Diplobune (Ancient Greek: (double) + (hill) meaning "double hill") is an extinct genus of Palaeogene artiodactyls belonging to the family Anoplotheriidae. It was endemic to Europe and lived from the late Eocene to the early Oligocene. The genus was first erected as a subgenus of Dichobune by Ludwig Rütimeyer in 1862 based on his hypothesis of the taxon being a transitional form between "Anoplotherium" secundaria, previously erected by Georges Cuvier in 1822, and Dichobune. He based the genus etymology off of the two-pointed pillarlike shapes of the lower molars, which had since been a diagnosis of it. However, in 1870, Diplobune was elevated to genus rank by Oscar Fraas, who recognized that Diplobune was a distinct genus related to Anoplotherium and not Dichobune. After several revisions of the anoplotheriids, there are currently four known species of which D. minor is the type species.
Diplobune was an evolutionarily derived medium to large-sized anoplotheriid with shared similarities to the sister taxon Anoplotherium; the differences mainly consisting of all species having specialized three-fingered limbs and various specific dental, postcranial, and brain anatomy differences. It was well-adapted for purely folivorous diets, with dentition capable of chewing through hard leaf material and an implied presence of tapered tongues for reaching branches similar to modern-day giraffids. Its limbs were very specialized of which there are no modern analogues, especially in artiodactyls, with implied powerful muscles for some extent of mobility in the form of bending its fingers, especially its left, shortest finger (finger II).
Such unique traits along with hints of slow-walking locomotion suggest a life of arborealism or semi-arborealism, where it was likely able to grasp onto hard objects for climbing them. These traits would have set it apart in lifestyle from Anoplotherium, the Palaeotheriidae, and most other mammals that it coexisted with. Although the sizes of several species are not described, D. secundaria of the late Eocene was estimated to weigh approximately and measure about in length and in shoulder height, whereas D. minor of the early Oligocene was much smaller with estimated weights of .
The evolutionary history of Diplobune is not complete, but it lived in western Europe back when it was an archipelago that was isolated from the rest of Eurasia, meaning that it lived in an environment with various other faunas that also evolved with strong levels of endemism. It, like Anoplotherium, arose long after a shift towards drier but still subhumid conditions that led to abrasive plants and the extinctions of the large-sized Lophiodontidae, becoming a regular component of late Eocene faunal communities. It survived through the Grande Coupure extinction event of western Europe in the earliest Oligocene but seemingly lost at least one species in the process. D. minor appeared in the early Oligocene as likely the last representative of the Anoplotheriidae, leaning towards specialization in forested, subhumid environments with freshwater bodies.
Taxonomy Early history
In 1862, Swiss palaeontologist Ludwig Rütimeyer discussed his hypothesis that Anoplotherium secundarium was a transitional species to the genus Dichobune. He noticed that the inner mounds of the molars of the studied species were distinctly bicuspid, the tips not being in equal size. Because of the molar morphologies being similar to those of both Dichobune and Anoplotherium, he created the Dichobune subgenus Diplobune, thinking that it was the subgenus that derived species of Dichobune descended from. Rütimeyer did not elaborate on which species belonged to the subgenus, however. While the etymology of Diplobune was not defined by Rütimeyer, it derives in Greek from "" (double) and "" (hill, usually referencing rounded cusps), meaning that the etymology of the genus name is "double hill."
However, the status of Diplobune as a subgenus of Dichobune did not last long. In 1870, German palaeontologist Oscar Fraas wrote about a mammal with numerous remains from the locality of Munich, its molars being similar but not identical to A. commune in terms of typical species diagnoses. He noticed the bicuspid characteristic and assigned the fossil materials to Diplobune. He also wrote that based on its dentition, Dichobune had no evolutionary relationship with the anoplotheriids, making Diplobune a distinct genus. Although Fraas was the sole author of his article, he credited his colleague Karl Alfred von Zittel for the name D. bavaricum, which the specimens belonged to.
Several species now attributed to Diplobune were historically not included in the genus initially due to either being erected far before the genus itself (Anoplotherium secundarium by Georges Cuvier in 1822) or after. In 1877, Henri Filhol reassigned A. secundarium to the genus Eurytherium on the genus diagnosis that it has three toes. He also made three species for the genus mostly based on dentition: E. modicum, E. quercyi, and E. minus. Filhol also argued that Eurytherium took priority over Diplobune because he thought that the three-toed diagnosis takes priority over dental diagnoses.
Several genus names that would eventually become synonyms of Diplobune were created in the late 19th century. In 1876, Paul Gervais named a newer genus Thylacomorphus based on skull that he thought to belong to an animal closely related to thylacines, the type species being T. cristatus. Thylacomorphus materials were later referred to the hyaenodont Cynohyaenodon by Max Schlosser. However, in 1901, William Diller Matthew determined that the back of the skull actually belonged to Diplobune quercyi.
