Ordovician graptolite evolutionary radiation: a review

Ordovician graptolite evolutionary radiation: a review

CHEN XU*, ZHANG YUAN-DONG and FAN JUN-XUAN

State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology,Chinese Academy of Sciences, Nanjing 210008, P.R. China

Based on the global graptolite genera and higher rank taxa, we propose three radiation stages through the Ordovician. Theisograptid type of development is present within anisograptids predominating in the Tremadocian. Thus, the evolutionaryradiation of the Anisograptid fauna from Tremadocian is proposed as the beginning of the Ordovician graptolite radiation. Thesecond graptolite radiation event is the radiation of the Dichograptid fauna, which began from the T. fruticosus Biozone. Thethird radiation event, the radiation of Diplograptid fauna began immediately after that of the dichograptids. This radiationincludes the peak in total diversity of all the Ordovician graptoloids in the Nemagraptus gracilis Biozone. The radiationextended through the Sandbian and Katian and then was interrupted by a major extinction during the early Hirnantian. Thus, theOrdovician graptolite radiation events coincide with those of the three graptolite faunas proposed by Bulman. The distributionand expansion of the Ordovician graptolites in South China may exemplify the graptolite origination pattern, which begins fromthe slope belt and expanded into both the shelf and oceanic zones. Copyright # 2006 John Wiley & Sons, Ltd.

Received 13 June 2005; revised version received 6 March 2006; accepted 24 March 2006

KEY WORDS graptolite evolutionary radiation; anisograptid fauna; dichograptid fauna; diplograptid fauna; Ordovician; South China

1. INTRODUCTION

The Ordovician faunal radiation became an important research field for Ordovician workers since the three major

faunas, the Cambrian, Palaeozoic and Modern faunas, were defined by Sepkoski (1981). It represents the

replacement from the Cambrian fauna to the Palaeozoic fauna. Investigations into the Ordovician radiation began

with the shelly faunas, in particular the Whiterockian shelly fauna (Hintze 1953; Cooper 1956). An abundant

change in the Whiterockian fauna, with a rapid diversity increase leads to the conclusion that the Ordovician faunal

radiation began during this time interval. This is in accord with a total diversity increase in many different fossil

groups (Webby 2004). Thus, many authors believe that the Ordovician faunal radiation began during the

Whiterockian.

There is no agreement about the beginning of the Ordovician graptolite radiation. The Tremadocian graptolites

are dominated by species of the Anisograptidae, which are recognized as taxa transitional between the dendroids

and graptoloids but included in the Dendroidea (Bulman 1949). Mu (1974, 1987) termed them Graptodendroids.

However, recent important contributions dealing with graptolite systematic palaeontology (Cooper and Fortey

1982; Fortey and Cooper 1986; Mitchell 1987) indicate the general acceptance of a modern palaeobiological

GEOLOGICAL JOURNAL

Geo. J. 41: 289–301 (2006)

Published online 11 August 2006 in Wiley InterScience

(www.interscience.wiley.com) DOI: 10.1002/gj.1051

*Correspondence to: Chen Xu, State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology,Chinese Academy, 39 East Beijing Road, Nanjing 210008, P.R. China. E-mail: [email protected]

Contract/grant sponsor: Chinese Academy of Sciences; contract/grant number: KZCX3-SW-149.Contract/grant sponsor:Major Basic Research Project of theMinistry of Sciences and Technology, China; contract/grant number: G2000077700.Contract/grant sponsor: National Natural Science Foundation; contract/grant numbers: 40372007, 40402003.

Copyright # 2006 John Wiley & Sons, Ltd.

concept in the new graptolite classification. Currently, only a few authors insist on a purely morphological

classification which emphasizes relations between stipes and even on the ‘virgula.’ However, a new,

comprehensive, cladistic phylogenetic analysis is to be adopted in the planned third edition of the graptolite

treatise work (Mitchell, oral communication). Thus, a study of graptolite radiation should not be based on an

inconsistent classification framework. A global Ordovician chronostratigraphic chart, though, has nearly been

completed, and high-resolution global Ordovician correlation provides an alternative basis for a graptolite radiation

study. Finally, the term ‘Evolutionary radiation’ (Mitchell 1990) is used in this paper, because graptolite radiation

mainly reflects large-scale evolutionary history.

