Reconstruction of the cranial musculature and masticatory function of the Pleistocene panamerican ground sloth Eremotheriumlaurillardi (Mammalia, Xenarthra, Megatheriidae)

Abstract

Cranial musculature, dental function and mandibular movement patterns in Eremotherium laurillardi were reconstructed from the examination of crania and dentitions. Size, shape and pattern of muscle divisions were reconstructed from the examination of bony rugosities indicating muscle attachments. Details of masticatory muscle structure and function were based on dissections of the tree sloths Bradypus and Choloepus. Among sloths, masticatory muscles in E. laurillardi demonstrate a different synergist–antagonist pattern, reflecting greater emphasis on mediolateral mandibular movements. Eight cranial character complexes (anterior facial, zygomatic arch, superficial masseter, deep masseter–zygomaticomandibularis, pterygoid, temporal, occipital and occlusal) determined by interrelated contributions of each component made to group functions were identified. An elongate anterior face and predental spout in E. laurillardi allowed protrusion of a long narrow tongue at small degrees of gape, reflecting a probably ancestral xenarthran condition. Gape minimisation, in conjunction with the mediolaterally directed masticatory stroke in E. laurillardi, was a unique solution to increase masticatory efficiency by permitting molariform tooth shearing surfaces to remain in or near occlusion for a greater percentage of each chewing cycle.

Keywords:

• Xenarthra
• Megatheriidae
• Eremotherium
• functional morphology
• mastication
• cranial reconstruction

Acknowledgements

We thank Chris McGowan for access to the Eremotherium specimens in the collection of the Royal Ontario Museum, and Fred Grady for access to E. rusconi ( = laurillardi) in the collection of the US National Museum of Natural History. Larry Martin, Dave Frailey and Greg McDonald were kind enough to read and comment on an earlier version of the manuscript and we appreciate their suggestions for improvements. Barbara Ball assisted with the graphics. This study is dedicated to Amber and Hou.