Metarhinus diploconus, (Osborn 1895)

Mader, Bryn J., 2008, A species level revision of Bridgerian and Uintan brontotheres (Mammalia, Perissodactyla) exclusive of Palaeosyops, Zootaxa 1837 (1), pp. 1-85 : 29-30

Species METARHINUS diploconus ( Osborn 1895)

= M. abbotti ( Riggs 1912)

= M. pater? Stock 1937

Holotype. AMNH 1863 , a skull lacking the nasals.

Referred specimens. CM 2866, CM 3098, CM 3510, FMNH P 12179 (type of M. abbotti , type species of Rhadinorhinus ), LACM ( CIT) 2037 (type of M. pater ).

Diagnosis. Large species of Metarhinus with nasals that are strongly tapered distally.

Discussion. The holotype of Metarhinus diploconus lacks the nasals and the conclusion that it represents the form with distally tapered nasals is based solely upon the large size of the type. If this conclusion should prove to be in error (either because Metarhinus diploconus represents the Metarhinus with spoon-shaped nasals or is a nomen dubium) then the next available name for the taxon with tapered nasals is Metarhinus abbotti (Riggs) , which was based on a skull (FMNH P 12179) that clearly exhibits this nasal morphology.

In his recently completed doctoral dissertation, Mihlbachler (2005) recognized the holotype of Metarhinus diploconus as representing an entirely new genus. Among the distinguishing characters were less prominent orbits, more strongly upturned rostrum, more deeply concave dorsal midcranial surface, and small premolar hypocone. In my opinion, however, all of these characters can be attributed to taphonomic deformation and individual variation. More compellingly, Mihlbachler noted a pair of large fossae in the floor of the rostrum and inside the nasal chamber that had not been observed by previous authors and seemed to distinguish the specimen from Metarhinus . The genus Metarhinus is characterized by a highly unusual nasal morphology, however, which has never been described, but is evident in a skull (UCM 44939) with distally flared nasals here referred to the species M. fluviatilis (see list of referred specimens for that species). Based on Mihlbachler’s description, it seems doubtful that the morphology of the holotype of M. diploconus shall be found to be significantly different from that of UCM 44939.

As indicated in the Discussion section for the genus Metarhinus , the type of Metarhinus pater Stock is provisionally referred to the species M. diploconus based on its size.