Diplacodon

Diplacodon elatus was described by Othniel Charles Marsh in 1875, based on the partial skull YPM 11180 from the Myton Member of the Uinta Formation in Utah. YPM 11180 is dorsoventrally (from the front to the back) crushed but preserves several largely undistorted teeth. Marsh considered YPM 11180 to exhibit traits intermediate between Limnohyus (now considered a synonym of Palaeosyops) and Brontotherium (now considered a synonym of Megacerops). The name Diplacodon means "double-pointed tooth", deriving from Ancient Greek διπλόoς (double), ἀκή (a point), and ὀδούς (tooth), and refers to upper premolars having two inner cones. In 1914, named the new species Diploceras osborni based on fossils from the Uinta Formation, including a partial skull and jaw (CMNH 2859, the type specimen) and another partial skull (CMNH 2858, designated as a paratype). CMNH 2858 had noticeably more rugose horns, which Peterson attributed to sexual dimorphism. In a series of studies between 1989 and 1998, Bryn J. Mader deemed YPM 11180 to be too crushed to be compared to the more complete fossils that had been discovered since 1875. Diplacodon elatus was thus designated as a nomen dubium. Mader noted that YPM 11180 was "extremely similar" to CMNH 2858, the paratype skull of E. osborni, Mader rejected Mihlbachler's revision and maintained Pseuodiplacodon and Eotitanotherium as distinct. Mader added an additional differentiating feature, that tooth rows tended to be shorter in Pseudodiplacodon, despite Pseusodiplacodon being larger than Eotitanotherium. Mader considered the variation exhibited in Duchesneodus minor in comparison to that he observed between Pseudodiplacodon and Eotitanotherium. Other scholars have followed Mihlbachler's revision and treated specimens formerly assigned to Pseudodiplacodon and Eotitanotherium as specimens of D. elatus. Taxonomy of D. gigan In 1982, a well-preserved brontothere skull (AMNH 117163) was discovered in the Wiggins Formation in Hot Springs County, Wyoming, most likely Uintan in age. Both Mihlbachler (2008) and Mader (2009) preliminarily proposed that the skull was from a new brontothere species. Mader described the specimen in 2009 and referred it to cf. Protitanotherium sp., based on similarities in size to the Protitanotherium type specimen (YPM PU 11242), similarities in the cross-sectional shape of the horn, and similar canine size. Mihlbachler disagreed with Mader's assessment in 2011, contending that many of the features used to refer the fossil to Protitanotherium (features of the canines and horns) were probably sexually dimorphic traits in brontotheres, and that Mader had not performed a phylogenetic analysis to show that the fossil fell within Protitanotherium. Mihlbachler instead referred AMNH 117163 to Diplacodon, noting that the skull shared a unique morphology of the nasal bone only with D. elatus. AMNH 117163 is distinct from other Diplacodon specimens in several respects. These differences include the skull's larger size, and that its dorsal (upper side) surface is more constricted by the parasagittal ridges (front-to-back bony ridges) than in D. elatus skulls. A phylogenetic analysis performed by Mihlbachler recovered AMNH 117163 as outside Protitanotherium and as the sister taxon of D. elatus. Mihlbachler designated the skull as the type specimen of the new species Diplacodon gigan. The name gigan derives from the kaiju Gigan and references D. gigan's greater size compared to D. elatus. == Description ==

Diplacodon elatus was about the size of a modern rhinoceros, small when compared to most other horned brontotheres. D. elatus was for instance smaller than the closely related Protitanotherium and Protitan grangeri, but bigger than Protitan minor. D. gigan was larger than D. elatus based on the size of its skull (the only part known of D. gigan). D. elatus skulls vary in size in the range , whereas the type skull of D. gigan measures . Like other brontotheres, Diplacodon had a saddle-shaped skull. Diplacodon had more elevated horns than many of its close relatives, such as Protitanotherium, Rhinotitan, and Protitan. The horns varied considerably in size from being very small rugose ridges to quite massive, though never as large as in some later brontoetheres (e.g. Megacerops). The orientation and position of the horns varied very little between individuals, projecting above the eye orbits at a 45° angle. The position and orientation of the horns in Diplacodon is more similar to derived brontotheres such as Aktautitan and Metatitan, than to the horns of Protitan and Protitanotherium. The nasal process of Diplacodon was also elevated higher than in Protitan and Protitanotherium, but not as highly as in Aktautitan and Metatitan. The most distinguishing anatomical feature of Diplacodon, present in both D. elatus and D. gigan to the exclusion of all other brontotheres, is the nasal bone, which is thickened and upturned at the lateral (towards the sides) margins. It is possible that D. gigan varied from D. elatus in a more elliptical cross-sectional shape of the horns and larger canines. These traits have an unclear usefulness in brontothere taxonomy and may also be sexually dimorphic. More fossils are required to assess individual variation within D. gigan. Diplacodon elatus had the dental formula . The incisors of Diplacodon were small, but not as reduced as they were in more derived brontotheres (e.g. Megacerops). == Classification ==

(1913), A: Palaeosyops, B: "Manteoceras" (=Telmatherium), C: Diplacodon, and D: "Titanotherium" (=Megacerops). Per Mihlbachler's 2008 revision, Diplacodon is classified as part of the brontothere subtribe Brontotheriina, alongside the genera Pachytitan, Protitan, Protitanotherium, and Rhinotitan. Brontotheriina also includes two infratribes of more derived brontotheres, Embolotheriita and Brontotheriita. Diplacodon and similar more basal members of the group have sometimes been informally referred to as "diplacodonts", though this is not a formal taxon. Diplacodon and other contemporary North American brontotheres (e.g. Protitanotherium) were part of an evolutionary radiation of brontotheres in North America and were descended from ancestral forms in Central Asia. Brontotheres that appeared during the Uintan raditation of the group in North America are believed to have later given rise to the larger forms in the Brontotheriita infratribe. In the 2011 description of D. gigan, Mihlbachler published three strict consensus trees of the Brontotheriina (and the sister taxon Epimanteoceras) as the results of his phylogenetic analysis. The analyses treated Protitanotherium differently to account for divering opinions on the diversity of that genus. Topology A below treats Protitanotherium as envisioned by Mihlbachler, whereas Topology C treats the specimen TMM 41723-3 as a separate taxon, which has been suggested by Mader. In Topology B, further specimens likely conspecific with TMM 41723-3 have also been included in an experimental additional taxon. Topology A: Protitanotherium emarginatum sensu lato (per Mihlbachler) Topology B: P. emarginatum specimens TMM 41723-3, 41723-6, and 41747-106 coded separately as "Sthenodectes australis". Topology C: P. emarginatum specimen TMM 41723-3 coded separately. == References ==