The Graptolithina is an extinct class of Phylum Hemichordata, and includes six orders (Bulman 1970), of

which the Dendroidea and the Graptoloidea are the most important. However, there is no agreement as to

whether the Anisograptidae belongs to the Dendroidea or the Graptoloidea. Most Cambrian graptolites are

dendroids with a few exceptions such as Dictyonema? wutinshanensis Mu 1955, which might be a primary

planktonic form occurring in latest Cambrian strata (Mu 1955). From the beginning of the Ordovician most

graptolites were planktonic with only a small group of dendroids. Zhang et al. (2005) concluded that the earliest

planktonic graptolites, the Rhabdinopora flabelliformis species group, might be derived from Airograptus,

which was a late Cambrian benthic form. A life strategy change from benthic to planktonic is the most important

change in graptolite evolutionary history. Noble and Danelian (2004) defined a similar life mode change in

radiolaria, which shows a transitional evolutionary stage from the late Cambrian to the Tremadocian, and then

changed their body plan in the Floian (Bergstrom et al. 2006), when true Ordovician radiolarians began. The

first occurrence of the planktonic graptolite Rhabdinopora was also considered by the Ordovician

Subcommission as a basic criterion on which to define the base of the Ordovician. Coincident with this

graptolite evolutionary development, the appearance of a nematophorous sicula in graptolites may have played

a much more important role than the reduction of stipes in both phylogeny and ecology. Fortey and Cooper

(1986) recognized the importance of this character in their diagnosis of the Graptoloidea. They defined the

Graptolithina as ‘retaining nematophorous sicula in adult, and with bilaterally symmetrical colonies,

secondarily lost in Monograptacea’. Although the anisograptids are similar to dendroids in possessing bithecae

and similar to graptoloids in possessing dichotomous branching, they essentially belong to the Graptoloidea.

Thus, the occurrence of anisograptids indicates that the Graptoloidea begins at the beginning of the Ordovician.

Fortey and Cooper (1986) designated the Anisograptidae as a ‘Suborder not assigned,’ which implies that this

group may attain a higher rank in taxonomy. Thus, we define herein the occurrence of the Anisograptidae as

marking the beginning of the Ordovician graptolite radiation.

The Suborder Virgellina was proposed by Fortey and Cooper (1986) based on the appearance of a virgella,

which developed on the metasicula (presumed to be sexually produced and to be stable in morphology). There

were four morphological phase changes of the metasicula through graptolite evolutionary history. The first is

the occurrence of the virgella; the second is the resorption porus from the prosicula in the Ordovician and the

change from resorption porus to primary porus in the monograptids in the early Silurian; the third is

the development of the retiolitid ancora and reduction of metasicula periderm in the Silurian retiolitids; and the

fourth is the sicula aperture enlargement in the upper Homerian and early Devonian. Although the Virgellina as

originally conceived is most likely polyphyletic, the virgellinids mainly flourished within the Diplograptacea,

which occurred later than most of the dichograptids. The flourishing of the Virgellina may indicate the peak of

the Ordovician graptolite radiation. We suggest that the occurrences of the Anisograptidae, Dichograptacea,

and Diplograptacea mark three graptolite radiation stages. Extended ranges of Dichograptacean species may

coincide with the peak of the radiation in the Darriwilian. The appearances of Anisograptidae, Dichograptacea

and Diplograptacea (of the Virgellina) principally reflect astogenetic pattern changes, which might have

accelerated graptolite macroevolution. The three radiation stages may be seen in geological range charts at

family, subfamily (Figure 1) and generic ranks (Figure 2a,b,c). Cooper et al. (2004) demonstrated their

graptolite species diversity pattern based on the data from Australia, Baltica and Avalonia. The peak of the

radiation from Australia is generally coincident with that of our dichograptid fauna, while the peak of Baltica

and Avalonia may be coincident with that of our diplograptid fauna. However, we employ only a graptolite

Copyright # 2006 John Wiley & Sons, Ltd. Geo. J. 41: 289–301 (2006)

DOI: 10.1002/gj

290 chen. x, zhang. y.-d and fan. j.-x

genus-level diversity analysis in the present study since the graptolite species number varies with the studies and

the authors.

