Diplacodon ("double-pointed tooth") is an extinctgenus of brontothere. Diplacodon fossils have been found in North America and date to the Middle Eocene, during the Uintan land mammal age. Two Diplacodon species are recognized, the type speciesD. elatus from the Uinta Formation and the larger D. gigan from the Wiggins Formation.
Diplacodon ranged in size from about the size of a modern rhinoceros to slightly larger. Diplacodon and other contemporary North American brontotheres, such as Protitanotherium, were part of an early radiation of brontotheres in North America during the Middle Eocene and were descended from Central Asian ancestors. It is believed that Diplacodon and its close relatives ultimately gave rise to the larger brontotheres of the Brontotheriita infratribe, such as the more famous Megacerops. Similar to some other brontotheres, fossils assigned to D. elatus vary in the size and morphology of the teeth and horns. These variations have been argued to represent species-level, or even genus-level, differences, but recent revisions favor them as the result of sexual dimorphism and other individual variation. D. gigan is known from a single skull, making studies on intraspecific variation impossible in that species.
History of research
YPM 11180, the type specimen of D. elatus
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.[1] YPM 11180 is dorsoventrally (from the front to the back) crushed but preserves some of the teeth without much distortion.[1] Parts of the skeleton associated with YPM 11180 were also recovered and described.[2] Marsh considered YPM 11180 to exhibit traits intermediate between Limnohyus (now considered a synonym of Palaeosyops) and Brontotherium (now considered a synonym of Megacerops).[2] Per Marsh's description, D. elatus resembled Brontotherium more in the form of the incisors and canines while it was closer to Limnohyus in the premolars and molars, and in parts of the skeleton, particularly the limbs and vertebrae. Marsh also questionably asserted that D. elatus was hornless.[2] The description of Diplacodon marked an important step in the study of brontotheres, since the fossil provided a link between more primitive brontheres found in the Green River Basin and more derived forms from the White River Group (e.g. Megacerops).[3]
Underside of YPM 11180, showing the preserved dentition
The name Diplacodon means "double-pointed tooth", deriving from Ancient Greekδιπλόoς (double), ἀκή (a point), and ὀδούς (tooth), and refers to the two inner cones of the upper premolars.[2] The species name, elatus, means "lofty" in Latin.[2] Marsh did not explain the meaning behind this name. In his 1929 brontothere monograph, Henry Fairfield Osborn speculated that Marsh used elatus either due to the large size of YPM 11180 or due to its apparent "advanced stage of evolution".[2]
Marsh's assertion that YPM 11180 lacks horns is unsubstantiable since the fossil is too damaged to determine if the skull once had horns or not.[1] In 1895, John Bell Hatcher described the new species Diplacodon emarginatus, based on another partial skull from the Uinta Formation (YPM PU 11242). D. emarginatus preserved small horns. According to Hatcher, this did not preclude its classification in Diplacodon since Marsh's description of D. elatus as hornless "is entirely conjectural, since his material does not show whether there were horns or not."[2] Hatcher considered D. emarginatus to otherwise agree with all the characters established by Marsh for Diplacodon. Hatcher noted that if D. elatus proved to actually be hornless, D. emarginatus may be reclassified in a new genus "which may be called Protitanotherium".[2] Osborn (1929) agreed that it was not determined if D. elatus was hornless, but considered D. emarginatus to belong to a distinct genus due to other characteristics, for which he considered Protitanotherium to be a valid name.[2]Protitanotherium has been maintained as a distinct genus by later researchers.[1][4]
In 1914, Olof August Peterson named the new species Diploceras osborni based on a partial skull and jaw (CMNH 2859). Another partial skull (CMNH 2858) was designated as the paratype specimen. Both specimens were found in the Uinta Formation. CMNH 2858 had noticeably more rugose horns, a difference attributed to sexual dimorphism by Peterson.[1] In the same year, Peterson discovered that the name Diploceras was preoccupied by a cephalopod and moved D. osborni to the new genus Eotitanotherium.[1] In his description of D. (E.) osborni, Peterson did not clearly distinguish the species from D. elatus, though he noted that Eotitanotherium had certain differences in the dentition, such as more distinct lingual cusps on P3 (premolar 3), a less ridge-like deuterocone (cusp) on P2, and blunter and shorter canines.[1]
