The Narial Anatomy of Extinct and Extant Sloths (Xenarthra, Folivora): Osteological Anomalies in the Extant Two-Toed Sloth Choloepus

The skeletal anatomy of the anterior narial region in mammals is complex, comprised of several bony and cartilaginous elements. Because it includes many cartilaginous components, preservation of this area in extant and extinct specimens is often incomplete. This, along with its complexity, means that this region of the cranium is generally understudied, with the exception of humans and a few select mammal species. The present study began with a cranial specimen of the extant Hoffmann's two-toed sloth, Choloepus hoffmanni (Xenarthra, Folivora, Megalonychidae), that possessed several unusual well-preserved narial anomalies. In order to determine whether such features are present in other extant sloths, and if so, how frequently, we examined two- and three-toed sloth specimens from a variety of museum collections. Additionally, we examined museum specimens of extinct sloths known to possess unusual osseous narial structures, with the goal of better understanding the anatomy of the anterior opening of the nasal cavity in all sloths, living and extinct. A diverse series of unusual narial elements were found, including an internasal bone, an ossified anterior nasal septum, an os nariale, an ossified processus lateralis ventralis of the nasal capsule, and an internarial bar of varying composition and completeness. All of these features are present in Choloepus, although they are preserved in museum skulls infrequently, but none were found to be present in the three-toed sloth, Bradypus. An internarial bar, anteriorly elongated ossified nasal septum, and ossified processus lateralis ventralis were observed in several species of extinct mylodontid and megatheriid sloths, and an os nariale was recorded in the extant vermilinguan anteaters Cyclopes and Tamandua. It is not known how much of the variation we document in these features is genuine and how much is due to preservation issues. Given such uncertainty, it is difficult to assess the phylogenetic or functional significance of these features. We believe, however, that this is an area of the skull in Pilosa (sloths and anteaters) that merits further study.

Automated summary

The skeletal anatomy of the anterior narial region in mammals is complex, and preservation of this area in extant and extinct specimens is often incomplete. A study found a diverse series of unusual narial elements in sloths, including internasal bone, ossified nasal septum, and ossified processus lateralis ventralis, but the significance of these features is still unclear due to preservation issues. Further research is needed to fully understand the variation and potential significance of these features in the skull of sloths and anteaters.