Page:The Osteology of the Reptiles.pdf/23

Rh Nevertheless, there were still so many inherited characters among both the amphibians and reptiles of early Permian times that nothing distinctive of either class can be found in the skeleton, except in the atlas and feet, with a considerable gap in the structure of their vertebrae. In the vertebral column there was a general change among the Temnospondyli from the embolomerous to the rhachitomous type, that is, from the more simply divided centrum of two disks to the tripartite centrum composed of wedges; while all reptiles had acquired a reduced embolomerous form with one disk, the centrum, and one wedge, the intercentrum.

Doubtless all amphibians of Lower Carboniferous times had embolomerous vertebrae, but only a very few of their stock persisted as late as the Permian. In general literature the Amphibia are distinguished from the Reptilia by the possession of two occipital condyles. The earliest amphibians doubtless all had a single occipital condyle, an inheritance from their ancestral fishes—all that we know from the Lower Carboniferous had—of which only one known descendant with that character survived to the Permian. The reptiles, however, retained the single condyle until the beginning of their evolution into mammals, when they too developed a double condyle. We relied, until recently, upon the widely open palate of the Amphibia as a final distinguishing character of their class, but we now know that some, if not all, of the earliest amphibians had a [closed] palate like that of the [earliest] reptiles, but of these none is known at the beginning of Permian times. In other words, a single condyle and a closed palate are more primitive characters of the tetrapods than those we had assumed as characteristic of the Amphibia. We know no amphibians with as many bones in the digits as the early reptiles possessed, and no reptiles with as many bones in the tarsus as the early amphibians had,