THE HOMOLOGIES AND EARLY EVOLUTION OF THE SHOULDER GIRDLE IN TURTLES

The homologies of the highly distinctive shoulder girdle of turtles ar e reinterpreted in the light of recent phylogenetic studies. The acrom ion process is an extension of the scapula blade: it is not a modified anterior coracoid; contrary to recent suggestions. In modern turtles, the acromion process articulates with the plastron (the ventral porti on of the turtle shell). However, recent phylogenetic analyses indicat e that the acromion arose in the common ancestor of pareiasaurs and tu rtles, long before the plastron evolved. In pareiasaurs and primitive turtles, the acromion projects anteriorly and forms a strong, mobile a rticulation between the shoulder girdle and the clavicle. In modern tu rtles, the acromion has changed its orientation and morphology, projec ting ventrally and contacting the plastron. Nevertheless, even in mode rn turtles the acromion still retains its original function: it suppor ts the shoulder girdle, and allows the shoulder girdle to pivot during locomotion, thus increasing stride length.