Earliest eutherian ear region revisited: a new Prokennalestes petrosal from the Lower Cretaceous of Mongolia

The earliest eutherian ear region was known previously from an isolated petrosal bone PSS-MAE 136 from the Lower Cretaceous (Aptian-Albian) Khovoor locality in Mongolia, attributed to Prokennalestes trofimovi, and from the complete skull of Cokotherium jiufotangensis from the Lower Cretaceous (Aptian) Jiufotang Formation of China. In this study, we present a comprehensive description of the second eutherian petrosal bone (PIN 3101/689) from the Khovoor locality based on high-resolution CT imaging. This petrosal exhibits a separate canal for the perilymphatic duct, a cochlear canal that is coiled for 360°, accompanied by ossified primary and secondary bony laminae, a tractus foraminosus, and a cochlear ganglion. PIN 3101/689 differs from PSS-MAE 136 in a number of plesiomorphic characters, including the lateral flange and anterior lamina extending the length of the pars cochlearis, dorsally open cavum supracochleare, and a low stapedial ratio (1.47 versus 1.71). We interpret these differences as evidence that the two petrosals from Khovoor belong to different eutherian taxa. PIN 3101/689 is within the size range for Prokennalestes minor, while the larger PSS-MAE 136 may not belong to Prokennalestes trofimovi, as originally suggested, but rather to the similarly sized Hovurleses noyon. Hovurlestes is more advanced than Prokennalestes in that it has a large, single-rooted canine, and it may have a more advanced petrosal morphology. The new Prokennalestes petrosal resembles Cokotherium but differs from PSS-MAE 136 in the presence of a stapedial sulcus crossing the fenestra vestibuli and a large central promontorial depression.