444-416 million years ago


After the acanthodian lineage diverged from that of bony fish, the skeletal system became more ossified and the bones they evolved provided the basis of the skulls and vertebrae of actinopterygians, sarcopterygians, and all tetrapods.

Although these primitive fish possess frontal and parietal bones and there is even a pineal foramen between the parietal bones (as in primitive tetrapods and amniotes), these bones lack many of the features seen in their homologs in higher vertebrates (Carroll, p.95). The frontal and parietal bones originated small flat plates. The frontal bones do not comprise the orbit nor form a postorbital bar as in humans. In fact, in primitive vertebrates these regions can be filled by several small bones (the prefrontal, postfrontal, and postorbital) which have been lost in the lineage which led to humans. The first frontal bones lacked sinuses and were not completely fused (and some did not even contact each other at the midline of the cranium). The skull roof is flat in pirimitive vertebrates, unlike the domed crania of higher vertebrates.
In bony fish, the area around the otic capsule is ossified with bones such as the prootic, epiotic (ophistotic), sphenotic. These fused to compose the petrosal component of the temporal bone in higher vertebrates (Kardong, 2002, p. 235). Bony fish also possess squamosal and angular bones which will form the squamous portion of the temporal bone (with the angular forming the support for the tympanum). Bony fish also possess the stapes, quadrate, and articular bones which form the mammalian auditory ossicles housed inside the temporal bone, although none of these are located inside the temporal bone in the ancestral condition.
In bony fish, a number of bones exist compose the occipital region of the skull including the supraoccipital, exoccipitals, basioccipital, tabular, and postparietal bones (Kardong, 2002, p. 235). In bony fish, the ethmoid region is ossified (Carroll, p. 87) and the parasphenoid forms ventral surface of sphenethmoid (Carroll, p. 142). In bony fish, the sphenoid region is composed of sphenethmoid, orbitosphenoid, basisphenoid, and pleurosphenoid components (the pleurosphenoid is absent in mammals) (Kardong, 2002, p. 235).

Bony fish developed a bony premaxilla and maxilla to compose the upper jaw (Carroll, p. 95) and the ossification of the palatoquadrate decreases through the groups of amphibians (Carroll). The early bony fish added bone to this region as well and possessed discrete nasal, lacrimal, and jugal (zygomatic) bones (Carroll, p. 95). Although these fish possessed nostrils, the nostrils functioned in smell only and did not open into the pharynx. In actinopterygians and sarcopterygians, dermal bones replaced Meckel's cartilage in the lower jaw to produce angular, surangular, splenial, postsplenial, prearticular, and multiple coronoid bones (Carroll, p. 97). Bony fish also developed a quadratojugal bone (Carroll, p. 95).Bony fish evolved a regular pattern of tooth replacement (Carroll, p. 89).

In actinopterygians, bony centra developed from the bases of neural arches (and intercentra may have resulted from the fusion of ventral arch bases (Romer, p. 181).
Higher bony fish evolved ribs and may even possessed dorsal and ventral sets of ribs. Primitive ribs possess 2 heads which were later modified to form the head and tuberculum (Kardong, 2002, p. 289). Primitively, the capitulum (head) articulated with an intercentrum and the tuberculum with a transverse process of the pleurocentrum (Romer, p. 189). Actinopterygians and sarcopterygians possessed a pectoral girdle composed of a cleithrum, scapulocoracoid, and clavicle, (Kardong, 2002, p. 325; Carroll, p. 145). Thus, the shoulder included elements of both endochondral and dermal bone, as in tetrapods (Carroll, p. 145).