Dichobune is the type genus of the Dichobunoidea, an extinct paraphyletic superfamily consisting of some of the earliest artiodactyls known in the fossil record. It was a primitive artiodactyl genus that was endemic to western Europe and lived from the Middle Eocene (or possibly the Early Eocene) to the Early Oligocene. The type species Dichobune leporina was originally described as a species belonging to Anoplotherium beginning in 1804 by the French naturalist Georges Cuvier, who noted its small size. Cuvier assigned it to its own subgenus Dichobune in 1822; later naturalists promoted it to genus rank and observed that it was not close to the Anoplotheriidae as previously thought. Today, there are five valid species within Dichobune.
Dichobune has a somewhat elongated skull with a lengthy snout (with the snout of D. jehennei being particularly lengthy), large and semi-centred orbits, and a complete dentition of 44 teeth (the maximum in placental mammals), which mainly consists of brachyodont (low-crowned) and bunodont (round-cusped) cheek teeth. Its dental morphologies suggest that it could have had a frugivorous diet, meaning that leaves were probably only a minor component of its diet compared to the likes of fruits and seeds. Its foot morphology was primitive with unfused foot bones including a total of four digits each, only two middle ones of which are functional for didactyl (two-toed) movements. Dichobunoids were generally small mammals, especially in comparison to modern artiodactyls, but Dichobune was medium-sized in comparison to its close relatives. Earlier species of Dichobune were smaller-sized while some later species were larger, with the late-appearing D. jehennei being the largest species of the genus.
The European subfamily Dichobuninae made its appearance by the Early to Middle Eocene, with D. aff. robertiana being among the earliest representatives. During much of its existence, western Europe was an archipelago that was isolated from the rest of Eurasia, meaning that Dichobune lived in a tropical-subtropical environment with various other animals that also evolved with strong levels of endemism. It survived multiple faunal turnover events within Europe, including the large Grande Coupure extinction event that drove many of its close relatives to extinction and replaced them with immigrant faunas from eastern Eurasia. Its survival into the Early Oligocene was not particularly long, but it likely descended into Metriotherium, a dichobunid that briefly coexisted with it and lasted up to the Late Oligocene.
Taxonomy Research history Early history
In 1804, the French naturalist Georges Cuvier named an artiodactyl genus Anoplotherium, which he referred multiple new species from Montmartre to. Among the species was Anoplotherium minus, which was known by an astragalus implying that its foot was comparable in size to that of a hare. The next year in 1805, Cuvier noted a lower jaw that he assigned to A. minus and described hare-sized postcranial material (forefoot and hindfoot bones) implying that it had long legs and short, tetradactyl (four-toed) feet that made it differ from the didactyl (two-toed) feet of A. commune. In 1807, he assigned additional postcranial material to it, namely a tibia, humerus, radius, and ulna. In 1812, he redescribed the lower jaw and wrote about additional postcranial material assigned to A. minus (i.e. a tibia and calcaneum) and proposed behaviors of the different species based on their sizes and anatomies; he suggested that A. medium was to a roe deer what A. minus was to a hare but also suggested that the two species shared the same terrestrial gracility.
In 1822, Cuvier again referenced the species for being smaller than A. gracile (formerly A. medium). He said that its head was smaller than that of a fox but bigger than that of a hare, possibly equal to that of a raccoon. He provided it the "provisional" name A. leporinum, replacing the previous name A. minus. Cuvier created the Anoplotherium subgenus Dichobune based on the "hill" (or cusp) pair arrangements on its four molars, assigning A. leporinum, A. murinum, and A. obliquum to it. The etymology of the name Dichobune is derived from the Ancient Greek words (two) and (hill, usually referencing rounded cusps), referencing the paired ridge arrangements on its back molars.
In 1841, the British naturalist Richard Owen, treating Dichobune as a subgenus of Anoplotherium, established the species D. cervinum from a lower jaw from the Isle of Wight in the United Kingdom. It was later in an 1848–1852 work that the French naturalist Paul Gervais validated Dichobune as a genus that was distinct from Anoplotherium, with the taxon Cainotherium being reranked as a subgenus of the former. Gervais considered D. leporinum, D. cervinum, D. murinum, and D? obliquum to all be valid species but suggested that the latter species be transferred into another genus or subgenus. He additionally erected D. suillum based on fossils found in limestone deposits from the French localities of Passy and Nanterre. In a second volume of the same study, he considered Cainotherium to instead be a distinct genus and erected another species D. robertianum based on a dental fossil from the limestone deposits of Nanterre, naming it after a geologist named M. E. Robert who discovered it there. He followed up by erecting Amphimeryx for the species D. murinum and questioned the placement of D? suillum. In 1855, the researchers François Jules Pictet de la Rive, Charles-Théophile Gaudin, and Philippe de La Harpe listed in their illustrated figures of fossils the name D. Campichii, credited solely to Pictet. Owen in 1857 supported Dichobune being a valid genus and created another species D. ovina using dental fossils that he felt were similar enough to D. leporina (emended from D. leporinum).
