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Russian Journal of Theriology

Peculiarities of the hind limb musculature in monotremes: an anatomical description and functional approach // Russian J. Theriol. Vol.1. No.1: 1-36 [in English].

ABSTRACT. A thorough description of the hind limb and pelvic muscles of two species of the monotremes is given. The homology of some muscles is discussed. During terrestrial locomotion the platypus shows a movement pattern which is optimal for swimming. Locomotion in the monotremes shows specific mechanical characteristics which differ from those in other tetrapods. Retraction of the femur is one of the main components of propulsive movement in majority of tetrapods; in sphenodontians, crocodiles, squamates it can reach more than 70, and in therians it is 5070. In the monotremes the terrestrial locomotion is executed mainly by pronation of the femur, retraction being no more than 15. A new hypothesis is proposed on the tuber calcanei evolutionary changes in the tetrapods. Enlargement of the tuber calcanei is usually believed to be strictly connected with the increasing role of flexion in the ankle joint. We consider tuber calcanei primary enlargement to be caused by increasing abduction in the ankle joint which helped the hind foot from slipping sideways during symmetrical-diagonal gaits in animals with widely extended extremities. Tuber calcanei enlargement in mammalian ancestors (as well as the analogous structure enlargement in crocodiles) resulted in increasing load on the lateral toes. In the mammalian evolutionary trend to use asymmetrical gaits, the primary function of the tuber calcanei changed gradually in a way that it could help increasing flexion in the ankle joint, transformation came to an end when mammalian extremities became parasagittal in position. There is an undoubted connection between the tuber calcanei and the fifth metatarsal in Monotremata as well as many advanced cynodonts. KEY WORDS. Monotremata, anatomy, musculature, locomotion.