Early vertebrates andpaleogeographic models

EARLY VERTEBRATES AND PAI,EOGEOGRAPHIC MODELS

GAVIN C. Y O U N G YOUNG G.C. 1995. Early Vertebrates and Palaeogeographic models. [Vertebras primitifs et modules pal~og~ographiques]. GEOBIOS, M.S. n ° 19 : 129-134.

ABSTRACT Studies on the distribution of early vertebrates indicate high endemism in the Early Devonian, and continental connection between West Gondwana and Euramerica in the Frasnian, and between East Gondwana and Asia in the late Famennian. The absence of a northern equivalent to the Siluro-Devonian Malvinokaffric Province is consistent with plate tectonics, but inconsistent with the Expanding Earth hypothesis. A detailed palaeogeography of land and sea areas based on analysis of sealevel, climate, tectonism, and other data provided by the rock record, is necessary for biogeography. Cladistic methods used in the phylogenetic analysis of early vertebrates can be adapted to accommodate various qualitative data sets (e.g., paleobiogeographic, paleoclimatic, paleogeographic information) relevant to the history of continental or terrane dispersion and accretion, permitting true data integration and parsimony analysis of competing palaeogeographic hypotheses. KEY-WORDS

: PALAEOZOIC,

VERTEBRATES,

BIOGEOGRAPHY,

PALAEOGEOGRAPHY,

CLADISTICS.

RESUME

L'~tude de la r~partition des premiers vertebras indique un fort taux d'end~misme au d~vonien inf~rieur, ainsi qu'un liaison continentale entre Gondwana-ouest et Euram~rique au Frasnien, entre Gondwana-est et Asie au Famennien sup~rieur. L'absence, au Siluro-D~vonien, d'un ~quivalent septentrional de la Province Malvino-Caffre est en accord avec la tectonique des plaques, mais pas avec l'hypoth~se d'une Terre en expansion. En biogSographie il est n~cessaire de disposer d'une pal~og~ographie d~taill~e des terres et des aires marines, fond~e sur l'analyse du niveau marin, du climat, de la tectonique et des autres donn~es fournies par les roches. La m~thode cladistique utilis~e pour l'~tude phylog~n~tique des premiers vert~b~rs peut 8tre adapt~e de fa~on h tenir compte d'ensembles varies de donn~es qualitatives, telles que les informations pal~obiog~ographiques, pal~oclimatiques et pal~og~ographiques, ayant trait/~ la d~r~ve ou ~ l'accr~tion des continents et des "terranes". cette m~thode permet une v~ritable integration des donn~es et une analyse parcimonieuse d'hypoth~ses concurrentes en pal~og~ographie. MOTS-CL]~S : PALI~OZOiQUE, VERTI~BRt~S,BIOGt~OGRAPHIE, PALI~OGt~OGRAPHIE,ANALYSE CLADISTIQUE.

INTRODUCTION S t u d i e s on t h e d i s t r i b u t i o n of e a r l y v e r t e b r a t e s , a n d of P a l a e o z o i c fossils generally, m a y i m p a c t on models of p a l a e o g e o g r a p h y at two levels : conc e r n i n g c o n t i n e n t a l r e c o n s t r u c t i o n h y p o t h e s e s as p a r t of m a j o r t h e o r i e s about E a r t h h i s t o r y (e.g., E x p a n d i n g E a r t h ; P l a t e Tectonics), a n d concern i n g m o r e d e t a i l e d analysis of t h e rock record to u n d e r s t a n d t h e i n t e r a c t i o n of sea level, climate, tectonism, a n d o t h e r factors which t o g e t h e r res u l t e d in p a r t i c u l a r global d i s t r i b u t i o n s of t h e div e r s e h a b i t a t s occupied b y p a s t biotas.

CONTINENTAL RECONSTRUCTION HYPOTHESES P l a t e tectonics is u n i f o r m i t a r i a n r e g a r d i n g proportions of the E a r t h ' s surface covered by past c o n t i n e n t s and oceans. Given similar proportions to m o d e r n geography, and a s s u m i n g a "supercontinent" configuration C G o n d w a n a " , "Pangaea") d u r i n g the Palaeozoic, t h e n m o r e t h a n h a l f of the Palaeozoic globe m u s t h a v e b e e n covered b y a m a j o r "proto-Pacific" ocean. U n d e r P l a t e Tectonic, theory, t h e m o d e r n Pacific O c e a n t h u s h a s an ancient history, e x t e n d i n g back to t h e beginning of

