Mi. Coates, THE DEVONIAN TETRAPOD ACANTHOSTEGA-GUNNARI JARVIK - POSTCRANIAL ANATOMY, BASAL TETRAPOD INTERRELATIONSHIPS AND PATTERNS OF SKELETAL EVOLUTION, Transactions of the Royal Society of Edinburgh. Earth sciences, 87, 1996, pp. 363-421
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.