2. EVOLUTIONARY RADIATION OF THE ANISOGRAPTID FAUNA: THE BEGINNING OF THE

ORDOVICIAN GRAPTOLITE RADIATION

Most palaeontologists define faunal radiation as reflecting the total diversity increase in a fossil group. While

increases in total diversity may indicate different stages in a radiation, the study of such a radiation is not restricted

to analysis of the total diversity increase. Webby (2004) defined three aspects of a radiation: (1) Taxonomic

diversity; (2) ecologic diversity; and (3) disparity or morphological diversity. As mentioned above, evolution of the

Cambrian benthic dendroids to Ordovician planktonic anisograptids indicates a graptolite life strategy change from

a limited benthic niche on shallow-water sea floors to a widespread planktonic water niche. The planktonic life

mode provided graptolites with the potential to occupy both the surface water layer and deep-water slope and basin

ecological regions. The total taxonomic diversity increase from the Cambrian dendroids to the early Ordovician

anisograptids was limited. However, the ecological space change associated with this faunal replacement was

considerable. The disparity change in this replacement was also considerable since the diversity in order rank

increased. Fortey and Cooper (1986) suggested that the anisograptids may be a Suborder. Thus, we suggest that the

earliest Ordovician should be considered as the beginning of the Ordovician graptolite radiation. It is interesting

that the graptolite disparity increase is similar to the diversity increases of some other fossil groups. Paris et al.

(2004) suggested that there was an initial major radiation of chitinozoans, which were similar to graptoloids in life

mode. The Tremadocian was also an important interval of faunal radiation in other fossil groups. Hu and Spjeldnaes

(1989) reported that the earliest bryozoan is found in Tremadocian strata (Fengshiang Formation) at Yichang,

China. Webby (2004) reported the first occurrence of tabulate corals also from the Tremadocian Stage.

Since Cooper and Fortey (1982, 1983) indicated that the isograptid type is the primitive mode of development in

graptolites, the ‘simple to complex’ progressive evolution of proximal development proposed by Bulman (1932,

Figure 1. A geological range chart of the families and parts of subfamilies of the Graptoloidea (modified from Fortey and Cooper 1986, fig. 11).Three new Stage names, Floian, Sandbian and Katian have been proposed by Bergstrom et al. (in press) very recently. I, II, and III indicate three

graptolite radiation events.


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ordovician graptolite evolutionary radiation 291

1a 1b 1c 1d 2a 2b 2c 3a 3b 4a 4b 4c 5a 5b 5c 5d 6a 6b 6c

?

?

?

?

Family Anisograptidae

Anisograptus

Chigraptus

Bryograptus

Ancoragraptus

Adelograptus

Paratemnograptus

Psigraptus

Aorograptus

Kiaerograptus

Araneograptus

Temnograptus

Tetragraptus

"Didymograptus"

Hunnegraptus

Cymatograptus

Schizograptus

Holograptus

Baltograptus

Etagraptus

Goniograptus

Kinnegraptus

Dichograptus

Loganograptus

Pseudophyllograptus

Sigmagraptus

Phyllograptus

Lexograptus

Suborder uncertain

Staurograptus

Radiograptus

Rhabdinopora

Triramograptus

Triograptus

Suborder Dichograptina

Superfamily Dichograptacea

Clonograptus

Paradelograptus

Expansograptus

Pendeograptus

Trochograptus

Corymbograptus

Trichograptus

Azygograptus

Zygograptus

Perissograptus

Xiphograptus

Yutagraptus

Didymograptellus

Orthodichograptus

Taxa Lower Middle UpperOrdovician

a)

Figure 2(a,b,c). A geological range chart of the global Ordovician genera (time slices after Webby et al. 2004).


