Morphology and evolution of sesamoid elements in bats (Mammalia, Chiroptera)

Abstract

Sesamoids are skeletal elements found within a tendon or ligament as it passes around a joint or bony prominence. Here we review the distribution of sesamoids in bats, the only mammals capable of powered flight. Our survey included bat species representing most extant families as well as two key Eocene fossil bats in which sesamoids are exquisitely preserved, Onychonycteris finneyi and Icaronycteris index. We identified 46 separate sesamoid elements (or sets of elements) from dissections of selected bat taxa, with no more than 23 of these present in any given species. Among the sesamoids identified in our survey, 12 have not previously been described in bats. We also identified seven sesamoids previously described in the literature that are not present in our sample of species. No sesamoids were found to be exclusive to the fossil taxa in our study; all the sesamoids observed in Onychonycteris and Icaronycteris have apparent homologs among extant species. We mapped the presence/absence of the 46 sesamoids onto a bat phylogeny. Based on these optimizations, we discuss homology issues and evolutionary history of some of the most taxonomically widespread sesamoids. Functional inferences regarding some sesamoids can be made based on what is known about bat musculoskeletal morphology, although further biomechanical studies are required to test the hypotheses proposed here. Sesamoids will continue to be a source of interesting insights about the evolution of bats and their unique locomotor abilities.