ORIGINAL PAPER Convergences in Scapula Morphology among Small Cursorial Mammals: An Osteological Correlate for Locomotory Specialization Lauren Seckel & Christine Janis Published online: 30 May 2008 # Springer Science + Business Media, LLC 2008 Abstract Small cursorial mammals, such as lago- morphs, elephant shrews, and the more cursorial caviomorph rodents, share both the similar locomotor gait of rapid half-bounding and a similar scapula anatomy of a long, slender, caudally projecting metacromion process. This scapular morphology is also present in some notoungulates (extinct endemic South American ungulates), in rabbit-like taxa such as Propachyruchos. In the rabbit Oryctolagus this elongated metacromion process serves to increase the moment arm of the acromiotrapezius and levator scapulae ventralis muscles, which we propose may aid in scapula stabilization and resisting ground reaction forces during the landing phase onto a single forelimb in half-bounding. A long, slender meta- cromion process is thus an osteological correlate of locomotor specialization, that of rapid half-bounding in small to medium-sized mammals. Keywords Lagomorpha . Caviomorpha . Notoungulata . Locomotion . Scapula . Half-bound Introduction The therian mammalian scapula is a unique morpho- logical adaptation among tetrapods, with an apparent single evolutionary origin (Sereno and McKenna 1995). The scapula of the rabbit shows a peculiar derived condition—that of a slender, elongated, caudally projected metacromion process (see Fig. 1), oft-pondered by the junior author over many years of teaching comparative anatomy. This paper investi- gates the distribution of this morphology among other therian mammals, and proposes a functional reason for this feature. In the evolution of the therian scapula from the more generalized tetrapod condition, the following changes are seen: the procoracoid and interclavicle were lost (although these structures are retained in neonate marsupials; Sears 2004); the coracoid is reduced and fused with the scapula; the scapular glenoid now points ventrally rather than laterally; and the scapula now possesses the new features of a scapular spine and supraspinous fossa (Jenkins and Weijs 1979). All these changes result in a scapula that is now free to rotate around its dorsal (vertebral) border, resulting in significant changes in the mode of therian locomotion, enabling the typical mammalian bounding gait. J Mammal Evol (2008) 15:261–279 DOI 10.1007/s10914-008-9085-7 L. Seckel : C. Janis (*) Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912, USA e-mail: christine_janis@Brown.edu L. Seckel e-mail: lauren.seckel@gmail.com L. Seckel 165 N. Michigan Ave, Pasadena, CA 91106, USA