Description of the stylohyal bone of a giant sloth (Lestodon armatus)

Abstract

The hyoid apparatus in fossil Xenarthrans is rarely preserved. Its largest bone, the stylohyal, is the most frequently found as an isolated element. It is known for some species of Pleistocene ground sloths (Megalonyx jeffersonii, Megatherium americanum, Paramylodon harlani, Nothrotheriops shastensis, Glossotherium robustum and Scelidotherium leptocephalum) but not for the largest mylodontid Lestodon armatus. Here we describe a right stylohyal found in the late Pleistocene megafaunal bonebed of the Arroyo del Vizcaino site, Uruguay, as two perfectly matched pieces that were within a few centimeters of each other. A small notch, possibly a conchoidal flaked perimortem fracture, is observed in the lateral face. The specimen, whose general form is a sinusoid cylinder with the proximal end expanded and compressed, must have belonged to a juvenile individual, since it lacks both the distal and proximal surfaces that articulate with the skull and the epihyal, respectively. Instead, both ends show an irregular surface as those seen in the contact with the growth cartilage. Additionally, its maximum length is 105 mm, larger than the equivalent part of the stylohyal of adult Glossotherium. At the expanded proximal end that articulates with the skull the large fossa found in Glossotherium and Scelidotherium cannot be observed, which could be due to a taxonomic difference, to the stage of growth or both of them. The bone shows marks, several interpreted as caused by trampling and at least one with features as those left by human tools. P. Sebastian Tambusso. Seccion Paleontologia, Facultad de Ciencias, Universidad de la Republica, Igua 4225, 11400 Montevideo, Uruguay pasebita@gmail.com H. Gregory McDonald. Museum Management Program, National Park Service, 1201 Oakridge Drive, Fort Collins, Colorado 80521 USA greg_mcdonald@nps.gov Richard A. Farina. Seccion Paleontologia, Facultad de Ciencias, Universidad de la Republica, Igua 4225, 11400 Montevideo, Uruguay dogor@netgate.com.uy