542-488 million years ago
The skeletal muscle of ancestral cephalochordates was organized to form V-shaped myotomes similar to those observed in fish. Homologs of the trapezius muscle also existed.
Myotomes first evolved in cephalochordates and are evident in the following
photos of modern lancelets (Cameron, 2000). Additional muscles in Amphioxus
surround the oral and pharyngeal areas and form an anal sphincter. A pair
of trapezius muscles on the dorsal surface helps to expel water from the
body. (Ruppert, from Harrison, 1997, p. 397-9).