The Carboniferous amphibian Proterogyrinus Scheelei Romer, and the early evolution of tetrapods

Abstract

Proterogyrinus scheelei Romer, from the Upper Mississippian of Greer, West Virginia, is one of the earliest known members of the am phibian 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 foramenrim 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 autapom orphies 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.