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Jiangnanograptus

Triaenograptus

Pseudotetragraptus

Pseudologanograptus

Pseudodichograptus

Paradidymograptus

Yushanograptus

Jishougraptus

Aulograptus

Allograptus

Anomalograptus

Atopograptus

Holmograptus

Tylograptus

Sinograptus

Didymograptus

Pterograptus

Nicholsonograptus

Pseudazygograptus

Superfamily Glossograptacea

Isograptus

Pseudotrigonograptus

Proncograptus

Pseudisograptus

Cardiograptus

Oncograptus

Exigraptus

Maeandrograptus

Procardiograptus

Skiagraptus

Apiograptus

Cryptograptus

Arienigraptus

Glossograptus

Paraglossograptus

Parisograptus

Bergstroemograptus

Lonchograptus

Kalpinograptus

Corynoides

Keblograptus

Mimograptus

Acrograptus

?Arachiclimacograptus

Suborder Virgellina

Superfamily Diplograptacea

Undulograptus

Eoglyptograptus

Haddingograptus

Proclimacograptus

Taxa Lower Middle UpperOrdovician

Protabrograptus

Wuninograptus

b)

Figure 2. (Continued).


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1936) has been modified. The isograptid type of development is present within anisograptids (Fortey and Cooper

1986; Zhang et al. 2005). If the occurrence of a virgella is the first important criterion for judging graptolite

taxonomy related to radiation, graptolite development type should be considered as the second imporant criterion.

Mitchell (1987 pp. 353–354) pointed out that graptoloid astogeny shows a striking parallelism with von Baer’s Law,

i.e. that primordial astogenetic features were not altered with great frequency but, rather, were highly conserved

during the evolution of graptoloid colonial design. Thus, the graptoloid astogenetic pattern may also have been

reflected within the graptolite radiation process.

This study provides a global graptolite range chart, illustrating the occurrences of graptolite genera through the

Ordovician (Figure 2a,b,c). This current range chart is based on new data from Australia (VandenBerg and Cooper

1992), China (Chen et al. 2000a,b, 2005; Chen and Bergstrom 1995; Mu et al. 2002; Zhang and Erdtmann 2004;

Taxa Lower n Middle UpperOrdovician


Diplograptus

Pseudamplexograptus

Amplexograptus

Urbanekograptus

Reteograptus

Hustedograptus

Pseudoclimacograptus

Normalograptus

Dicaulograptus

Gymnograptus

Dicellograptus

Orthograptus

Abrograptus

Dinemagraptus

Parabrograptus

Jiangshanites

Amphigraptus

Nemagraptus

Dicranograptus

Climacograptus

Hallograptus

Leptograptus

Syndyograptus

Geniculograptus

Lasiograptus

Neurograptus

Rectograptus

Phormograptus

Pipiograptus

Diplacanthograptus

Plegmatograptus

OrthoretiolitesPeiragraptus

Nymphograptus

Archiretiograptus

Pleurograptus

AnticostiaAppendispinograptus

Sinoretiograptus

Orthoretiograptus

Yinograptus

Archniograptus

Yangzigraptus

Paraplegmatograptus

Pseudoreteograptus

Tangyagraptus

Paraorthograptus

Diceratograptus

Sunigraptus

Neodiplograptus

Sudburigraptus

c)

Figure 2. (Continued).


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Fortey et al. 2005), the Baltic region (Cooper and Lindholm 1990), British Isles (Williams 1982a,b, 1994, 1995;

Williams et al. 2003; Zalasiewicz and Tunnicliff 1994; Toghill 1970), Bohemia (Boucek 1973) and North America

(Williams and Stevens 1987, 1988, 1991; Finney et al. 1999). The left hand of the range chart is based on time slices

used by Webby (2004).