130 the Phanerozoic and beyond. The fact that its oldest known ocean floor (Jurassic) is about the same age as t h a t in the much younger Atlantic Ocean (also Jurassic), m u s t be attributed to coincidental subduction of older oceanic sediments in the Pacific (this can be called "Nelson's Paradox", after Gareth Nelson). The hypothesis of an Expanding E a r t h has many fewer supporters amongst earth scientists, but continues to attract the attention of biologists to explain anomalous patterns of distribution (e.g., biogeographic tracks across the Pacific ; Sluys 1994). Resolving geological evidence critical to testing the Expanding E a r t h hypothesis is difficult if both subduction and Earth expansion operated in the past, but evidence of expansion should be accentuated in older rocks. A much smaller earth would have been covered in the early Palaeozoic by the major continental blocks, thereby eliminating the proto-Pacific ocean. Norable differences in Palaeozoic faunal distribution would be expected compared to modern distribution patterns. A major feature of Palaeozoic distribution patterns is the cool-cold w a t e r Malvinokaffric Province, which is well documented for the Late Silurian Early Devonian in South America, southern Africa, and Antarctica (Boucot 1985). This province cannot be attributed to global cooling, because w a r m w a t e r faunas of the same age are also well documented, indicating latitudinal differentiation in climate. There is no evidence in Palaeozoic strata of a cold w a t e r province for the opposite ("northern") pole to the Malvinokaffric Province, implying an oceanic hemisphere (protoPacific ocean) for the Palaeozoic globe, in which the opposite cold polar region was located, a conclusion inconsistent with the expanding earth hypothesis. PAI,AEOGEOGRAPHIC

RECONSTRUCTION Biologists and biogeographers plot distributions on standard continental reconstruction models, but these provide only a partial framework for analysis of real palaeogeography - they indicate only that oceanic areas were presumably largely devoid of emergent land, whilst continental crust m a y or m a y not have been inundated by shallow seas. Habitats m a y be subdivided at the coarsest level into "marine" and "nonmarine" (the latter including terrestrial, freshwater aquatic, etc.), a distinction in the geological past crucial to biogeography. Less accessible to non-geologists are extensive data sets on palaeoclimate, changing

sealevel, and timing of tectonic events, all of which m u s t be t a k e n into account to distinguish past oceans and seaways from emergent land. Giyen the paucity of data, this is the level of palaeogeographic analysis which must be tackled at present.

EARLY VERTEBRATE DISTRIBUTION PATTERNS For fossil fish distributions, the dispersal capacity of nonmarine versus marine taxa or faunas has been much discussed, but a cladistic approach, concerned with pattern rather t h a n process, recognises only two ecological groupings of fishes - continental and oceanic - with assignment to either group based on distribution in relation to phylogeny, and in relation to the distribution pattern of other organisms (Rosen 1974 : 323 ; Young 1987 : 284). The Expanding Earth hypothesis has some appeal to explain widely distributed early vertebrate taxa, but familiar forms are generally the first to be identified in new areas, so the current global data base probably has a systematic bias towards widespread forms, with endemic elements still poorly known. Conventional continental reconstructions for the Palaeozoic generally show a dispersal of small continental blocks from Eurasia (e.g., Blieck & Janvier 1993), combined with supercontinent configuration for the southern continents. This is rather different from modern geography, but some aspects of early vertebrate distributions show surprising conformity with modern geographic relationships between areas. The "Gondwana" pattern between South America and Australasia evident in m a n y modern groups (e.g., Talent 1984) is seen at the beginning of the vertebrate fossil record in the close relationship between Ordovician arandaspid agnathan faunas of these two regions (e.g., Gagnier 1989). Recent discoveries of Devonian fishes in Kazakhstan and Tarim, both interpreted as separate blocks during the Palaeozoic, suggest affinity with South China, and the same applies to the North China block, which contains galeaspid agnathans and sinolepid antiarchs, b u t supposedly had a separate geological history. All these regions show an "Asian" faunal element, which is equally consistent with the modern geography of a "composite" Asia, as with the many Palaeozoic and Mesozoic terranes inferred from the geological evidence of major sutures (e.g., Metcalfe 1990). Michaux (1989) considered it unlikely that novel predictions regarding geological hypotheses might

