Earliest Eutherian Ear Region: A Petrosal Referred to Prokennalestes from the Early Cretaceous of Mongolia

Abstract

A right petrosal from the ?Aptian or Albian Khoobur locality is referred on the basis of size and morphology to Prokennalestes trofimovi, the earliest eutherian previously known only from dentigerous elements. The petrosal shows a mosaic of primitive and derived features, bearing on the purported therian and eutherian morphotypes. Among the primitive features shared with the Early Cretaceous prototribosphenidan Vincelestes and other more basal taxa that are modified in later eutherians and metatherians are the pattern of basicranial arterial and venous circulation, including a prootic canal and an intrapetrosal inferior petrosal sinus; a vertical paroccipital process; and a fenestra semilunaris, an incomplete wall between the cavum epiptericum and cavum supracochleare. Among the derived features shared with therians is a cochlea coiled through a minimum of 360°, with Prokennalestes extending the range of the oldest occurrence of such a coiled cochlea by at least 10 million years. Shared with Late Cretaceous eutherians is a shallow internal acoustic meatus with a thin prefacial commissure. The petrosal referred to Prokennalestes is intermediate in having a reduced anterior lamina and lateral flange, both of which are well developed in Vincelestes and essentially lacking in later eutherians and metatherians. Features previously held to be part of the therian and eutherian morphotypes, such as the absence of the anterior lamina and lateral flange, may have been lost independently in metatherians and in post-Prokennalestes eutherians.