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The Devonian Tetrapod Acanthostega Gunnari Jarvik: Postcranial Anatomy, Basal Tetrapod Interrelationships and Patterns of Skeletal Evolution M

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The Devonian Tetrapod Acanthostega Gunnari Jarvik: Postcranial Anatomy, Basal Tetrapod Interrelationships and Patterns of Skeletal Evolution M Transactions of the Royal Society of Edinburgh: Earth Sciences, 87, 363-421, 1996 The Devonian tetrapod Acanthostega gunnari Jarvik: postcranial anatomy, basal tetrapod interrelationships and patterns of skeletal evolution M. I. Coates ABSTRACT: The postcranial skeleton of Acanthostega gunnari from the Famennian of East Greenland displays a unique, transitional, mixture of features conventionally associated with fish- and tetrapod-like morphologies. The rhachitomous vertebral column has a primitive, barely differentiated atlas-axis complex, encloses an unconstricted notochordal canal, and the weakly ossified neural arches have poorly developed zygapophyses. More derived axial skeletal features include caudal vertebral proliferation and, transiently, neural radials supporting unbranched and unsegmented lepidotrichia. Sacral and post-sacral ribs reiterate uncinate cervical and anterior thoracic rib morphologies: a simple distal flange supplies a broad surface for iliac attachment. The octodactylous forelimb and hindlimb each articulate with an unsutured, foraminate endoskeletal girdle. A broad-bladed femoral shaft with extreme anterior torsion and associated flattened epipodials indicates a paddle-like hindlimb function. Phylogenetic analysis places Acanthostega as the sister- group of Ichthyostega plus all more advanced tetrapods. Tulerpeton appears to be a basal stem- amniote plesion, tying the amphibian-amniote split to the uppermost Devonian. Caerorhachis may represent a more derived stem-amniote plesion. Postcranial evolutionary trends spanning the taxa traditionally associated with the fish-tetrapod transition are discussed in detail. Comparison between axial skeletons of primitive tetrapods suggests that plesiomorphic fish-like morphologies were re-patterned in a cranio-caudal direction with the emergence of tetrapod vertebral regionalisation. The evolution of digited limbs lags behind the initial enlargement of endoskeletal girdles, whereas digit evolution precedes the elaboration of complex carpal and tarsal articulations. Pentadactylous limbs appear to have stabilised independently in amniote and amphibian lineages; the colosteid Greererpeton has a pentadactylous manus, indicating that basal amphibian forelimbs may not be restricted to patterns of four digits or less. KEY WORDS: East Greenland, phylogeny, axial skeleton, girdles, limbs, polydactyly. Acanthostega gunnari, from the Late Fammenian, Upper and Coates (1995). Details of the dermal mosaic surrounding Devonian of Gauss Halve, East Greenland, is probably the the external naris, a full description of the palate and most completely known of the three Devonian tetrapods associated dentition, the choana, and the neurocranial ventral preserved in any substantial detail. The first specimens, two surface are described and discussed in Clack (1994a). The incomplete dermal skulls, were discovered by Save-Soderbergh condition of the stapes and discussions of the potential on Wiman Bjerg, Gauss Halvo, in 1932. However, ill health stapedial function in early tetrapods, are provided in Clack and Save-Soderbergh's untimely death delayed the publication (1989, 1992, 1993). Further details of the hyobranchial of their description for a further 20 years (Jarvik 1952, 1980). skeleton were described in Coates and Clack (1991), and an No further material was collected until 1970, when John attempted reconstruction appears in Clack and Coates (1993). Nicholson discovered numerous specimens while mapping the The neurocranium is now known in exceptional detail; a stratigraphy of Stensio Bjerg, Gauss Halve (Friend et al. preliminary description, including details of the fenestra 1976). Surprisingly, the significance of these finds, relative to ovalis, in Clack (1994b), has been superseded by a more the extreme rarity of any Devonian tetrapods whatsoever, detailed study including a discussion of the evolution of the^ went unrecognised until they were studied in the mid-1980s auditory region in tetrapods (Clack in press). A detailed by Dr Jenny Clack (1988). Nicholson's discoveries then description of the mandible is currently in an advanced state provided the impetus for further field-work, completed during of preparation (Clack & Ahlberg, personal communication). an Anglo-Danish expedition in 1987 (Bendix-Almgreen et al. Previous publications concerning the postcranial anatomy of 1988; Panchen 1988). Acanthostega include Coates and Clack (1990), and Coates Subsequent analyses of the combined Nicholson and Clack (1991, 1993, 1994a,b, 1995), on the origin, early diversification, collections have vastly