Morphology and phylogenetic implications of the inclinatores dorsales for the galeomorph sharks (Chondrichthyes: Elasmobranchii)

Elasmobranchs have been studied in anatomical terms for nearly 200 years, but several elements of their anatomy, such as the dorsal fin musculature, have not been completely addressed and still lack detailed descriptions. In this context, the present study investigates the anatomical variation of the muscles inclinatores dorsales across galeomorph sharks, shedding new light on their evolution. We have observed that the inclinatores dorsales have two distinct components, being composed of a profundus and a superficialis component. Additionally, we have uncovered, through an anatomical comparative analysis, that the variation present in these muscles is considerably greater than previously considered, indicating some characteristics never described before, for instance the presence or absence of the inclinatores dorsales at the free rear tip of the first dorsal fin. Moreover, our findings for the inclinatores dorsales are different from their previous interpretation in the context of the hypnosqualean hypothesis, reinforcing the need for a reevaluation of previous morphological characters. Lastly, we discuss our findings in relation to the most recent interrelationships of elasmobranchs.