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arise from biogeographic analysis, but this is not the case for Devonian fishes. During the Early Devonian at least five distinctive vertebrate faunas characterised by endemic taxa can be recognised (Euramerica, Siberia, Tuva, China, and East Gondwana). There were also some widespread groups with higher dispersal ability. By Late Devonian time evidence of these vertebrate faunal provinces is obscured by widespread taxa indicating greater faunal communication between Gondwana, Euramerica and China. Widespread taxa also occur mostly in apparent nonmarine deposits, but some are also known from marine rocks, suggesting dispersal along continental shelves and across shallow marine barriers. The marked change in pattern between the Early and Late Devonian may be attributed to intrinsic (evolutionary) or extrinsic causal factors. Dispersal capabilities of aquatic vertebrates may have increased during the initial gnathostome radiation of the Devonian (Burrett et al. 1990), but a predominantly extrinsic cause (e.g., global change in geography or climate) is required to explain

the similar pattern for marine invertebrate faunas, which also indicate Early Devonian endemism and Late Devonian cosmopolitanism. Current issues in the analysis of Devonian vertebrate biogeography concern the nature and timing of barriers and connections between (a) East Gondwana and eastern Asia ; (b) West Gondwana and Euramerica ; and (c) East and West Gondwana. Brief comments in these aspects follow. EAST GONDWANA AND EASTERN ASIA Invertebrate and other evidence suggests geographic proximity between east Gondwana and China during the Early Palaeozoic (Burrett e t al. 1990), with a highly endemic vertebrate fauna established on the South China block during the Silurian-Early Devonian. Compelling evidence for continental isolation comes from endemic higher taxa like yunnanolepid and sinolepid antiarchs, and galeaspid agnathans. Some indication of faunal affinity between east Gondwana and eastern Asia during the Middle-

132 Late Devonian can be attributed to shallow marine dispersal (e.g., pituriaspid agnathans ; Young 1991 ; species groups of the widespread antiarch Bothriolepis ; Young 1988). However the only representative of the highly endemic Asian fish fauna known to occur outside Asia is the sinolepid antiarch Grenfellaspis from the Lachlan Foldbelt of eastern Australia (Ritchie et al. 1992). This occurrence (latest Devonian ; Fig. 1) may reflect continental rearrangement associated with the Early Carboniferous closure across the Song Ma suture between the South China and Indochina terranes proposed by Metcalfe (1990). Some of the smaller terranes in southeast Asia have also yielded vertebrate faunas (Long 1990, 1993 ; Janvier et al. 1994), but the complexity of this region requires a more rigorous assessment of competing hypotheses of terrane history (see below). WEST GONDWANA AND EURAMERICA A dispersal hypothesis for continental fishes from Gondwana into Euramerica was initially based on disparities between the northern and southern stratigraphic ranges of some key Late Devonian macrovertebrate taxa (Young 1981). Supporting evidence has come from earlier occurrences in East Gondwana of freshwater xenacanth sharks, phyllolepid placoderms, rhizodontid crossopterygians, early tetrapods, etc., and other evidence from independent datasets of various invertebrates (summarised by Young 1987, 1989, 1990). An alternative hypothesis recently proposed by Li et al. (1993) involves Asian migration routes to get Famennian~'tetrapods from Gondwana to Euramerica, because close proximity between West Gondwana and Euramerica is contradicted by palaeomagnetic data, which requires a major equatorial ocean between these landmasses (e.g., Kent & Van Der Voo 1990). Detailed biostratigraphic analysis (Young 1993) highlights a chronological separation of three different vertebrate distribution patterns in the Late Devonian, which makes the alternative Asian migration routes unacceptable (Fig. 1). Although still distinctive during the early Late Devonian (Frasnian Stage), the Euramerican and Gondwanan faunas of Famennian age show a close similarity. Euramerican vertebrate endemism was terminated with the replacement of psammosteid heterostracans by phyllolepid placoderms at or near the FrasnianFamennian stage boundary, but it is only after phyllolepids have disappeared from the vertebrate succession of eastern Australia that the similarities with Asian blocks discussed above indicate faunal exchange between these areas (?latest Famennian-Tournaisian). Phyllolepids are a r e a -

dily recognised placoderm group represented in almost all Frasnian-Early F a m e n n i a n fish assemblages in the Lachlan Foldbelt of eastern Australia, and associated with tetrapod occurrences both here and in the northern hemisphere (east Greenland), but they have never been found in China. The biogeographic/biostratigraphic evidence indicates two separate patterns and different dispersal events during the Late Devonian, the first with Euramerica, and the second with Asia, perhaps due to separate palaeogeographic rearrangements at the margins of Gondwana. Failure to distinguish these different patterns within the "Late Devonian" has caused confusion in assessment of biogeographic data ; for example, Colbath's (1990) analysis of Frasnian phytoplankton is completely consistent with vertebrate evidence for the Frasnian j u s t outlined, r a t h e r than the "minority view" that he suggested. EAST AND WEST GONDWANA The early vertebrate faunas of South America and Australia-Antarctica m a y be contrasted as representing extremes in the faunal differentiation to be expected across the Gondwana supercontinent. The predominance of chondrichthyans in South America (Leli~vre et al. 1993) m a y be a vertebrate equivalent of the cool-water Malvinokaffric invertebrate faunal province of the SiluroDevonian. Although chondrichthyans are well represented in the Aztec fish fauna of Victoria Land, Antarctica (Young 1982 ; Long & Young in press), this fauna is dominated by placoderms, as is typical for East Gondwana, and exemplified by the endemic Middle-Late Devonian wuttagoonaspid-phyllolepid assemblage known throughout eastern and central Australia. Evidence of shallow marine dispersal at this time between East Gondwana and Asia has been noted above, but representatives of the wuttagoonaspid-phyllolepid assemblage have never been found in Asia, so it is assumed this group was constrained by narrow marine barriers. However, phyllolepids occur along the northern Gondwana margin (Turkey, Venezuela) from which they evidently dispersed into Euramerica (see above). ANALYTICAL