In a posthumous publication in 1934, Peterson named another species of brontothere from the Uinta Formation, this time classified within Diplacodon, D. progressum. This species was based on a nearly complete skeleton (CMNH 11879), with the skull (CMNH 11879A) designated as the type specimen.[1] Peterson did not clearly explain why D. progressum was referred to Diplacodon.[3] He distinguished D. progressum from D. elatus by two features: a more molarized (development of molar-like features) premolar series in D. progressum than in D. elatus, the more "perfectly quadrates" shape of individual teeth of D. progressum (especially P2), and D. elatus having a longer "facial region" than D. progressum.[1]D. progressum was distinguished from E. osborni by shorter and heavier nasal bones, and a "noticeably shorter" alveolar border of the premaxilla.[1]
Additional brontothere specimens from the Uinta Formation continued to be assigned to Eotitanotherium osborni and Diplacodon progressum over the course of the 20th century. These fossils vary mosty in the robustness of the skull, particularly the thickness of nasal bones and size of the horns, and by whether the lingual (tongue-side) cusps of the premolar teeth are separated or connected by a small lingual crest.[1]
Taxonomy of D. elatus
CMNH 2859 (A) and CMNH 2858 (B), the type and paratype specimens of "Eotitanotherium osborni"
Osborn (1929) considered Diplacodon, and D. elatus, to be a valid taxon and based on an "excellent type".[2] Osborn noted substantial similarities between D. elatus and E. osborni, but considered "it best to let [Eotitanotherium] stand provisionally as a distinct genus" due to the characteristics of the premolars.[5] In one place, Osborn nevertheless designated E. osborni as Eotitanotherium (Diplacodon?) osborni.[6]
In a series of studies between 1989 and 1998, Bryn J. Mader considered YPM 11180 to be too crushed to be compared to the more complete fossils that had been discovered since 1875, and thus designated Diplacodon elatus as a nomen dubium.[3][7] Mader noted that YPM 11180 was "extremely similar" to CMNH 2858, the paratype skull of E. osborni,[3] and thus suggested the possibility that Diplacodon elatus was a senior synonym of Eotitanotherium osborni.[7] Because Mader considered Diplacodon to be a nomen dubium, he created the new genus Pseudodiplacodon in 2000 for D. progressum.[3] Mader considered Pseudodiplacodon to be distinct from Eotitanotherium based on features of the teeth and horns. Per Mader, the lingual cusps on P4 were poorly separated in Pseudodiplacodon and well-separated in Eotitanotherium. Mader also made the case that fossils referred to Pseudodiplacodon had horns that were more circular in their cross-sectional shape than the horns of Eotitanotherium fossils, which were more elliptical in their cross-sectional shape.[3]
In 2005, Matthew C. Mihlbachler disagreed with Mader's assessment and referred all fossils previously assigned to Eotitanotherium and Pseudodiplacodon to a single taxon,[7] a conclusion Mihlbachler maintained in a 2008 major revision of the Brontotheriidae.[1] If all fossils are the same taxon, the oldest available name takes priority and Mihlbachler thus considered Diplacodon to be a valid species and the valid name for the entire fossil assemblage.[1] Mihlbachler questioned all of the features used by Peterson to distinguish D. elatus, D (P.) progressum, and E. osborni. Some of Peterson's differences could be attributed to the different sizes and degrees of completion of the specimens, several features were deemed to not be taxonomically relevant, and some of Peterson's observations could not be confirmed.[1]
Mihlbachler considered the features used by Mader to separate Eotitanotherium and Pseudodiplacodon to most likely be the result of individual variation. Per Mihlbachler, the horns of several of the specimens assigned to Pseudodiplacodon by Mader were intermediate in size between the horns of the E. osborni and D. progressum holotypes, Pseudodiplacodon specimens seemed to vary in the separation of the lingual cusps, and the nasal bones of a specimen referred to Eotitanotherium by Mader were as thick as the same bones in Pseudodiplacodon specimens.[1] Mihlbachler thus concluded that Eotitanotherium and Pseudodiplacodon could not be separated into distinct units that could be validly diagnosed.[1] Mihlbachler thus designated Eotitanotherium osborni and Pseudodiplacodon progressum as junior synonyms of D. elatus.[1] Per Mihlbachler, apparent differences in horn sizes between Pseuodiplacodon and Eotitanotherium were consistent with intraspecific variations in other brontothere genera, such as Megacerops and Duchesneodus.[7]
