Ichthyostega

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.

Abstract: Proterogyrinus scheelei Romer, from the Upper Mississippian of Greer, West Virginia, is one of the earliest known members of the amphibian infraorder Embolomeri. The primitive, eogyrinid-like skull conforms to Panchen's allometric plot of the British eogyrinids. It has no identifiable autapomorphies, but shares with two other, as yet undescribed genera attributed to the same family, a unique skull table with a raised pineal foramen rim and an acuminate median ridge on the skull table posteriorly, flanked on each side by a deep depression. The kinetic junction extends from the otic notch anteriorly to the posterodorsal corner of the orbit. The braincase, bearing no ossified roof, is composed of separate otic-occipital and sphenethmoid units, the latter bearing no sagittal or parasagittal septa. The vertebrae are similar to those of other embolomeres, except that the pleurocentrum retains a dorsal suture in adults, and the intercentrum, unossified dorsally in adults, is a ventral cresent. The presacral count is 32. The atlas-axis is reptiliomorph. The limbs and girdles are similar to those of Archeria, except that they are slightly stouter. Humerus structure is primitive. Range of limb movement, at least in the pectoral limb, is very restricted. The possession of few autapomorphies indicates that Proterogyrinus is a member of the stem family (Proterogyrinidae) of the infraorder Embolomeri. Although most of the putative autapomorphies of anthracosaurs that were formerly thought to preclude them from reptile ancestry are not considered to be valid, there are no undisputed synapomorphies with reptiles either, making it impossible to support close relationship between the two groups.

Abstract: Well preserved material of the crossopterygian fish Eusthenopteron enables fresh reconstructions and interpretations of its postcranial skeleton to be given. Comparisons throughout with other bony fishes show that it may be primitive in many features. Similarities with early amphibians such as the screw-shaped glenoid, the form of the humerus (on which an attempt to restore the pectoral musculature is based), the dorsal bicipital ribs and the possibility of a sacral attachment, throw much light on the origin of the tetrapod postcranial skeleton, particularly of the cheiropterygium. A functional analysis of the skeleton of Ensthenopteron is attempted, suggesting that it resembled the pike (Esox) in its mode of life and that it may have been capable of short journeys “walking” overland. The possible selective factors stimulating the evolution of such a fish, and further evolution to the tetrapod stage are discussed.