Postcranial Morphology of Adalatherium hui (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar

ABSTRACT

The holotype and only known specimen of Adalatherium hui from the Late Cretaceous of Madagascar preserves the first postcranial skeleton for the Gondwanatheria. It represents only the fourth Mesozoic mammaliaform described from articulated postcranial material from Gondwana (the others being the morganucodontans Megazostrodon and Erythrotherium and the stem therian Vincelestes). The postcranial skeleton of Adalatherium is virtually complete, and its preservation is exceptional; even costal cartilages and manual and pedal sesamoids are preserved. Adalatherium exhibits a variety of derived and plesiomorphic traits, probably reflective of a long evolutionary history in geographic isolation on Madagascar. Among the bizarre features are a mediolaterally compressed and anteroposteriorly bowed tibia, an unusually high number of trunk vertebrae (at least 16 thoracic and 12 lumbar vertebrae), a short tail, and a trochleated navicular facet on the astragalus. Aside from these features, Adalatherium displays derived and plesiomorphic characteristics in its pectoral girdle: the coracoid is well developed and contributes to half of the glenoid fossa, a separate procoracoid is absent, and the sternoclavicular joint appears to have been mobile. A ventrally facing scapular glenoid and the well-developed humeral trochlea suggest a relatively parasagittal forelimb posture. Notable features of the hind limb and pelvic girdle include a large obturator foramen similar in size to that of therians, a large parafibula, and the presence of epipubic bones. Articulation between the pelvic girdle and hind limb is in contrast to that between the pectoral girdle and the forelimb in that it is indicative of a sprawling posture.

ACKNOWLEDGMENTS

We gratefully acknowledge V. Heisey (Stony Brook University) and J. Groenke (Ohio University) for mechanical preparation of the skeleton of UA 9030; B.-A. S. Bhullar (Yale University), J. Groenke (Ohio University), M. Hill (AMNH), P. O’Connor (Ohio University), and K. Hurdle (New York Institute of Technology) for µCT imaging of the skeleton and individual postcranial elements; J. Groenke (Ohio University) for assistance with digital imaging; J. Neville and M. Stewart (both Stony Brook University) for taking photographs; L. Betti-Nash (Stony Brook University) for executing the drawings in Figures 56 and 57 and editing photographs of several other figures; J. Molnar (New York Institute of Technology) for assistance in reconstructing thoracic and lumbar vertebrae for Figures 7 and 10; and K. Pan (New York Institute of Technology) for digitally articulating the postcranial skeleton. We also thank the following curators, collection managers, and colleagues for allowing access to specimens as well as for their accessibility and hospitality during visits: J. Galkin, J. Meng, and E. Westwig (AMNH); Q.-J. Meng (BMNHC); C. Jin, H. Wang, and Y. Wang (IVPP); P. Brewer and J. Hooker (NHM); C. Zhou (PMOL); T. Martin (University of Bonn); P. Gill (University of Bristol); Z.-X. Luo (University of Chicago); R. Asher, J. Clack, M. Lowe, and R. O’Meara (University Museum of Zoology, Cambridge); and Ł. Fostowicz (ZPAL). We also thank countless individuals for fruitful discussions on Mesozoic mammaliaform evolution in the context of postcranial morphology but here single out in particular M. Chen (NJU), J. Meng (AMNH), Z.-X. Luo (University of Chicago), and E. Seiffert (University of Southern California). Finally, we are immensely grateful to M. Chen (NJU) and L. Gaetano (National Scientific and Technical Research Council), whose thorough reviews of the manuscript have improved it considerably. This research was supported by the National Science Foundation (grants DEB-1501497 and MRI-1828305 to S.H. and EAR-9706302, EAR-0106477, EAR-0446488, EAR-1122642, EAR-1528273, and EAR-1664432 to D.W.K.) and a grant from the Office of the Vice President for Research, Stony Brook University (2005–2007), to support the late Y. Hu as a postdoctoral associate.