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ORDOVICIAN PERIOD

488-444 million years ago

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fish eating

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).