In the same year, Filhol created a genus "Hyracodon" ("hyrax tooth") after he noticed that the species' teeth, originating from the locality of Caylux in Quercy, were similar to those of hyraxes, creating the type species H. primavus. Filhol was apparently unaware that the genus name "Hyracodon" was already reserved by American palaeontologist Joseph Leidy in 1856 for a rhinocerotoid initially, later changing the genus name to Hyracodontherium in 1877. Filhol named another species H. crassum in 1882, and a third species H. filholi was named by Richard Lydekker in 1889. The genus name was eventually synonymized with Diplobune by a palaeontological textbook in 1925 and Johannes Hürzeler in 1938, the latter noticing the similarities of the type species to the older genus. Hürzeler reclassified H. filholi to the new anoplotheriid genus Ephelcomenus in 1938 and discussed about being unsure of the status of "H. crassum" since it was based on a fragment of a mandible that was only briefly described and not illustrated. "H. crassum" and "H. primaevum" were not directly invalidated in 1938 but have not been recognized as valid species since.
Anoplotheriidae revisions
In 1883, Max Schlosser made Eurytherium a synonym of Anoplotherium because he argued that the limb anatomies and dentitions were specific differences in characteristics rather than major ones that defined an entire genus. Sclosser pointed out that all species of Anoplotherium in some form had three indexes despite A. commune having less developed third indexes than A. latipes. As a result, Diplobune became the senior synonym for the species D. bavaricum, D. modicum, D. quercyi, and D. minus. He also mentioned that D. modicum was similar to D. bavaricum with the differences lying in specific dental characteristics. Schlosser noticed that A. secundarium appeared to have been related to Diplobune that he considered it "almost advisable" to reclassify it to the genus.
In 1885, Lydekker synonymized Diplobune with Anoplotherium because he felt that differences in the lower dentition were not worthy of generic distinctions, transferring species of the former genus to the latter genus. He also named a new species A. cayluxense. The demotion of Diplobune as a synonym of Anoplotherium was not followed by subsequent authors, however, as they recognized it as a valid genus. Also, von Zittel synonymized Plesidacrytherium and Mixtotherium with Diplobune in 1891–1893, but Hans Georg Stehlin in 1910 instead synonymized Plesidacrytherium with Dacrytherium while revalidating Mixtotherium.
The same year that he touched upon the two other genus names, Stehlin also argued that Diplobune was morphologically similar to Anoplotherium but were generically distinct from each other. The German palaeontologist reclassified Anoplotherium secundaria to Diplobune plus synonymized D. modicum/D. modica with D. bavarica and A. cayluxense with D. secundaria. The synonymy of "A. secundaria" with D. secundaria was agreed upon by Marcellin Boule and Jean Piveteau in 1935.
Classification
Diplobune belongs to the Anoplotheriidae, a Palaeogene artiodactyl family endemic to western Europe that lived from the middle Eocene to the early Oligocene (~44 to 30 Ma, possible earliest record at ~48 Ma). The type species of the genus is D. minor, first described long after the genus name was first created. The exact evolutionary origins and dispersals of the anoplotheriids are uncertain, but they exclusively resided within the continent when it was an archipelago that was isolated by seaway barriers from other regions such as Balkanatolia and the rest of eastern Eurasia. The Anoplotheriidae's relations with other members of the Artiodactyla are not well-resolved, with some determining it to be either a tylopod (which includes camelids and merycoidodonts of the Palaeogene) or a close relative to the infraorder and some others believing that it may have been closer to the Ruminantia (which includes tragulids and other close Palaeogene relatives).
The Anoplotheriidae consists of two subfamilies, the Dacrytheriinae and Anoplotheriinae, the latter of which is the younger subfamily that Diplobune belongs to. The Dacrytheriinae is the older subfamily of the two that first appeared in the middle Eocene (since the Mammal Palaeogene zones unit MP13, possibly up to MP10), although some authors consider them to be a separate family in the form of the Dacrytheriidae. Anoplotheriines made their first appearances by the late Eocene (MP15-MP16), or ~41-40 Ma, within western Europe with Duerotherium and Robiatherium. By MP17a-MP17b, however, there is a notable gap in the fossil record of anoplotheriines overall as the former two genera seemingly made their last appearances by the previous MP level MP16.
By MP18, Anoplotherium and Diplobune made their first appearances in western Europe, but their exact origins are unknown. The two genera were widespread throughout western Europe based on abundant fossil evidence spanning from Portugal, Spain, United Kingdom, France, Germany, and Switzerland for much of pre-Grande Coupure Europe (prior to MP21), meaning that they were typical elements of the late Eocene up until the earliest Oligocene. The earlier anoplotheriines are considered to be smaller species whereas the later anoplotheriines were larger. Not all species of Diplobune were medium to large-sized however, as at least D. minor is known for having small weight estimates. Anoplotherium and Diplobune are considered the most derived (or evolutionarily recent) anoplotheriids based on dental morphology and achieved gigantism amongst non-whippomorph artiodactyls, making them some of the largest non-whippomorph artiodactyls of the Palaeogene as well as amongst the largest mammals to roam western Europe at the time (all species of Anoplotherium were large to very large whereas not all species of Diplobune were large).