Variation and disparity within the inner ear and trigeminus of the Tenrecomorpha

How does the evolution of specialized sensory systems relate to adaptive radiation? Does adaptation to different ecological niches correlate significantly with changes in sensory anatomy? The afrotherian clade Tenrecomorpha, represented today by the African potamogalines and the Malagasy tenrecines, is of particular interest in this regard because of its extraordinary variety: its membership reportedly includes fossorial, arboreal, semiaquatic, and even echolocating taxa. To investigate their sensory ecology further, here we provide geometric morphometric analyses of inner ear endocasts (i.e., the bony labyrinth) of 24 tenrec species. We expand this data set with iodine-stained specimens of an additional 9 species, to provide information on cerebral and trigeminal organization. Although tenrecomorphs display cross-taxon differences in structures that may relate to sensory ecotype, our analyses distinguish signals of conflicting strength and direction within the tenrec ear. We found no single factor that might explain a substantial portion of the observed variation when controlling for phylogenetic signal. This is in marked contrast to prior studies of the tenrec cranial endocast, where sensory ecotype and habitat are more strongly associated with shape change. Iodine-enhanced scans of the trigeminal nerve are nonetheless consistent with this and other studies based on bony anatomy. The disparate patterns of shape evolution within the Tenrecomorpha and the contrasting signals of variation exhibited by the inner ear and trigeminal nerve provide a nuanced portrait of neurosensory adaptation and a departure from expectations set by other mammalian groups.

Methods
Skulls of 24 tenrecomorph species were scanned on the GE Phoenix Vtome x SMicroCT scanner in the AMNH Microscopy and Imaging Facility (MIF), at a target resolution between 18-56 μm. For all taxa, the left bony labyrinth of adult specimens was segmented. For 4 specimens where this structure was damaged or otherwise unsuitable for analysis, the right cochlea was segmented and mirrored prior to analysis. For rendering, specimens were reconstructed with the 3DSlicer software suite and semilandmarks placed using the SlicerMorph package. For fixed landmarks, landmarks were collected, translated from a template (Nesogale talazaci) using the ALPACA module, and corrected/resampled by hand where necessary. Cochlear and semicircular landmarks were placed and resampled manually for each specimen, and all landmarks were merged to form the final dataset.