Proterogyrinus scheeleri (Romer 1970) ~322 mya, 1.3-2.5 m, Late Mississippian is considered an early reptilomorph, one of several taxa leading toward the Reptilia. Derived from a sister to Ichthyostega, Proterogyrinus was a sister to Eoherpeton. They phylogenetically preceded Seymouria and Silvanerpeton.
Distinct from Ichthyostega, the skull of Proterogyrinus was relatively smaller. The nostrils had moved anteriorly and slightly further from the jawline. The skull was less flattened with steeper cheeks. The jugal separated the maxilla from the quadratojugal. The dentary did not ascend to the coronoid process (the high point) of the lower jaw. The teeth that rimmed the jawline were shorter.
A definite neck appeared enabling the skull to turn in any direction. More than 30 presacral vertebrae produced a longer torso. The dorsal ribs were narrow and no longer overlapped. They permitted rib-breathing by expansion. Lepidotrichia (tail fins) had disappeared.
The elements of the pectoral girdle were no longer coossified. The cleithrum rose high above the ossified portion of the scapula. The elbows were bendable. The ulna was longer than the radius. The carpals remained poorly ossified. The digits were reduced to five with this phalangeal formula: 2, 3, 4, 5, 4.
The ilium had a short dorsal and a long posterior process. The pubis and ischium were not so deep. The knees were bendable. The tarsus was well-ossfied. The digits were reduced to five with this phalangeal formula: 2, 3, 4, 5, 5.
Proterogyrinus was fully capable of terrestrial locomotion, but in considereation of its size and relatively short limbs, it probably preferred to stay underwater.
A reptilomorph family tree is here. A more complete family tree of the Reptilia and their ancestors is here. |