In 1862, Swiss palaeontologist Ludwig Ruetimeyer hypothesized that Anoplotherium secundarium was a transitional species to Dichobune based on dental morphology and established the subgenus Diplobune under Dichobune. He also erected the species D. mülleri based on additional dental fossils. The British zoology lecturer Charles Carter Blake in 1863 erected the genus Didymodon and its only species Didymodon Vauclusianum using a dental specimen from a fossil collection in the Natural History Museum in London, arguing that the molars' forms closely resembled that of Dichobune but differed from all known fossil artiodactyl genera based on specific dental anatomies. He explained that the genus name derived from (twofold) and (tooth) while the species name derived from the French department of Vaucluse where the specimen originated from. In 1870, German palaeontologist Oscar Fraas argued that Dichobune had no evolutionary relationship with the Anoplotheriidae, then recognizing the anoplotheriid Diplobune as a distinct genus. In 1885, British naturalist Richard Lydekker emended Dichobune to Dichobunus, making Didymodon a synonym of it; he also listed Anoplotherium minus and Didymodon vauclusianus as synonyms of D. leporinus and referenced D. robertiana as being the smaller species of the genus. Lydekker, furthermore, reclassified D. ovinus into Dacrytherium and D. cervinus into Dichodon. In 1891, Ruetimeyer, using the name "Dichobune", recognized D. leporinum, D. Robertianum and D. Mülleri as valid species did not discuss the validity of D. Suillum. He additionally erected the species D. langii and D. pygmaea using additional dental material from the Swiss locality of Egerkingen.
Late history
In 1902, German palaeontologist Max Schlosser described an upper jaw from mineral deposits in the German locality of Eselsberg that was held in State Museum of Natural History Stuttgart, comparing it in size to that of D. Campichi. Based on dental differences, he erected the species D. Fraasi. Later in 1906, Swiss palaeontologist Hans Georg Stehlin validated the taxonomic statuses and placements of D. leporina, D. robertiana, and D. Langi. Stehlin also studied dental fossils from the French phosphorite deposits of Caylux, assigning them to another named subspecies D. leporina major (or D. leporina var. major). He also erected two species: D. nobilis, basing it off of a maxilla fragment with molars from Egerkingen; and D. spinifera using a partial maxilla from Mormont in the Natural History Museum of Basel. He also suggested that D. Mülleri should be reclassified to a different genus. In 1908, Stehlin transferred "D." mülleri into Haplobunodon and tentatively reclassified both "D." Campichii and "D." suillus into Cebochoerus. He then followed up by synonymizing D. pygmaea with Pseudamphimeryx schlosseri and reclassified both D. nobilis and D. spinifera into their own genus Hyperdichobune in 1910. Stehlin also provisionally reclassified "D." obliquus into Haplomeryx.
In 1972, French palaeontologist Jean Sudre relisted "D." langi as a species of Hyperdichobune. He later erected D. sigei in 1978, having named it after fellow palaeontologist Bernard Sigé and designated its holotype based on an upper molar from the French locality of Lavergne. He also classified within Cebochoerus siullus the subgenus Gervachoerus; Gervachoerus has later been considered to be a distinct cebochoerid genus.
In 1980, Michel Brunet and Sudre studied a nearly complete skull from the French commune of Villebramar that dated to the Early Oligocene and was held at a fossil collection at the University of Poitiers. They designated the name D. jehennei to it, deriving its etymology after Yves Jehenne, who was a major contributor to fossil collections from Villebramar. In 1986, British palaeontologist Jerry J. Hooker reclassified "Cebochoerus" campichii into another cebochoerid genus Acotherulum. In addition to European specimens designated as Dichobune sp., one other from the Lushi Province of the Chinese province of Henan has been assigned the same name. A lower jaw from the Heti Formation of Henan that was previously assigned to ?Dichobune sp. has since been reassigned to another artiodactyl genus Limeryx.
Classification
Dichobune is the type genus of the Dichobunidae, an extinct early artiodactyl family within the superfamily Dichobunoidea. The Dichobunoidea is a paraphyletic group of basal artiodactyls appearing in the Early Eocene that gave way to various other artiodactyl clades, extant and extinct. The Dichobunoidea is considered by researchers to consist of seven families: Cebochoeridae, Diacodexeidae, Dichobunidae, Helohyidae, Homacodontidae, Leptochoeridae, and Raoellidae (although not all researchers agree that the Raoellidae is a dichobunoid family as referenced by Abhay Rautela & Sunil Bajpai in 2023). Despite the consensus that the Dichobunoidea is a paraphyletic group, researchers are still investigating the extent to which certain members are stem taxa to other major artiodactyl clades. At least some dichobunoid families are thought to be monophyletic while others are considered to be paraphyletic or polyphyletic; this means some clades require further reassessment.