In 2008[7] and 2009,[8] Mader rejected Mihlbachler's revision and maintained that Pseuodiplacodon and Eotitanotherium were distinct, based on the separation of the lingual cusps on P4, the cross-sectional shape of the horns, and horn sizes. Mader also noted a new observation, that tooth rows tended to be shorter in Pseudodiplacodon, despite the Pseusodiplacodon being larger than Eotitanotherium. Mader considered the variation exhibited in Duchesneodus minor in comparison to that he observed between Pseudodiplacodon and Eotitanotherium, and also rejected Mihlbachler's synonymization of several genera into Megacerops.[7] In the 2011 description of D. gigan, Mihlbachler maintained his conclusion that the logical intepretation of the fossils was that purported differences between Pseudodiplacodon and Eotitanotherium were due to sexual dimorphism. Mihlbachler noted that the canine sizes of the fossils could be used to group the fossil assemblage into two clusters, most likely representing male and female specimens. If grouped in this way, it was apparent that the cross-sectional shape of the horns did not appear to vary consistently with either the size of the canines or the size of the horns, suggesting that Mader's Pseudodiplacodon and Eotitanotherium were not separable taxa.[9] In later research, other scholars have followed Mihlbachler's revision and treated specimens formerly assigned to Pseudodiplacodon and Eotitanotherium as specimens of D. elatus.[10][11]
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 in deposits of Uintan age. Both Matthew C. Mihlbachler in 2008 and Bryn J. Mader in 2009 preliminarily proposed that the skull was from a new brontothere species.[9] Mader described the specimen in 2009 and referred it to cf. Protitanotherium sp., based on similarities in size between AMNH 117163 and YPM PU 11242 (the Protitanotherium type), similarities in the cross-sectional shape of the horn, and similarities in the size of the canines.[8]
In 2011, Mihlbachler disagreed with Mader's assessment, 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.[9] Mihlbachler instead referred the specimen to Diplacodon, noting that AMNH 117163 had a unique morphology of the nasal bone, shared only with D. elatus. AMNH 117163 was distinct from other D. elatus specimens in several respects, notably its larger size, and that the dorsal (upper side) surface of the skull is constricted more by the parasagittal ridges (front-to-back bony ridges) than in skulls of D. elatus.[9] A phylogenetic analysis performed by Mihlbachler recovered AMNH 117163 as outside Protitanotherium and as the sister taxon of D. elatus.[9] Mihlbachler designated the skull as the type specimen of the new species Diplacodon gigan. The name gigan was derived from the fictional kaiju Gigan and references D. gigan's larger size when compared to D. elatus.[9]
Description
Restoration of the head of D. elatus
Diplacodon elatus was about the size of a modern rhinoceros,[12] small when compared to most other horned brontotheres.[1]D. elatus was for instance smaller than the closely related Protitanotherium and Protitan grangeri, but bigger than Protitan minor.[1]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 50–60.7 centimetres (19.7–23.9in), whereas the type skull of D. gigan measures 72 centimetres (28in).[9]
Like other brontotheres, Diplacodon had a saddle-shaped skull.[1]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).[1] 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.[1] The nasal process of Diplacodon was also elevated higher than in Protitan and Protitanotherium, but not as highly as in Aktautitan and Metatitan.[1]
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.[9] 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.[9]
Diplacodon elatus had the dental formula 3.1.4.33.1.4.3 .[2][9] The incisors of Diplacodon were small, but not as reduced as they were in more derived brontotheres (e.g. Megacerops).[9]
Per Mihlbachler's 2008 revision, Diplacodon is classified as part of the brontothere subtribe Brontotheriina, alongside the genera Pachytitan, Protitan, Protitanotherium, and Rhinotitan.[1] Brontotheriina also includes two infratribes of more derived brontotheres, Embolotheriita and Brontotheriita.[1]Diplacodon and similar more basal members of the group have sometimes been informally referred to as "diplacodonts", though this is not a formal taxon.[8]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.[9]
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.[9]
Topology A: Protitanotherium emarginatum sensu lato (per Mihlbachler)
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