increased our knowledge of the and developmental evolution of the earliest digited limbs. formerly nebulous Acanthostega, and have yielded a wealth Aspects of the axial skeleton have been discussed in Clack of new data concerning the so-called fish-tetrapod transition. and Coates (1993, 1995), Coates (1994a), and Coates and Clack (1988) revised previous descriptions of the dermal skull Clack (1995); pectoral and pelvic girdle structure in Clack of Acanthostega (Jarvik 1952, 1980); and more recent and Coates (1993, 1995), and Coates and Clack (1995). Early reconstructions can be found in Clack (1992, 1994a, in press), tetrapod palaeoecology, and previous reconstructions of Clack and Coates (1993—this includes an inaccurate sutural Acanthostega appear in Clack and Coates (1993), and Coates pattern on the external surface of the lower jaw), and Clack and Clack (1995). 364 M. I. COATES Until recently, most information about Devonian tetrapods an aquatic animal, occupying the active parts of a vegetation- was based upon Ichthyostega, Acanthostegas more famous choked, meandering fluvial channel system. Published data contemporary, from Gauss Halva localities and Celsius Bjerg, concerning the palaeoenvironments of other Devonian tetra- Ymers 0 (Ymers Island; detailed summaries and maps of the pods are limited, but indicate some diversity. Tulerpeton is original localities are provided in Jarvik 1952 & 1996; the latter known from shallow marine deposits (Lebedev 1984, 1985, reference also contains a valuable historical review of the 1990), while the apparently terrestrially-adapted Hynerpeton various expeditions to East Greenland of relevance to the originates from non-marine floodplain sediments (Daeschler collection of Devonian tetrapods). Ichthyostega was first et al. 1994) rich in faunal and floral remains (Woodrow et al described by Save-Soderbergh in 1932, and subsequently 1995). Concise summaries and discussions of these and further researched by Jarvik (summarised in Jarvik 1980, 1996; in the new data concerning questions of tetrapod origin, terrestrialis- present work Ichthyostegopsis, Save-Soderbergh 1932, known ation, and the changing picture of Upper Devonian terrestrial from an incomplete dermal skull, is considered as probably environments can be found in Shear (1991), Lombard and representing a small Ichthyostega specimen). However, many Sumida (1992), Ahlberg and Milner (1994), Coates and Clack apparently specialised features seemed to preclude it from the (1995), and Daeschler and Shubin (1995). main path of tetrapod evolution (Jarvik 1980). As a conse- The focus of this paper, however, is restricted to the detailed quence, attempts to resolve the interrelationships of tetrapod- description of the postcranial skeletal anatomy of like fishes and fish-like tetrapods tended to shift attention Acanthostega, including the vertebral column, caudal fin, ribs, to the increasing diversity of seemingly less specialised girdles, and limbs. These data are (then) combined with an Carboniferous taxa (e.g. Milner et al. 1986; Long 1990). expanded database developed from that which was assembled Nevertheless, other Devonian tetrapod taxa were already being for the analysis of the Tulerpeton postcranium (Lebedev & discovered in new field localities or found lurking unrecognised Coates 1995), and used to generate a phylogeny incorporating in existing collections. The third Devonian tetrapod known in all of the recently discovered Upper Devonian tetrapods plus significant detail, Tulerpeton curtum (Lebedev 1984), was selected Carboniferous and Permian species. Character choice, collected from the Late Devonian of the Tula region, Pre-Ural as in the Tulerpeton analysis, places greater emphasis on Russia. Although consisting mostly of limb and girdle material, postcranial features, an area which has often been neglected detailed description (Lebedev & Clack 1993; Lebedev & Coates in previous works. The contentious question of what actually 1995) and preliminary phylogenetic analysis already suggests constitutes a tetrapod is re-examined (cf. Lebedev & Coates that it is more derived than either of the well known East 1995), and a 'total group' (Patterson 1993a,b) definition is Greenland taxa (which do not constitute a monophyletic taxon: used throughout the text. contra Lombard & Sumida 1992). Formerly misidentified fragments plus new material from the Upper Frasnian of Scat Craig in Scotland were recognised as tantalising evidence of an 1. Materials and methods earlier and perhaps more primitive tetrapod, Elginerpeton The Acanthostega gunnari material described in this paper (Ahlberg 1991a, 1995, in press). Another fish-like tetrapod, comes from Stensio Bjerg of the Britta Dal Formation of the Ventastega, was discovered in the Upper Devonian of Latvia Remigolepis Group sensu Nicholson and Friend (1976). The (Ahlberg et al. 1994), and this in turn corroborated the Remigolepis Group forms part
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