The endocranial cavities of sloths (Xenarthra, Folivora): Insights from the brain endocast, bony labyrinth, and cranial sinuses

Abstract

Folivora (sloths) constitutes a taxonomically rich group comprised of many extinct representatives and a few living forms. The clade also shows remarkable skeletal disparity that has been studied by paleontologists for more than two centuries. Accordingly, the first published information on endocranial morphology in fossil sloths, recovered though natural or reconstructed internal casts or broken specimens, date to the nineteenth century. Today, new computed tomography (CT) and three-dimensional digital reconstruction techniques allow observation in great detail of many endocranial structures, such as the brain endocast and cranial nerve trajectories, as well as the bony labyrinth of the inner ear and patterns of cranial pneumatization. These analyses have been recently applied to modern (Bradypus and Choloepus) and extinct (Catonyx, Glossotherium, and Megatherium) sloth skulls. General patterns include a conservative brain anatomy among sloths, a strong phylogenetic imprint on the bony labyrinth, and the presence of extensive cranial pneumatization in some extinct mylodontids. The present work comprises a synthesis of both historical and recently published data on endocranial structures in Folivora. Further studies of the endocranial cavities in other extinct sloths have the potential to enhance understanding of several aspects of the clade's evolutionary history, including phylogeny and paleobiology.