A Mylagaulus (Mammalia, Rodentia) with nasal horns from the Miocene (Clarendonian) of western Oklahoma

ABSTRACT

A partial skeleton of an adult mylagaulid rodent was collected in 1959 near Durham, Roger Mills County, Oklahoma, but the specimen was not prepared until 2010–2011. The specimen is a part of the Durham local fauna, of late Clarendonian (late Miocene) age. Several other isolated premolars previously published from the same locality give some indication of dental and morphological variation in the population sample. The new skeleton includes a complete cranium with two large horns on the nasals, associated dentaries, and some of the postcranial skeleton. The only mylagaulids previously reported to have nasal horns are Ceratogaulus rhinocerus, C. hatcheri, C. anecdotus, and Ceratogaulus (or Mylagaulus) minor (Hibbard and Phillis). There is discord about the classification of horned mylagaulines; problematically, an abundance of isolated teeth and paucity of skulls are found as fossils and sometimes were named as new taxa. Some authors place all horned mylagaulines in Ceratogaulus, whereas others place the taxon minor in Mylagaulus, indicating that at least one species (M. minor) of that genus has horns. The Oklahoma specimen has P4s with occlusal features considered diagnostic of Mylagaulus and is referred to that genus. Dentally the new specimen is most similar to the type specimen of Epigaulus minor Hibbard and Phillis, which is now variously placed in the genus Mylagaulus or Ceratogaulus. The nasal horns on the Durham cranium are distinct from the horns of all known horned mylagaulids in bearing large longitudinal furrows posteriorly. Accordingly, the Durham specimens are placed in a new species, Mylagaulus cornusaulax.

ACKNOWLEDGMENTS

I express my thanks and dedicate this paper to the late David B. Kitts, under whose supervision the OMNH specimens were collected but the preparation of which could not be completed in 1959. Abundant thanks go also to OMNH volunteer R. Tempelmeyer, who meticulously and expertly prepared the specimen under the direction of our preparator K. L. Davies, while also photographically documenting the relative positions of the bones as they were freed from the matrix. Thanks also to K. L. Davies for prioritizing preparation of this and associated plaster jacketed blocks that confirmed the provenance of the specimens from the Durham local fauna. I extend my appreciation to R. Burkhalter for his genius in photographing the cranium and jaws using a bellows camera, stacking hardware and software, and computer, and to S. Westrop for the use of this photographic rig. Desui Miao of the University of Kansas Museum of Natural History kindly provided a cast of the type cranium of Epigaulus minor. Reviewers W. W. Korth and S. S. B. Hopkins kindly read and improved the manuscript.

Handling editor: Thomas Martin

Notes

^aMeasurements of KUVP 6886 (holotype of Epigaulus minor) taken from Hibbard and Phillis (1945), and Korth (2000a), and by the author.

^bMeasurements of USNM 5485 (holotype of Epigaulus hatcheri) from Gidley (1907).

^cMeasurements of AMNH 9456 (holotype) from Matthew (1902).

^dMeasured by the method of Korth (2000a:fig. 1).

^eIn M. cornusaulax the occiput is broken ventrally and only the dorsal-most 7 mm of the occiput is intact at the midline, this angle was measured along the intact surface.

^fRe-measured by the author.

^aWhether measurement indicates greatest dimension of tooth or occlusal surface dimension was not specified by original authors.

^bFrom holotype (KUVP 6886) after Hibbard and Phillis (1945) and measurements made by the author, means of lefts and rights.

^cFrom holotype P4 (AMNH 8329) after Cope (1878) and Matthew (1902); and from referred p4 (AMNH 8330) after Matthew (1902) and Webb (1966).

^dMeasurements represent the Observed Ranges for the series of P4s (N = 3) and p4s (N = 7) studied by Baskin (1980).

^eFrom a partial P4 (UF 11553) and holotype p4 (UF 11552) after Webb (1966); these apparently represent measurements of the occlusal surfaces.

^fFrom holotype (AMNH 9456; Matthew, 1902) and several referred specimens, after Korth (2000a:table 6). Only the overall observed range of measurements provided by these authors is given here.

^gFrom holotype (USNM 5485) after Gidley (1907).

^hFrom Korth's (2000a;table 6) observed range of measurements.