絶滅哺乳類デスモスチルスの生体力学的姿勢復元

Abstract

The Desmostylus, known to have lateral-type limbs like a reptile, was a large mammal that lived about 15 million years ago. Most of its bones have already been discovered; due to the peculiar shape of its skeleton, however, adequacy of its posture restoration had not been verified, and a paleoecological restoration closely related to the posture had never been attempted. In this study, we constructed a three-dimensional musculo-skeletal model of the Desmostylus based on Inuzuka's restored skeleton, which laterally extending the limbs, and verified the adequacy of the peculiar posture by estimating muscular loads to sustain its posture. The musculo-skeletal model was constructed by referring to data obtained from three-dimensional measurement of the restored skeleton and a whole body model made by Inuzuka. Its body was represented by total of 16 rigid links: 3 links in each leg and 4 in the torso. The rigid link properties, such as mass and center of gravity of each segment, were calculated by representing the body shape as elliptical plates. A total of 49 muscles were modeled, including torso muscles. In order to sustain its body weight efficiently, a nonlinear elastic element, like a ligament, was attached around each joint. Body length of the model was 2.6 meters and its body weight was 1.2 tons. We calculated muscular loads to support the body at posture by varying four parameters defining its posture (height of hip joint; inclination angle of a motion plane composed by scapula, upper and fore limbs; directions of toe tips; and positions of the toe tips). In order to maintain the posture with limbs extending laterally, the elastic element at each joint became very important to reduce the muscular loads, instead of the structural support of the posture by bone. Therefore, we determined the posture so as to minimize the change of supporting moment and to maximize the step length. Constant moment can be easily exerted by passive elements. The results were as follows: 1) The calculated posture of Desmostylus is 0.45 meter hip height, 1.06 meters lateral distance between right and left forelimbs, and 0.7meter for that of hindlimbs; 2) muscle cross-sectional area to support the body weight exceeded its permissible leg thickness, therefore, abdomen should touched the ground during daily locomotion; 3) it is more feasible to extend its forelimbs forward, and hindlimbs backward, and the step length is 0.65 meter; 4) the hindlimbs were able to land at a different position from that of the forelimbs; 5) hands and feet were adapted to scratch backward if they were positioned perpendicular to the ground; 6) by inclining the neck downward about 20 degrees from the estimated posture, it could place its mandible parallel to the ground; 7) when in such a low posture, the total muscular load may be comparable with the load of a rat-type posture. We concluded that the lateral posture of Desmostylus touching its abdomen to the ground was adequate and adapted the mammal to live on tidelands.