Mosaic evolution of cochlear characters seen in meridiolestidan mammal from the latest Cretaceous of northern Patagonia

Abstract Meridiolestida is an endemic South American clade of non-therian mammals known to display a range of derived craniodental characteristics, such as complete cochlear coiling. Here we describe the morphology of an isolated petrosal we refer to Meridiolestida. This specimen was recovered from the Upper Cretaceous Allen Formation and includes an excellently preserved and fully coiled cochlear canal. However, both high resolution µ-CT imaging and a subsequent comparative geometric analysis of these data indicate the presence of an innervated lagenar sensory endorgan located at the apex of the cochlear canal. Since the lagenar macula is a plesiomorphic characteristic of the amniote inner ear (seen only in monotremes among extant mammals), its presence in a lineage close to crown therians highlights the evolutionary lability of inner ear characters in even late-diverging stem therians. The fossilizable bony correlates of cochlear coiling and lagenar innervation are therefore found to be more homoplastic and ambiguous than previously recognized.