3. RADIATION OF DICHOGRAPTID FAUNA

Based on new data arising from IGCP project 410, Webby (2004) revised the Ordovician part of the faunal diversity

distribution diagram constructed by Sepkoski (1984) for the Cambrian, Palaeozoic and more recent faunal diversity

distributions. The revised pattern of the Ordovician radiation in all fossil groups (Webby 2004, figure 1B, figure 4b

in the present paper) indicates that the radiation was not a simple-increase process but a stepwise event, including

two major radiations. This pattern may correspond to the distribution of the three graptolite evolutionary radiation

stages, that agree with the three graptolite faunal originations, i.e. the Anisograptid, Dichograptid and Diplograptid

faunas suggested by Bulman (1970) (Figure 3, 4a,b). The last of these was also defined as the Dicranograptidae-

Diplograptidea-Orthograptidae fauna (DDO fauna) by Melchin and Mitchell (1991). Mu (1974) suggested five

Ordovician faunas, i.e. Anisograptid, Didymograptid, Dicellograptid, Axonocrypta and Diplograptid faunas. From

the view of geological distribution, Mu (1974) suggested three Ordovician graptolite faunas, i.e. Anisograptid,

Didymograptid, and Dicellograptid faunas. However, these revised definitions in reality show little significant

difference from Bulman’s (1970) analysis. Thus, the three faunas of Bulman (1970) are followed in the present

study.

The Dichograptacea total 75 genera. Among them, 27 (36%) originate in the Floian Stage (upper stage of the

Lower Ordovician), 21 (28%) from the un-named Stage (early Middle Ordovician). These two groups, comprise

64% of the genera within the Dichograptacea, and point to the dichograptid faunal radiation, i.e. the second

Ordovician graptolite radiation. The second radiation interval begins from the T. fruticosus Biozone (2b,

corresponding to A. filiformis and D. eobifidus biozones in the Yangtze region) and ranges to the H. teretiusculus

Biozone (4c, top of the Darriwilian). Thus, the long duration of the Dichograptid faunal radiation overlaps with the

succeeding one, the Diplograptid faunal radiation (Figure 2c).

Radiations of other Ordovician marine organisms may not completely agree with those of the graptolites. Webby

(2004) indicated the first radiation peak of Ordovician marine organisms to be at the beginning of the third

0

5

10

15

20

25

30

35

40

45

Ordovician

Upper OrdovicianMiddle OrdovicianLower Ordovician


Anisograptidfauna

Dichograptidfauna

Diplograptidfauna

I II III

Num

ber

of g

ener

a

Figure 3. Three Ordovician graptolite evolutionary radiation stages (time slices after Webby et al. 2004).


DOI: 10.1002/gj


un-named Stage or early Darriwilian. Among the fossil groups, the total specific diversity of the chitinozoans agrees

with that of the graptolites with its first radiation in the late Early Ordovician (Paris et al. 2004). The record of the

total diversity of conodonts is not complete. However, the increase of the specific total diversity indicates that its

first radiation occurred during the late Early Ordovician (Albanesi and Bergstrom 2004). The generic diversity of

the Ibex trilobite fauna decreased after the Early Ordovician while that of the Whiterock trilobite fauna diversity

increased simultaneously, suggesting the replacement of one trilobite fauna by another (Adrain et al. 2004). The

Ordovician brachiopods include mainly three groups, i.e. the linguliformeans, the craniiformeans and the

rhynchonelliformeans. Their total diversities gradually increased from the Early Ordovician to the early Middle

Ordovician (Harper et al. 2004). The normalized diversity of the Ordovician nautiloids reached its first generic

radiation peak during the late Early Ordovician (Frey et al. 2004). Based on the total diversity of the five Ordovician

gastropod groups (selenimorpha, trochomorpha, mimospirina, euomphalomorpha and macluritoidea), a gradual

increase through the Early Ordovician is evident (Fryda and Rohr 2004). Similar gradual increases of the total

diversities are showed by the crinozoans (Sprinkle and Guensburg 2004), acritarchs (Servais et al. 2004) and the

1,000

800

600

400

200

0

200400 0600

Apoorly preserved families

Cm

Pz

Md

V C O S D C P J KTR T

Num

ber

of fa

mili

es

2,000

1,600

1,200

800

400

0M U T Ar Ln L C As Ly LvW

PaleozoicEF

Modern EFB

Geological time(Ma)

Cambrian EF

I

II

III

Time interval

CambrianLower Middle Upper

Silurian

1a-d

3a-b 4a-c 5a-d

Ordovician

510 489 472 460.5 443 419 Ma

2a-c

6a-c

Num

ber

of g

ener

a

Figure 4. A. A comparision of the diversity of the Phanerozoic margine organisms (Sepkoski 1984, fig. 1) and B. the diversity of the Palaeozoicmarine organisms (Webby 2004, fig. 1.1). Cm, Pz and Md indicate the Cambrian, Paleozoic and Modern faunas. EF means evolutionary fauna.


