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.
In amphibians, the ribs became longer, reinforcing the lateral sides of the body wall (Carroll). After Acanthostega, sacral ribs specialized to attach to the pelvis, allowing the legs to support the weight of the body (Carroll). The sternum first evolved in tetrapods (Kardong, 2002, p. 291).
In amphibians, the shoulder lost its connection to the skull as a number of ancestral skull bones were lost, such as the supracleithrum, anocleithrum, extrascpaular (medial and lateral), and most of the opercular series (Carroll, p. 160). As a result, the arms could move without simultaneously moving the head which probably improved the function of the head's sensory structures. The glenoid and acetabulum faced laterally rather than posteriorly (Carroll, p. 162) and the scapulocoracoid was composed of two centers of ossification, as opposed to one element in fish (Kardong, 2002, p. 329).
In amphibians, the humerus was modified so that it could be held out from the body, allowing a greater range of movement including rotation (Carroll).
Amphibians possessed five wrist bones, including a pisiform and centralia near radius and distal carpals (Carroll). In anthracosaurs, the number of phalanges (finger bones) of the hand were 2,3,4,5,3 (Carroll). Amniotes possessed 11 wrist bones (Carroll).
In Ichthyostega, the pelvis was composed of three bones (ilium, ischium, and pubis) as would be typical of all later tetrapods. The acetabulum for the head of the femur forms where these 3 bones meet, a pubic symphysis joins the two pubic bones, and (after the most primitive amphibians such as Acanthostega), sacral vertebrae attached to the pelvis to enable the legs to bear the weight of the body (Carroll). In amphibians, the femur was held out from body; allowing a greater range of movement including rotation (Carroll). Amphibians evolved ankle which utilized these bones plus additional tibiale, centralia, and 5 distal tarsal bones in the foot. The primitive amphibian ankle permitted ankle flexion, extension and twisting (Carroll). Primitive amphibians originally had 3 rows of tarsal bones. As amphibians evolved into the first reptiles, the os fibulare and 1-2 os centrale fused to form the calcaneus while the os tibiale, os intercentrum, and a variable number of os centrale fused to form the talus/astragalus (Isidro, 2002; Carroll). The first amphibians possessed more than 5 digits on their hands and feet. Amphibians and other groups may possess a pretarsal bone (called the prehallux) which has been regarded as a vestigial sixth digit (Isidro, 2002).