The last time I taught vertebrate anatomy one student always liked to ask me what my favorite term was on our list that day. Maybe he was just fishing for questions on the impending quiz, but it did challenge me to come up with a good reason for why my favorite shark nerve was the vagus. On the week we covered shark musculature, I didn’t hestitate. My favorite muscle is the cucullaris. At the start I just liked how it sounded, but the cucullaris was also interesting because it was a muscle unique to gnathostomes (jawed fishes). In sharks the cucullaris elevates both the gill arches and the pectoral (“shoulder”) girdle. The pectoral girdle itself is a novel feature of gnathostomes and the cucullaris was one of a number of muscles to evolve in association with the development of the neck and paired fins in gnathostomes. We know this by looking at extant fishes more basal than gnathostomes that lack girdles, such as lampreys. No pectoral girdle. No neck region. No cucullaris.
Why all this talk about my favorite shark muscle? This week an article was published in science demonstrating the presence of the cucullaris muscle in an extinct group of gnathostomes–the placoderms. Placoderms lived more than 35o million years ago and are most notable morphologically for their remarkable head shields and sharpened bony jaws. Like other gnathostomes, they have a pectoral girdle and “neck”, so it is consistent with our current understanding that they too have a cucullaris muscle. However, the function of the cucullaris, inferred from its origin and insertion on fossilized specimens, is quite different than in sharks. The cucullaris apparently depressed the head at the hinge joint between the head and pectoral girdle. Such a joint is not present in extant gnathostomes, but the phylogenetic distribution of traits suggest that it is possible that our distant ancestors possessed head shields, possibly with the cucullaris to depress the head. Humans today still possess homologs of the cucullaris muscle in our neck and shoulder (trapezius and sternocleidomastoid complexes).
K. Trinajstic et al. 2013. Fossil musculature of the most primitive jawed vertebrates. Science. 341: 160-164.