359-299 million years ago

paleothyris skull

amphibian and reptile skull


Ancestral amniotes increased the sizes of the frontal and parietal bones and the components of the temporal and occipital regions were modified.

In the first reptiles, the frontal bones had become larger and composed part of the orbit. The parietal also became larger (Carroll, p. 194). The semicircular canals were enclosed by the supraoccital, anterior prootic, and posterior opisthotic regions of the petrosal (Carroll). The derivatives of the ancestral palatoquadrate were reduced to a small epipterygoid bone and the quadrate (Carroll). In amniotes, cranial nerves IX, X, and XI exit the skull through jugular foramen (which is called posterior lacerate in reptiles) (Carroll, p. 403).In ancestral amniotes, the postorbital, tabular, and supratemporal bones were smaller and no longer visible at the top of the skull. The supraoccipital expanded to join these bones to the exoccipitals and otic capsule (Carroll). Amniotes lost the labyrinthine folds typical of the teeth of primitive amphibians (although some of the earliest amniotes still possessed these folds; (Carroll, p. 195). Amniotes evolved teeth equivalent to canines and lost the palatal fangs present in early amphibians (Carroll).