A COMPLETE CRANIUM CONFIRMS THE PRESENCE OF MERYCOCHOERUS MAGNUS IN OREGON

Both Merycochoerus magnus and Merycochoerus matthewi are reported from Oregon, in the John Day Formation, the former in the Rose Creek Member and the latter from the Warm Springs local fauna. The identification of these taxa in the Rose Creek Member was particularly important, as Merycochoerus magnus was used biostratigraphically to date the Rose Creek Member to the earliest Hemingfordian. However, in both locations the material was fragmentary, and lacked almost all of the cranial characters used to diagnose species of Merycochoerus. In this research we describe a newly discovered complete cranium of Merycochoerus magnus from the Rose Creek Member and verify the previous identification of this species.

The attribution to Merycochoerus is based on diagnostic features of the genus, including presence of a preorbital rather than a full-facial fossa, nasal retraction, and a robust premaxilla. The new specimen is a nearly complete skull, but it belonged to an old individual and suffered some taphonomic event that compressed the skull dorso-ventrally, and sheared it antero-posteriorly. The deformation makes it difficult to accurately measure overall length and width of the skull, but in most of the undeformed measurements it was closest to M. magnus.

One key diagnostic difference between species of Merycochoerus is the extent of nasal retraction (a character indicative of some development of a vestibular proboscis). Our specimen showed retraction of the nasal notch to the molars, consistent with M. proprius and M. magnus but further back than in M. matthewi. The specimen possesses a well-marked ridge on the braincase, which is commonly observed in M. magnus and in old individuals of M. proprius, but is not present in M. matthewi. The sigmoidal curve to the nasals found in our specimen are also observed in M. magnus and M. proprius, but not in M. matthewi. The lamboidal crest of our specimen converges much farther posterior than in M. proprius or M. matthewi, confirming a designation of M. magnus for this specimen. Although previous literature cited M. magnus in Rose Creek, this is the first description of a specimen with the diagnostic cranial characters to confirm its presence.