardCounty, western Utah.

Discussion.Dimeropygiella fillmorensis was compared withD. caudanodosa and D. ovata above. It is possible to assign tho-racic segments to the species because of the characteristic denselycrowded tubercles, reflected on the thoracic axial rings and pos-terior pleurae. The basis for association of sclerites was also dis-cussed above.

DIMEROPYGIELLA BLANDA Hintze, 1953Figures 13, 14, 15.1–15.11

Dimeropygiella blanda HINTZE, 1953, p. 155, pl. 19, figs. 6–8.Ischyrotoma blanda (Hintze, 1952 [sic]); YOUNG, 1973, p. 102, pl. 2, fig.

2 (non pl. 2, fig. 1 5 D. ovata).

Ischyrotoma ovata (Hintze, 1952 [sic]). YOUNG, 1973, P. 102, pl. 2, figs.6, 7 (non pl. 2, figs. 3, 4 5 D. fillmorensis).

Diagnosis.Cranidium with strong sagittal convexity; anteriorpart with tubercles effaced in mature holaspides; long preglabellarfield; librigenal field nearly smooth in large specimens; librigenallateral border furrow narrow and incised; lateral border with tu-bercles completely effaced and raised lines on ventrolateral as-pect; thoracic segments lacking dorsal sculpture; pygidia tall, withvery little transverse flexure; three and rarely a minute fourthpygidial axial rings and 5 1 0 pairs of pygidial pleural ribs; ribsbecoming subdued distally near contact with pygidial border; bor-der strong and rim-like.

Material and occurrence.Figured specimens SUI 94349–94370, horizon H-434, Fillmore Formation, Ibex Section H, Ib-exian (Blackhillsian; Zone H/Trigonocerca typica Zone), MillardCounty, western Utah.

Discussion.Dimeropygiella blanda is compared with D.mccormicki n. sp. below. Early cranidial growth stages (Fig. 14)show considerable differences with those of the ‘‘caudanodosa’’group (Fig. 9, 11). In particular, the transverse shape of the an-terior border is in place from the earliest stages known, in whichit resembles a generalized, ‘‘normal’’ border (Fig. 14.3), and thepreglabellar field is also present throughout ontogeny. This indi-cates that these features are likely to be plesiomorphic. The ma-terial does show that the axial furrows were bounded by a rowof fine tubercles, as in the ‘‘caudanodosa’’ clade. Smaller cranidiashow the inverted ‘‘W’’ shape to the bottom of the anterior bordercharacteristic of the genus (Fig. 13.15), though weakly. In largerspecimens (Fig. 13.9) the bottom of the border is nearly trans-verse.

DIMEROPYGIELLA MCCORMICKI new speciesFigure 15.12–15.33

Diagnosis.Long, unfurrowed preglabellar field; cephalonwith dense tuberculate sculpture, very slightly subdued on ante-rior part of glabella; long genal spine retained in mature holas-pides; pygidium with three axial rings, 6 1 0 pleural ribs that arevery subdued distally, and tab-shaped, well spaced terminal axialnodes.

Etymology.After Tim McCormick.Material and occurrence.Holotype, cephalon, SUI 94373

(Fig. 14.16, 14.21, 14.30, 14.31), paratypes SUI 94371, 94372,94374–94379, horizon H-573, Fillmore Formation, Ibex SectionH, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone),Millard County, western Utah.

Discussion.Dimeropygiella mccormicki shares several fea-tures with D. blanda, including the presence of a long preglabellarfield, more transversely straight connective sutures, at least partialsuppression of sculpture on the anterior of the glabella in maturespecimens, the suppression of tuberculate sculpture on the libri-genal field and dorsal surface of the pygidium, and the completeabsence of tubercles on the librigenal lateral border. It differs inthe generally denser cranidial tuberculate sculpture, the presenceof a more pronounced inverted ‘‘W’’ shape to the bottom of the


