THE DEVONIAN TETRAPOD ACANTHOSTEGA-GUNNARI JARVIK - POSTCRANIAL ANATOMY, BASAL TETRAPOD INTERRELATIONSHIPS AND PATTERNS OF SKELETAL EVOLUTION

The postcranial skeleton of Acanthostega gunnari from the Famennian of East Greenland displays a unique, transitional, mixture of features c onventionally associated with fish- and tetrapod-like morphologies. Th e 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 prolifer ation and, transiently, neural radials supporting unbranched and unseg mented lepidotrichia. Sacral and post-sacral ribs reiterate uncinate c ervical and anterior thoracic rib morphologies: a simple distal flange supplies a broad surface for iliac attachment. The octodactylous fore limb and hindlimb each articulate with an unsutured, foraminate endosk eletal girdle. A broad-bladed femoral shaft with extreme anterior tors ion and associated flattened epipodials indicates a paddle-like hindli mb function. Phylogenetic analysis places Acanthostega as the sister-g roup of Ichthyostega plus all more advanced tetrapods. Tulerpeton appe ars to be a basal stem-amniote plesion, tying the amphibian-amniote sp lit to the uppermost Devonian. Caerorhachis may represent a more deriv ed stem-amniote plesion. Postcranial evolutionary trends spanning the taxa traditionally associated with the fish-tetrapod transition are di scussed in detail. Comparison between axial skeletons of primitive tet rapods suggests that plesiomorphic fish-like morphologies were re-patt erned in a cranio-caudal direction with the emergence of tetrapod vert ebral regionalisation. The evolution of digited limbs lags behind the initial enlargement of endoskeletal girdles, whereas digit evolution p recedes the elaboration of complex carpal and tarsal articulations. Pe ntadactylous limbs appear to have stabilised independently in amniote and amphibian lineages; the colosteid Greererpeton has a pentadactylou s manus, indicating that basal amphibian forelimbs may not be restrict ed to patterns of four digits or less.