A small-bodied species of Brontotheriidae from the middle Eocene Nut Beds of the Clarno Formation, John Day Basin, Oregon

Abstract

We diagnose a new species of Brontotheriidae from a middle Eocene locality, the Clarno Nut Beds, from the Clarno Formation, John Day Basin, Central Oregon. Though renowned for its richness in fossil flora, fossil vertebrates are rare in the Clarno Nut Beds and this new species is the most abundantly represented mammal. Radiometric dating constrains the age of the Nut Beds fauna to about 43.76 Ma within the Uintan North American Land Mammal Age. This new taxon, represented by numerous cranial, mandibular, and dental specimens, is comparatively small for a brontothere and notable for its cranio-caudally shorted nasal incision, a trait shared with three larger-bodied middle Eocene species,Metatelmatherium ultimum,Wickia brevirhinus, andSthenodectes incisivum. Phylogenetic analysis suggests the sister taxon of the Nut Beds brontothere could be one of two species—Wickia brevirhinusfrom the Sand Wash Basin of Colorado and Washakie Formation of Wyoming, orMetatelmatherium ultimum, a pan-Beringian species known from the Uintan Formation of Utah (and other Uintan age deposits) and the “Irdin Manha” Formation of Inner Mongolia, China. Phylogenetic results also indicate that the Nut Beds brontothere is a dwarf taxon. Though brontotheres are renowned for having evolved very large body sizes, this new brontothere is one of several discovered in recent decades that suggest evolutionary reductions in body size may have been relatively common in Brontotheriidae.

Eocene (Duchesnean and earliest Chadronian) brontotheres (Brontotheriidae), Protitanops curryi and cf. Parvicornus occidentalis, from west Texas and Mexico

“…Characters 1-92 are described in previous publications (Mihlbachler, 2008(Mihlbachler, , 2011Mihlbachler and Samuels, 2016). Two additional characters (93, 94) are described here.…”

“…This analysis is based on 242 character states distributed among 94 characters and coded for 59 species including two outgroups, "Hyracotherium" (=Xenicohippus osborni sensu Froehlich, 2002), Pachynolophus livinierensis Savage et al, 1965, early brontotheriioids Danjiangia pingi Wang, 1995, andLambdotherium popoagicum Cope, 1880 (Appendix 1). The present dataset is derived from Mihlbachler and Samuels (2016) with additional newly described species, Maobrontops paganus Averianov et al, 2018 andEpimanteoceras mae Li, 2018. The dataset includes all taxa belonging to Brontotheriidae that produce unique combinations of character data. All brontotheres were included as species except Palaeosyops due to uncertainty about the species-level taxonomy of this genus (Gunnell and Yarborough, 2000;Mihlbachler, 2008;Mader, 2009).…”

“…A parsimony analysis was conducted in PAUP 4.0a (Swofford, 2003) using the heuristic search, with equally weighted, ordered multistate characters. Like prior equally weighted heuristic searches of this brontothere data matrix (Mihlbachler, 2007b(Mihlbachler, , 2008(Mihlbachler, , 2011Mihlbachler and Deméré, 2009;Mihlbachler and Samuels, 2016), resolution was uncovered from a poorly resolved strict consensus by removing wildcard taxa aposterior using reduced consensus techniques. Here, another approach was adopted using implied weighting measures (Goloboff et al, 2008) in TNT v. 1.5 software.…”

The first Eocene rodents from the Pacific Northwest, USA

“…Similarly, the late Uintan (Ui3) or earliest Duchesnean age Hancock Mammal Quarry shares taxa considered characteristic of multiple land mammal ages (Robinson et al, 2004). Interestingly, both of the previously described Eocene faunas from Clarno have distinct Asian affinities, sharing taxa like Teletaceras, Zaisanamynodon, and Plesiocolopirus with similar age faunas in Asia (Radinsky, 1963;Lucas, 1992;Hanson, 1996;Lucas, 2006;Mihlbachler, 2007;Mihlbachler and Samuels, 2016).…”

“…The mid-Uintan age Clarno Nut Beds represent the oldest Cenozoic vertebrate fauna from the Pacific Northwest (Robinson et al, 2004;Fremd, 2010). Vertebrate fossils are rare and fragmentary from the Clarno Nut beds (Stirton, 1944;Hanson, 1989Hanson, , 1996Fremd, 2010); specimens from the fauna are mainly from the dwarf brontothere, Xylotitan (Mihlbachler and Samuels, 2016), along with a few specimens of Orohippus, Hyrachyus, Patriofelis, Hadrianus, and a crocodile (Stirton, 1944;Hanson, 1996). The second vertebrate locality from Clarno, the Hancock Mammal Quarry, represents the only vertebrate quarry deposit in the region (Pratt, 1988;Hanson, 1996;Fremd, 2010).…”