The Devonian tetrapodAcanthostega gunnariJarvik: postcranial anatomy, basal tetrapod interrelationships and patterns of skeletal evolution

Abstract

The postcranial skeleton of Acanthostega gunnari from the Famennian of East Greenland displays a unique, transitional, mixture of features conventionally associated with fishand 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 stemamniote 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.

On the squamation of Australerpeton cosgriffi Barberena, a temnospondyl amphibian from the Upper Permian of Brazil

“…No amphibian known so far shows dermal denticles (odontodes) in the squamation. Amphibians and higher vertebrates apparently lost the capacity, seen in fishes, of the trunk neural crest cells to make teeth and odontodes; only the cranial neural crest presumably expresses itself forming oral teeth and, in certain groups like the urodels and some temnospondils, tooth-bearing pharyngeal plates (Graveson et al 1997, Coates 1996. Consequently, the dermal bones and squamation of tetrapods are made up exclusively of bone tissue.…”

Evolution of vertebrate appendicular structures: Insight from genetic and palaeontological data

“…distal endoskeleton (distal to wrist bones) in the pectoral appendage of sarcopterygian fish (Tiktaalik & Panderichthys), there is no direct morphological relationship among these distal bones and digital elements of primitive tetrapod Acanthostega (Coates, 1996). If the digital elements of modern tetrapods are evolved from distal radial bones of tetrapodomorph fish, then this evolutionary trajectory must have entailed considerable morphological restructuring and developmental repatterning (Figs.…”

“…1 and 3). For instance, the Devonian tetrapods were polydactylous (6-8 digits), the elements of carpus/tarsus are patterned oddly, whereas the metacarpals/tarsals and phalanges are morphologically weakly differentiated (Coates and Clack, 1990;Coates, 1996). The transformation from five distal radials of Tiktaalik pectoral fin to modern autopodial elements of tetrapods in late and post-Devonian times, might have involved the origin of additional endoskeleton elements and joints distal to the intermedium and ulnare and proximal to five radials of Tiktaalik, as well as extensive multiplication of radial or digital elements and joints, which should have occurred to evolve an intermediate polydactylous limb condition during the late-Devonian (exemplified by Acanthostega).…”

The pectoral fin of Panderichthys and the origin of digits

“…1a-c). In contrast to early tetrapods, in which the limb projects at an angle from the body wall 11 , the fin of Panderichthys is oriented anteroposteriorly in line with the body axis. In distal cross section (Fig.…”

“…The axis of the fin comprises two more elements distal to the ulnare, and the distal radials are arranged pinnately around this axis. In contrast, in Panderichthys and tetrapods, the ulna is much longer than the ulnare, the ulnare is the last axial element, and the distal radials/digits are arranged in a transverse fan shape 11,21 (Fig. 3).…”