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).