The Terrestrial Posture of Desmostylians

Abstract

An attempt to reconstruct a skeleton of Paleoparadoxia Reinhart, 1959 (Mammalia, Desmostylia), suggests that desmostylian terrestrial posture deviated from that of typical ungulates much less than has been supposed by other authors. Desmostylians probably had a quadrupedal stance, with the body welt off the ground and the limbs more or less under the body; a strongly arched spine and steeply inclined pelvis; slightly abducted elbows and more strongly abducted knees; and a digitigrade foot posture with an extended but not hyperextended wrist and hyperextended toes, the front toes pointing anterolaterad and the hind toes pointing forward. Most peculiarities of the skeleton have parallels in certain large, slow-moving terrestrial mammals, such as ground sloths and chalicotheres. The desmostylian skeleton was apparently well suited to supporting the body’s weight on the hindquarters, perhaps while the animal clambered slowly over very uneven ground. This most likely occurred while it foraged for marine algae or sea grasses in rocky intertidal areas of the North Pacific shoreline, and while it crossed these areas en route to and from the water. Locomotion in the water probably resembled that of polar bears, with alternate pectoral paddling as the principal means of propulsion and the hind limbs used for steering. Surprisingly, desmostylian- like features of the tibia and ankle also are found in many other primitive ungulates and deserve closer study.