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).