416-359 million years ago
The skeletal differences which separated early amphibians from their rhipidistian ancestors occurred gradually and the earliest amphibians demonstrate intermediate conditions.
In the skull, the frontal bones were larger and fused in the early amphibians. In amphibians, the braincase became more solid and the ancestral joint between the parietals and postparietals was lost (Carroll).Amphibians lost most of the opercular series of bones (Carroll, p. 160) although the earliest amphibians, such as Ichthyostega, retained several of them such as the preopercular and subopercular (Carroll). In early amphibians, ossification in the otic capsule formed the opisthotic and anterior prootic components of the petrosal (Carroll). In anthracosaurs, the braincase and otic capsule were more attached to the dermal bones of the skull. There is also evidence that a tympanum existed in the temporal region in early amphibians (Carroll, p. 174). Amphibians possessed occipital condyle and, after Ichthyostega, the notochord no longer passed through the occipital region to enter the skull (Carroll, p. 167). In anthracosaurs, the braincase and otic capsule became more attached to dermal bones of skull. The length of the snout increased in amphibians (Kemp, 1982, p. 18). The early amphibians decreased the number of bones in the face and the face increased in length (Carroll).
Beginning in early tetrapods, the dentary gradually composed a larger
and larger portion of the lower jaw. In Ichthyostega, the notochord was
unrestricted and processes (zygapohyses) existed between neighboring neural
arches (Carroll, p. 159). In anthracosaurs, the pleurocentrum became larger,
and the intercentrum became smaller (Carroll, p. 167). Fish lack any real
specialization of the atlas/axis like that seen in tetrapods. This modification
of the first two vertebrae occured in tetrapods after the most primitive
members (such as Ichthyostega). (Carroll). Sacral vertebrae were modified
for their attachment to the hip in early amphibians. In the most primitive
known amphibian, Acanthostega, the pelvis is not attached to the vertebral
column and, as a result, the leg could not support the weight of the body.
In later amphibians and reptiles, the vertebrae with which the pelvis
interacted fused to form a solid bone called the sacrum which attached
to the pelvis at the sacroiliac joint.