THE SKELETAL SYSTEM
The early gnathostomes
evolved cartilaginous structures in their skulls which higher tetrapods still
retain as parts of the ethmoid, sphenoid, temporal, and occipital bones. Pectoral and pelvic girdles evolved and the
precursors of vertebrae were composed of bone.
The early gnathostomes evolved cartilage to form the orbital region, nasal
capsules, ethmoid region, and an otic capsule around inner ear. Parachordal
cartilage and trabeculae from around notochord contribute in development of
braincase (Romer, p. 192). In humans, the chondrocranium contributes to the
occipital bone (supraoccipital, exoccipital, and basioccipital regions),
ethmoid bone (mesethmoid and turbinals), sphenoid bone (prespheniod,
orbitosphenoid, and basispheniod regions), and temporal bone (petrosal and
mastoid process). Cartilaginous fish possess a number of cartilaginous
regions of the braincase, including an otic capsule around the ear (Romer, p.
192). In cartilaginous fish, the occipital region of the skull is
cartilaginous but contains a formamen magnum and occipital condyle (Romer, p.
192), Placoderms possessed a cartilaginous floor of their braincase which
included a parasphenoid component (Carroll, p. 48, 65). Cartilaginous fish
possess also possess a cartilaginous floor of the braincase and a
cartilaginous ethmoid region as well (Romer, p. 192). Acanthodians possessed
homologs of the parasphenoid and basisphenoid (Carroll, p. 87). Since the
early gnathostomes, cartilage has existed in the nasal and orbital region
(Romer, p. 192).
In placoderms, the neural
arches were ossified (Carroll, p. 47).
Placoderms were the first fish to evolve pectoral and pelvic
girdles. The pectoral girdle included
a scapulocoracoid element (Kardong, 2002, p. 323).