Inner ear morphology of Adalatherium hui (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar

ABSTRACT

The cochlear canal of Adalatherium hui, from the Late Cretaceous of Madagascar, exhibits a combination of features previously unknown among mammaliaforms. The cochlear canal of Adalatherium is short and ‘C’-shaped (curved through at least 210°). A presumptive lagenar nerve canal extends from the apex of the cochlea to the internal acoustic meatus. In conjunction with an apical expansion of the cochlear canal, this morphology suggests that Adalatherium retained a lagenar macula. Adalatherium is derived among mammaliaforms in possessing primary and secondary osseous laminae, a cribriform plate, and a distinct cochlear ganglion canal. Strikingly, Adalatherium shows a single-layered primary osseous lamina that lacks internal, radially oriented canaliculi for auditory nerve fibers. In this respect, the primary lamina of Adalatherium is structurally different from the double-layered primary lamina containing habenulae perforatae of extant therians. Furthermore, Adalatherium resembles the gondwanatherian Vintana in possessing a secondary canal running parallel to the cochlear ganglion canal. Although its exact role and homology are uncertain, the morphology of this secondary canal and its tributary canaliculi is most consistent with a vascular function. The inner ear of Adalatherium augments a growing list of characters (or character combinations) in which gondwanatherians are not only different from the inferred plesiomorphic condition for mammaliaforms but also distinct from the derived characters of therians. Collectively, the new morphological and phylogenetic data support homoplastic transformation of such structures as the cribriform plate and primary osseous lamina in mammals and independent loss of the lagenar macula within gondwanatherians.

ACKNOWLEDGMENTS

We thank J. Wible (Carnegie Museum of Natural History) and G. Rougier (University of Louisville) for discussion of petrosal morphology and cochlear canal orientation; Z.-X. Luo (University of Chicago) and one anonymous reviewer whose comments greatly improved the manuscript; and D. Krause (Denver Museum of Nature & Science) for insightful comments and edits on previous versions of the manuscript. We further thank M. Colbert, D. Edey, J. Maisano, and R. Racicot (University of Texas at Austin) and J. Groenke (Ohio University) for assistance in µCT imaging of UA 9030. This research was supported by grants from the National Science Foundation (EAR-1122642, EAR-1528273, and EAR-1664432 to D. Krause and DEB-1501497 to S.H.).