Endocranial and Inner Ear Morphology of Vintana Sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar

ABSTRACT

We present the first digital reconstruction of the endocranial cavity and endosseous labyrinth of the Late Cretaceous gondwanatherian mammal Vintana sertichi from the Maevarano Formation of Madagascar. The Malagasy specimen is exceptionally well preserved and represents the only described cranium known for Gondwanatheria, an enigmatic clade from the Late Cretaceous and Paleogene of Gondwana. The endocranial cast of Vintana is relatively small for an animal of its estimated body mass. Its encephalization quotient is 0.28–0.56 for a range of body mass estimates, which is similar to that of basal mammaliaforms. The olfactory bulbs are very large, occupying over 14% of the endocranial volume. The cerebral hemispheres are only slightly expanded, more similar to the condition in Morganucodon than to that in multituberculates and monotremes. Unlike the condition in other Mesozoic mammaliaforms, the endocast is greatly flexed at the circular fissure. The osseous labyrinth displays a mixture of derived and primitive features. The cochlear canal is only slightly curved and short compared with that of extant therians. The ratio between total cochlear canal length and maximum cranial length is smaller than in basal mammaliaforms and approximates that of non-mammaliaform cynodonts. By contrast, the presence of both primary and secondary osseous laminae, the tractus foraminosus, and Rosenthal's canal represent derived characteristics of the mammalian inner ear typical of cladotherians. A modern innervation of the cochlea has either evolved independently in Vintana and cladotherians or was already present in the last common ancestor of both clades.

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ACKNOWLEDGMENTS

The following individuals are gratefully acknowledged: L. Betti-Nash (Stony Brook University [SBU]) for assistance with the interpretive drawing (Fig. 4) of the brain and general figure composition; W. Holloway (Ohio University) and J. Groenke (SBU) for input on digital processing; T. Rowe (University of Texas at Austin) and digimorph.org for generously providing μCT scans of monotreme crania; N. Jeffery (University of Liverpool) for generously providing μCT scans of several placental heads; J. Diehm and B. Ruether (Avonix Imaging, Plymouth, Minnesota) for providing expert assistance in μCT scanning of the cranium; N. Thompson (SBU) for assistance in calculating semicircular canal orthogonality; and M. Borths (SBU), J. Groenke (SBU), W. Holloway (Ohio University), Z.-X. Luo (University of Chicago), G. W. Rougier (University of Louisville), I. Ruf (Universität Bonn), and N. Thompson (SBU) for discussions. T. Macrini (St. Mary's University) and I. Ruf (Universität Bonn) provided useful comments for the revision of this paper. This research was supported by grants from the National Science Foundation (EAR-0446488, EAR-0933619, and EAR-1123642), the National Geographic Society (8597-09), the Ohio University Office of Research and Sponsored Programs, and the Ohio University Heritage College of Osteopathic Medicine. S.H. was further supported by the Turkana Basin Institute (SBU).

Handling editor: Jeffrey A. Wilson