Sloth cranial osteology

41 p. : ill. ; 26 cm.Includes bibliographical references (p. 36-38).Fossil and Recent sloths show great diversity in the structure of the skulls and dentition. Many of the characters seen in adult sloths are growth related, or depend on the mechanical relationships of skull elements. The teeth in sloths are of persistent growth and erupt as evenly spaced simple cones. These teeth acquire 'cuspid' occlusal surfaces with both growth and wear. In Choloepus the anterior teeth are caniniform; in adults they are separated from the cheek teeth by a diastema which develops in juveniles with rapid growth of the most anterior aprt of the maxilla. The pattern of the tooth 'cusps' differs between Choloepus and Bradypus. In Choloepus the teeth alternate; in Bradypus they are more directly apposed. In both sloths the biting and chewing functions are separated, the mandible is positioned more anteriorly in the glenoid fossa for biting with the caniniform teeth (Choloepus) or anterior chisel-shaped teeth (Bradypus), and moves posteriorly into position for chewing. This is analagous to the shift in mandibular position in rodents. Choloepus and some megalonychid ground sloths resemble carnivorans in that the cranio-mandibular joint (CMJ) is close to the occlusal plane of the cheek teeth. In Bradypus a raised CMJ results in an improvement in the mechanical advantage of the masseter and medial pterygoid muscles and changes the path of mandibular movement, emphasizing forward motion. This is also true of herbivores, where it is beneficial to optimize the mechanical advantage of the masseter and medial pterygoid muscles. Analysis of both the pattern of wear facets and the muscles shows that jaw movement in the power-stroke in anteromedially directed. In sloths, the retention or loss of elongate anterior teeth and the ramifications that follow from these changes can be regarded as the most important factors in explaining the differences seen in cranial structure between Choloepus and some megalonychids and Bradypus and some megatheriids"--P. [1]