METHODS

Analyses of palaeomagnetism, palaeoclimatology, palaeobiogeography all contribute to Palaeozoic palaeogeographic reconstructions. Although organised differently, each data set relies on the same principle of concordance with a general pattern (Young 1990). The extent to which a hypothesis based on one data set explains unrelated patterns in another is a primary means of testing

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Figure 2 - Comparison of diverging and converging cladograms and their supporting evidence. A, biological ctadogram (diverging through time) showing relationships of four taxa (W-Z) supported by three synapomorphies (1-3). More general attributes of taxa (1) historically preceding the less general (3). Data matrix beneath scored for three characters and four taxa (binary coding : 0, absent ; 1, present). B, coalescing area cladogram (converging through time) showing accretion of three terranes (A-C) to a craton (CR), again supported by three special shared attributes (e.g., an overlap assemblage, or other geologic, palaeomagnetic or biogeographic data indicating connection). In this case the less general attributes (1) historically precede the more general (3). For the purposes of analysis, attribute 2 can be inferred to be present in terrane A and the craton, and attribute 3 can be inferred to be present in terranes A and B and the craton, on the evidence of less general attribute 1. Data matrix beneath scored for three attributes and four areas (binary coding : 0, absent ; 1, present). Scores in brackets can be inferred from the distribution of attribute 1. Comparaison de cladogrammes diuergents et conuergents, avec leur argumentation. ,4, cladogramme biologique, divergent au cours du temps, rnontrant les relations de 4 taxa W-Z, gtablies ~ partir des 3 synapomorphies 1-3. Le caract@re gdndralis4 1 prdc~de historiquement le caract@re moins ggngral 3. La matrice de caract~res/taxa est reprdsentge en dessous (eodage binaire : O, absent ; 1, prdsent). B, cladogramme d'aires conuergents au cours du temps, montrant l'accr6tion des trois "terranes" A-C p a r rapport au craton CK, dtabIie & partir des trois attributs partag~s 1-3 (par exempIe, une translation faunique ou tout autre donnge gdologique, palgomagngtique ou bioggographique indiquant une relation). Dans ce cas, c'est le caract@re le moins gdngralisg 1 qui prgc@de historiquement le plus gdn4ralisg 3, Le caract@re 2 peut @tre supposg prdsent sur le "terrane A" et le craton CR, le caract~re 3 peut @tre supposg prdsent sur les "terranes" A-B et le craton parce que le caract~re 1 est moins g6n@raIis@. Matrice caract~res/ "terranes" repr4sentde en.dessous (codage binaire : O, absent ; 1, prdsent). Les caract6res entre crochets sont infdrds.

the hypothesis. The well-established apparent polar wander path representation of palaeomagnetic data facilitates testing against those palaeoclimatic or palaeobiogeographic data providing evidence of palaeolatitude, but is inappropriate for biogeographic and various qualitative geological data sets which provide evidence of connections or barriers between regions. This can be achieved through cladistic representation of hierarchically organised data sets (Young 1986).