FIGURE 15—1–11, Dimeropygiella blanda Hintze, 1953, from the Fillmore Formation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/Trigon-ocerca typica Zone), Millard County, western Utah. All figures are 310. 1, 5, 9, pygidium, SUI 94367, dorsal, posterior, and right lateral views.2, 6, 10, 11, pygidium, SUI 94368, dorsal, posterior, anteroventral, and ventral views. 3, 7, pygidium, SUI 94369, dorsal and posterior views. 4,8, pygidium, SUI 94370, dorsal and posterior views. 12–33, Dimeropygiella mccormicki n. sp., from the Fillmore Formation, Ibex Section H,thrombolitic buildups at H-573, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah. All figures are 310.12, 14, cranidium, SUI 94371, dorsal and left lateral views. 13, 15, cranidium, SUI 94372, dorsal and anterior views. 16, 21, 30, 31, cephalon,holotype, SUI 94373, dorsal, anterior, oblique, and right lateral views. 17, 18, 22, 23, cranidium, SUI 94374, dorsal, ventral, left lateral, and anteriorviews. 19, 20, 24, left librigena, SUI 94375, external, ventrolateral, and internal views. 25, pygidium, SUI 94376, dorsal view. 26, 27, 32, 33,pygidium, SUI 94377, dorsal, left lateral, posterior, and ventral views. 28, right librigena, SUI 94378, external view. 29, right librigena, SUI 94379,external view.


FIGURE 16—Dimeropygiella n. sp. aff. D. blanda, from the Ninemile Formation, Little Rawhide Mountain, south end of Hot Creek Range, Nevada.1, 2, 4, cranidium, It. 25035, dorsal, oblique, and anterior views, 37.5. 3, 6, 7, pygidium, It. 25036, dorsal, left lateral, and posterior views, 310.5, right librigena, It. 25037, external view, 310.

anterior border; retention of a long genal spine in large specimens,and the presence of 6 1 0 versus 5 1 0 or 5 1 1 pygidial pleuralribs. By reference to Pseudohystricurus and the outgroup, thedenser sculpture and long genal spines are very likely plesiom-orphic conditions relative to the more effaced and shortened statesin D. blanda. Dimeropygiella mccormicki, however, occurs some45 m above D. blanda in section H, a testament to the indepen-dence of stratigraphic versus phylogenetic data, and of the irrel-evance of the former to the latter. Dimeropygiella mccormicki hasnever previously been reported from the Ibex area and none ofits sclerites have been figured. It is the most common trilobite ina collection derived from the sponge-algal reef which forms thebasal unit of Hintze’s (1973) Calathium calcisiltite member of theFillmore Formation.

DIMEROPYGIELLA new species aff. D. BLANDA Hintze, 1953Figure 16

Material and occurrence.Figured specimens It. 25035–25037, from section at Little Rawhide mountain, south end of HotCreek Range, central Nevada (Siewers in Droser and Sheehan,1995, fig. 31). The lower part of the section in the NinemileFormation consists of dominant shales with micritic laminatedlimestone beds, which is succeeded by 21 metres of rubbly bed-ded nodular limestones with shale partings. This is succeeded bybedded limestones. The collection was made 2 m from the baseof these limestones.

Discussion.This taxon is obviously closely related to Dimer-opygiella blanda, though nevertheless clearly distinct. It resem-bles D. blanda in the cranidial effacement, general dimensions,and presence of a preglabellar field. The available cranidium islarge, however, yet it retained distinct tubercles on the frontalarea. These are effaced even in smaller specimens of D. blanda.The pygidium is also differentiated, with a very strong 4 1 1 rib

count and smaller, more rim-like border. The small librigenafound (Fig. 16.5) could well belong to another taxon, given thattwo or more species of the Ischyrotoma group occur together atmany Ibex horizons.

DIMEROPYGIELLA sp.Figure 7.51–7.53

Material and occurrence.Single pygidium, SUI 94291, fromhorizon J-42, Wah Wah Formation, Ibex Section J, Ibexian(Blackhillsian; Zone J/Pseudocybele nasuta Zone), Millard Coun-ty, western Utah.

Discussion.A single large pygidium from J-42 is clearly dis-tinct from co-occurring Dimeropygiella caudanodosa and almostcertainly represents a new, rare species. It is shorter (sag.; post.)relative to its width than pygidia of D. caudanodosa, has muchstronger dorsal tubercles, and less fin-shaped pleural ribs. Its over-all morphology and terminal axial tubercles indicate that it likelyrepresents a member of the Ischyrotoma group, and its similarityin axial ring and rib counts to D. caudanodosa suggest Dimero-pygiella. However, it does not closely resemble any species thusfar described.

ACKNOWLEDGMENTS

This work was supported by NSF grant EAR 9973065. Costsof earlier fieldwork were met by a Natural Sciences and Engi-neering Research Council (Canada) operating grant to S.R.W.,held at Brock University, and by Natural Environment ResearchCouncil and Leverhulme Trust research grants to R.A.F. We aregrateful to T. McCormick for assistance in the field and with acidprocessing.

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