Evolutionary and functional morphology of the shoulder region and stay-apparatus in fossil and extant horses (Equidae)

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Abstract
In advanced members of the Equidae, the shoulder joint evolved a unique morphology about 5 Ma ago. Anatomically this diarthrosis is notable for its lack or weak development of ligaments. In fact, the normal role of ligamentous structures in bracing the joint has been taken over by muscles. One of these, the biceps brachii, exhibits morphological specializations consistent with preventing collapse of the shoulder joint during long periods of quiet standing characteristic of extant horses, thus reducing the muscular effort required to stand for prolonged periods. Osteologically, the specialization of the biceps brachii in horses is reflected by the presence of a well-developed intermediate tubercle (INT) on the proximal aspect of the humerus and by a prominent muscle origin on the supraglenoid tubercle of the scapula. Primitive horses and both fossil and extant outgroups of diverse sizes (e.g., tapirs and titanotheres) lack the intermediate tubercle and have an intertubercular sulcus that spirals medially (passing proximal to distal) with respect to the humeral head. In the derived equid state the intermediate tubercle is well-developed, bordered by distinct greater and lesser tubercles, and it is aligned parallel to the long axis of the humerus. The derived state of this character complex is incipient in Dinohippus (ca. 5 Ma) and is first well-developed in Pliocene (ca. 3.5 Ma) extinct species of Equus; it is thus a synapomorphy to unite this monophyletic clade. The function of this musculoskeletal complex is discussed in terms of body size trends and the distribution of various character states in other clades.