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Figure 5. Comparison of the generic and species diversity curves of Ordovician major fossil groups. The graptolte trajectory is based on presentstudy, while the rest are based on the results presented by a variety of authors inWebby et al. (2004), see the text for details. The asterisk indicates

a normalized diversity curve for the fossil group.


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Baltic ostracodes (Braddy et al. 2004), as well as by the increase of the normalized diversities of the bryozoans

(Taylor and Ernst 2004), sponges (Carrera and Rigby 2004), bivalves (Cope 2004) and receptaculitids (Nitecki et al.

2004). All of these fossil groups showed small radiation peaks during the late Early Ordovician (Figure 5).

Therefore, Ordovician fossil groups diversified at different time intervals. Although some of these began during

the time interval of the dichograptid fauna, most were delayed by comparison with the dichograptids (Figure 2b,c).

Very recently, Zhan et al. (2004) published an account of the Ordovician brachiopod radiation based on data from

South China, in particular from the Yangtze region. The brachiopod radiation in South China began from the

Tetragraptus approximatus Biozone and reached its peak at the Didymograptellus eobifidus Biozone at generic to

order ranks. It appears that the brachiopod radiation of South China is coeval with the dichograptid faunal radiation.

4. RADIATION OF THE DIPLOGRAPTID FAUNA—PEAK OF THE ORDOVICIAN GRAPTOLITE

RADIATION

The concept of the diplograptid fauna, as relates to graptoloid systematic classification, is not agreed upon among

some Chinese graptolite workers (Mu et al. 2002). The faunas listed in this paper are based mainly on the

Diplograptacea Lapworth 1873 emend. Mitchell 1987. In most cases, detailed structural and developmental

similarities in early astogeny among graptoloids are homologies (Mitchell 1987). Thus, the dicellograptid fauna

proposed by Mu (1974) should be included within the current diplograptid fauna and the graptoloids of the

Axonocrypta Mu and Zhan, 1966 should be included within the current dichograptid fauna. Focussing on the

criterion of using the astogentic pattern as a primitive character leads to an entirely different classification of the

graptoloids. Thus, using different interpretations gives rise to different explanations of the temporal distributions of

the graptoloids.

Radiation of the diplograptid fauna began shortly after that of the dichograptids. Immediately following the peak

of the dichograptid radiation, the diplograptid fauna radiation began (Figure 2, 4b). This radiation includes a peak in

total diversity of all the Ordovician graptoloids in the Nemagraptus gracilis Biozone (5a, Figure 2b,c). The

radiation extended through the Sandbian Stage and the Katian Stage and then was interrupted by a major extinction

beginning in the early Hirnantian (Chen et al. 2005). Based on data from the Yangtze region, China, we have

defined a Katian graptolite radiation (Chen et al. 2004; Chen et al. 2005) during the Dicellograptus complexus

Biozone to Tangyagraptus typicus Subzone of the late Katian. This is essentially an extension of the diplograptid

faunal radiation. High diversity of the late Katian graptolites in the Yangtze region was due to the presence of a

partly isolated marine environment, where many endemic genera and species arose (Chen et al. 2003).

The duration of the diplograptid radiation is different from that of the dichograptids, and is consistent with the

second Ordovician radiation of many other marine organisms (Figure 4b). The total species diversity of the

chitinozoans reached its second radiation peak in the late Darriwilian, completely coinciding with that of

the graptolites. It was also the maximum diversity of the chitinozoans during the Ordovician (Paris et al. 2004). The

Whiterock trilobite fauna first reached its radiation peak in the late Darriwilian and then extended to the end of

the Ordovician (Adrain et al. 2004). The brachiopods reached their radiation peak in Darriwilian and extended

to the Katian, followed by a rapid decrease during the late Katian to Hirnantian (Harper et al. 2004). The normalized

diversity of the Ordovician nautiloids showed two peaks in late Darriwilian and the late Katian. However, the

