An articulated pectoral girdle and forelimb of the abelisaurid theropod Majungasaurus crenatissimus from the Late Cretaceous of Madagascar

ABSTRACT

Abelisaurid theropods are common members of Cretaceous Gondwanan faunas and are characterized by a bizarre, highly reduced forelimb. Unfortunately, forelimb elements are rarely preserved and thus the basic structure of the abelisaurid forelimb remains poorly understood. Until recently, the Upper Cretaceous Maevarano Formation of northwestern Madagascar has produced numerous exceptional specimens of the abelisaurid theropod Majungasaurus crenatissimus but comparatively little forelimb material. A recently discovered articulated skeleton of Majungasaurus preserves a virtually complete pectoral girdle and forelimb, which, along with additional isolated forelimb elements, affords important new insights into the structure of these elements. New specimens of the scapulocoracoid and humerus allow more detailed description of their morphology, and antebrachial and manual elements are described for the first time. The radius and ulna are approximately one-quarter the length of the humerus and both have expanded proximal and distal articular surfaces relative to their narrow diaphyses. The manus consists of four digits, each composed of a short metacarpal and one (digits I and IV) or two (digits II and III) phalanges. No ossified carpals are present. The proportions of the brachium and antebrachium are stout, more similar to the condition in Carnotaurus than in Aucasaurus. We reinterpret manual digit identities in Aucasaurus and Carnotaurus based on new information provided by the manus of Majungasaurus. Overall, the morphology of the forelimb in Majungasaurus reveals that abelisaurids share an extremely reduced, unique morphology that is dissimilar to the more typical theropod condition seen in other ceratosaurs.

ACKNOWLEDGMENTS

We thank D. Krause for the opportunity to work on the Mahajanga Basin Project and the Majungasaurus forelimb materials. We also acknowledge the members of the 2005 and 2007 field expeditions for the collection of these specimens, and A. Rasoamiaramanana of the Université d’Antananarivo, B. Andriamihaja and his staff of the Madagascar Institute pour la Conservation des Ecosystèmes Tropicaux, and the villagers of Berivotra for logistical support in the field. We thank J. Groenke, V. Heisey, and the volunteers of the Science Museum of Minnesota for their skilled preparation of these materials and J. Sertich and W. Simpson for curation. Assistance with CT scanning was provided by J. Georgi, J. Groenke, B. Patel, and J. Sertich. J. Bonaparte, F. Novas, A. Kramarz (MACN) and R. Coria (MCF) generously provided access to specimens in their care. We thank D. Krause and A. Turner for comments on early drafts of the manuscript, and D. Krause, J. McCartney, P. O’Connor, and J. Sertich for helpful discussions. We also thank R. Benson and J. Choiniere for their insightful reviews. Funding for the Mahajanga Basin Project has been provided by the National Science Foundation (DEB-9224396, EAR-9418816, EAR-9706302, EAR-0106477, EAR-0446488, EAR-1123642), the Dinosaur Society (1995), and the National Geographic Society (1999, 2001, 2004, 2009). Support for S.H.B. was provided by a National Science Foundation Graduate Research Fellowship and a Stony Brook University Graduate Council Fellowship. Support for M.T.C. was provided by the National Science Foundation (DEB-9904045). Translations of Bonaparte and Novas (1985), Buffetaut et al. (1988), Depéret (1896a, 1896b), and Lavocat (1955) are available on the Polyglot Paleontologist Web site (www.paleoglot.org).

Handling editor: Hailu You