Complex hypotheses such as those concerned with terrane dispersal or accretion require rigorous analysis of supporting evidence to resolve the most parsimonious explanation of all available data. Appropriately defined, various geological, geophysical, and biological data used to support hypotheses of fragmentation or fusion history can be organised hierarchically (Young 1990). However, unlike phylogeny reconstruction, where a single common ancestor is assumed, only subsets of geological data, involving discrete episodes of either dispersion or accretion, can be analysed as hierarchical data sets. Terrane fragmentation is equivalent to phylogenetic splitting of biological taxa, and standard algorithms for parsimony analysis may be directly applied to a data matrix (Fig. 2A). For terrane accretion the supporting evidence also forms a hierarchical data set, but with two main differences (Fig. 2B). The less general attributes historically precede the more general, the reverse of the situation in phylogeny reconstruction. In addition, the branching points on the coalescing area cladogram represent identifiable geological provinces, in contrast to the hypothetical common ancestors in phylogeny reconstruction, whose attributes can only be inferred from the distribution of attributes amongst the terminals (known biological taxa). Because the end product of terrane accretion has a composite geological structure which can be investigated, juxtaposition of terranes may eliminate some of the possible historical sequences which led to its formation (Young, in press). This method permits qualitative paleobiogeographic, paleoclimatic, and paleogeographic information to be represented as binary codes in a data matrix, which can then be subjected to parsimony analysis (Fig. 2). In this way, cladistic analysis allows integration of both qualitative and quantitative data in a context emphasising inconsistencies in the evidence, and thereby exposing competing hypotheses to falsification.

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seum, Records. METCALFE I. 1990 - Allochthonous t e r r a n e processes i n s o u t h e a s t Asia. Philosophical Transactions of the Royal Society, Series A, 331 : 625-640. MICHAUX B. 1989 - G e n e r a l i s e d tracks and geology. Systematic Zoology, 38 : 390-98. RITCH1E A., WANG S.T., YOUNG G.C. & ZHANG G.R. 1992 - The Sinolepidae, a family of antiarchs (placoderm fishes) from the D e v o n i a n of South C h i n a a n d e a s t e r n Australia. Records of the Australian Museum, 44 : 319-370.

ROSEN D.E. 1974 - Phylogeny and zoogeography of salmoniform fishes. Bulletin of the American Museum of Natural History, 153 : 265-326. SLUYS R. 1994 - E x p l a n a t i o n s for biogeographic tracks across the Pacific Ocean : a challenge for paleogeography and historical biogeography. Progress in Physical Geography, 18 : 42-58. TALENT J.A. 1984 - A u s t r a l i a n biogeography p a s t and p r e s e n t : d e t e r m i n a n t s and implications, pp. 57-93. In VEEVERS J.J. (ed) : Phanerozoic E a r t h history of Australia. 418 p. Clarendon Press, Oxford. YOUNG G.C. 1981 - Biogeography of D e v o n i a n vertebrates. Alcheringa, 5 : 225-243. YOUNG G.C. 1982 - Devonian sharks from south-eastern A u s t r a l i a a n d Antarctica. Palaeontology, 25 : 817-843. YOUNG G.C. 1986 - Cladistic methods i n Paleozoic cont i n e n t a l reconstruction. Journal of Geology, 94 : 523-537. YOUNG G.C. 1987 - Devonian palaeontological data a n d the Armorica problem. Palaeogeography, Palaeoclimatology, Palaeoecology, 60 : 283-304. YOUNG G.C. 1988 - Antiarchs (placoderm fishes) from the Devonian Aztec Siltstone, s o u t h e r n Victoria Land, Antarctica. Palaeontographica, A, 202 : 1125. YOUNG G.C. 1989 - The Aztec fish f a u n a of s o u t h e r n Victoria Land - evolutionary a n d biogeographic significance. In CRAME J.A., (ed) : Origins a n d Evolution of the Antarctic Biota. Geological Society of London, Special Publication 47 : 43-62. YOUNG G.C. 1990 - D e v o n i a n v e r t e b r a t e d i s t r i b u t i o n p a t t e r n s , and cladistic analysis of palaeogeographic hypotheses. In MCKERROW W.S. & SCOTESE C.R. (eds) : Palaeozoic biogeography a n d palaeogeography. Geological Society of London Memoir, 12 : 243255. YOUNG G.C. 1991 - The first a r m o u r e d a g n a t h a n vert e b r a t e s from the Devonian of Australia. pp. 67-85. In CHANG M.M., LIu Y.H. & ZHANGG.R. (eds) : Early Vertebrates a n d Related Problems of Evolutionary Biology. 514 p, Science Press, Beijing, China. YOUNG G.C. 1993 - Middle Palaeozoic m a c r o v e r t e b r a t e biostratigraphy of e a s t e r n Gondwana. C h a p t e r 9, p. 208-251. In LONG J.A. (ed) : Palaeozoic V e r t e b r a t e Biostratigraphy and Biogeography. 369 p. Belhaven Press, London. YOUNG G.C. in press - Application of cladistics to terr a n e history - p a r s i m o n y analysis of q u a l i t a t i v e geological data. Southeast Asian Journal of Earth

Sciences.

G.C. YOUNG Australian Geological Survey 0rganisation P.O. Box 378 Canberra ACT, 2601 Australia