Ordovician nautiloid diversity change was modest (Frey et al. 2004). The five groups of gastropods bear two

diversity peaks similar to those of the nautiloids (Fryda and Rohr 2004). The bivalves reached their diversity peak in

the mid Darriwilian, and continued more or less until the Hirnantian, at which time the diversity plunged (Cope

2004). The diversity changes of the Baltic ostracods (Braddy et al. 2004), stromatoporoids (Webby 2004),

bryozoans (Taylor and Ernst 2004) and sponges (Carrera and Rigby 2004) are different from those of the other fossil

groups. They reached their diversity peaks during the Katian, and in the Hirnantian, diversity dropped sharply. The

crinozoans (Sprinkle and Guensbury 2004) and the receptaculitids (Nitecki et al. 2004) reached their diversity

peaks during early Katian, but decrease rapidly during the Hirnantian (Figure 5).


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In summary, the second radiation of non-graptolite Ordovician organisms agrees with that of the diplograptid

radiation as indicated by Webby in his diagram (2004, Figure 4b in the present paper).

5. DISTRIBUTION OF THE ORDOVICIAN GRAPTOLITE RADIATION IN SOUTH CHINA

Sepkoski (1981, 1991), Sepkoski and Sheehan (1983) and Sepkoski and Miller (1985) summarized the global

historical diversity distribution of the trilobites, brachiopods and molluscs, showing a seaward movement pattern in

their evolutionary faunal replacement, which indicates a seaward direction, i.e. replacement from inner shelf to

outer shelf. However, graptolites as a marine planktonic organism did not follow this pattern. Cooper (1999)

proposed that the earliest planktic forms occurred in the continental-slope belt region and were likely the ancestors

of later graptoloids. Later forms occupied a wider range of habitats, expanding into both the shelf and oceanic

zones. Distribution and expansion of the Ordovician graptolites in South China may provide a good example.

The earliest anisograptids, such as Staurograptus and Rhabdinopora, may be seen in Newfoundland, on the eastern

slope belt of the Laurentian continent (Fortey et al. 1982, Erdtmann 1988), Dayangcha, Jilin, east marginal belt of the

North China Block (Chen JY et al. 1985), and the JCY (Jiangshan-Changshan-Yushan) area, Jiangnan slope belt in

South China Block (Chen 1985). Later, anisograptids such as Kiaerograptus and Adelograptus penetrated into

epicratonic deposits of the Yangtze platform during the late Tremadocian. The earliest dichograptids from the

Tetragraptus approximatus Biozone occur from Pinnan, Guangxi of the Zhujiang basin (Chen personal data) as well

as the JCY area (Chen et al. 1983) and Sandu, Guizhou (Li and Chen 1962) of the Jiangnan slope belt. The earliest

dichograptids reached the Yangtze platform region in the succeeding Acrograptus filiformis Biozone.

The graptolite distribution pattern changed from deeper-to shallow-water habitat within the Undulograptus

austrodentatus Biozone of the early Darriwilian. A more rapid global sea-level rise occurring during this interval

brought the undulograptid fauna onto the Yangtze Platform (Chen and Yang 1988). Earliest diplograptids such as

Undulograptus sinodentatus (Mu and Lee) derived from exigraptids (Chen and Finney 1985; Fortey et al. 2005). They

appeared almost simultaneously on the Jiangnan slope, Zhujiang basin and Yangtze platform regions (Chen et al.

2001). Diversity of the undulograptid fauna is low in the Yangtze region with only a few species. However, in the JCY

area of the Jiangnan slope belt, the diversity of the undulograptid fauna at species level is much higher (Zhang 1993).

ACKNOWLEDGEMENTS

The present paper is a contribution to the IGCP project 503. The authors express their thanks to Professor R. A.

Fortey and Professor A. C. Lenz for reading and polishing the manuscript, to Professor M. J. Melchin and Professor

J. Zalasiewicz for reviewing the manuscript and providing many valuable comments.

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Ordovician graptolite evolutionary radiation: a review

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