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ORDOVICIAN PERIOD

488-444 million years ago

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fish chasing trilobites

THE MUSCULAR SYSTEM

Early gnathostomes increased the complexity of their musculature which included homologs of muscles found in terrestrial vertebrates.


In the early gnathostomes, a horizontal septum formed to divide epaxial and hypaxial (dorsal and ventral) musculature along the sides of the body (Romer, p. 282). The eye muscles became standardized (Romer, p. 291), dorsal and ventral fin musculature developed (Romer, p. 292), and the branchiomeric musculature became better developed (Romer, p. 306). Gnathostomes evolved a cucullaris muscle (homologous to the trapezius in tetrapods; the only branchial bar levator which tetrapods retain) (Romer, p. 307). The hyoid arch and its musculature were modified to form the operculum (Romer, p. 309). Gnathostomes developed an adductor mandibulae (which would divide in tetrapods to form the temporalis, masseter, and pterygoid muscles) (Kardong, p. 393; Romer, p. 312) and an epihyoideus (which contributes to mammalian facial musculature including platysma) (Kardong, p. 393).