Dichobune

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 of D. leporina in upper view, 1906 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 , who erected Dichobune in 1822 cf. gabineaudi'', an early dichobunine 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. In 2023, Abhay Rautela and Sunil Bajpai created an analysis on the phylogenetic relationships between basal artiodactyls by compiling a matrix of dental remains of 34 artiodactyl species; most of these artiodactyl species are dichobunoids (Diacodexeidae, Dichobunidae, Homacodontidae, Cebochoeridae, Leptochoeridae, Raoellidae), but some are members of the Pakicetidae and one other species is a member of the Helohyidae (the basal placental mammal Protungulatum is the outgroup taxon in the analysis). One clade pairs Dichobune with Homacodon, Buxobune, and Gobiohyus based on specific dental traits. Based on the cladogram, Rautela and Bajpai defined Diacodexis, the Diacodexeidae, and Dichobunidae as all polyphyletic taxa. In the case of the dichobunines, this is because they are more closely paired with non-dichobunids than with the lantianiines (Eolantianus, Elaschitotherium) and hyperdichobunines (Mouillacitherium). == Description ==

Skull The Dichobuninae is diagnosed as having slightly elongated snouts. Dichobune is typically defined in part by several cranial traits, among them a somewhat elongated skull with a rounded neurocranium, large and semi-centred orbits, well-developed mastoid parts of the temporal bones, and unossified tympanic parts of the temporal bones. Unlike with several other basal artiodactyls like Mouillacitherium, Cebochoerus, and Amphimeryx, the brain of Dichobune is slightly less simple due to an additional small groove on the neocortex, in which the suprasylvian sulcus (or suprasylvia) extends farther from the rhinal region. Among the traits making the fissuring of Dichobune more complex includes a visible presylvia fissure on the neocortex's upper side. This is the case for genera of the Dichobunidae like Dichobune, whose teeth are not much separated by diastemata and are bunodont (low and rounded cusps). Except for some of the oldest genera, dichobunids are also described as having molars (M/m) that generally have five to six tubercles (or cusps) each. Dichobune has been defined as both brachyodont (low-crowned) and bunodont in dentition, although D. leporina has higher crowns in comparison to its earlier relatives. In Dichobune, the lower incisors (I/i) are thin and sharp and the premolars are simple in form, the latter of which all have a paraconid cusp and metaconid cusp individually. The upper molars (M/m) are quadrangular in shape and have six bunodont tubercles (except for M3 with five of them). Three of the upper molar cusps are positioned in a mesially (centred) while three others are within distal positions. The buccal side cusps are crested. The paracone cusp is as large as the metacone cusp. The lower molars are made up of four cuspids, including two lingual ones that are globular in shape and one buccal ones that have slight crescent-shaped ridges developed on them. The metaconid cusp on them is large and usually swollen in its front area, and the entoconid is separate from the enamel ridges between it. The talonid basin region lacks a preentocristid crest on it. The early representative, D. aff. robertiana from the French localities of Aumelas and Saint-Martin-de-Londres, has primitive morphologies in the forms of simple-patterned premolars and smaller upper molars. While Dichobune (more specifically D. robertiana) shares dental traits with the basal dichobunid Messelobunodon, the two have distinct dentition to the point where the former may have not descended from the former, contrary to earlier hypotheses. The dental morphologies of Dichobune and Metriotherium suggest that the former genus split into at least two different evolutionary branches that existed by the Oligocene, with one potentially ending in D. jehennei and the other descending into Metriotherium. Schlosser also said that the middle metacarpals were about 70% as long as the middle metatarsal bones. The morphology of the foot bones of Dichobune were close in resemblance to those of other dichobunoids like Diacodexis and Messelobunodon, although today the former's postcranial fossils are only known from old illustrations and descriptions. Size The Dichobuninae is described as consisting of larger dichobunids; Dichobune in particular is recorded as being medium compared to its close relatives. Its close relatives and possible descendants Synaphodus and Metriotherium are diagnosed as being larger dichobunids in comparison. == Palaeobiology ==

The Dichobuninae is thought by researchers to have a very different morphology from its other dichobunoid relatives. This is evident by the dichobunines having stockier body builds like in carnivorans, traits known also in the European endemic families Cebochoeridae and Choeropotamidae; this, along with the dental morphologies, may imply more suid-like feeding habits as opposed to the Diacodexeidae. The dentitions of the dichobunines, according to Jessica M. Theodor et al., suggest omnivorous to herbivorous diets. Based on his studies on the dentitions of D. cf. robertiana and other early artiodactyls, Leonie C. Schwermann hypothesized that Dichobune and Gobiohyus are part of a cranial and dental morphotype in which the jaw's chewing movements (power stroke) would have been similar to the basal Diacodexis but differed by the facets on the hypocone that seemingly made chewing functions more efficient. The hypocone itself has no direct impact on how either genera chew, however. A frugivorous diet is assumed in Dichobune, meaning that it probably consumed higher proportions of the likes of fruit, seeds, and nuts. In comparison, leaves probably formed only minor components of its dietary habits. This type of feeding habit would have differed from that of another early dichobunid Messelobunodon, whose fossilized gut contents revealed that it had mixed diets consisting of fungi, seeds, and leaves. The crushing functions of the molars needed for frugivorous diets is supported by the more rounded cusps and the increased crushing efficiencies from the hypocones. == Palaeoecology ==

Early to Middle Eocene of Europe and Asia during the Middle Eocene with possible artiodactyl and perissodactyl dispersal routes. For much of the Eocene, a hothouse climate with humid, tropical environments with consistently high precipitations prevailed. Modern mammalian orders including the Perissodactyla, Artiodactyla, and Primates (or the suborder Euprimates) appeared already by the Early Eocene, diversifying rapidly and developing dentitions specialized for folivory. The omnivorous forms mostly either switched to folivorous diets or went extinct by the Middle Eocene (47–37 Ma) along with the archaic "condylarths". By the Late Eocene (approx. 37–33 mya), most of the ungulate form dentitions shifted from bunodont (or rounded) cusps to cutting ridges (i.e. lophs) for folivorous diets. Land connections between western Europe and North America were interrupted around 53 Ma. From the Early Eocene up until the Grande Coupure extinction event (56–33.9 mya), western Eurasia was separated into three landmasses: western Europe (an archipelago), Balkanatolia (in-between the Paratethys Sea of the north and the Neotethys Ocean of the south), and eastern Eurasia. The Holarctic mammalian faunas of western Europe were therefore mostly isolated from other landmasses including Greenland, Africa, and eastern Eurasia, allowing for endemism to develop. The earliest representative of Dichobune is D. aff. robertiana from the French localities of Aumelas and Saint-Martin-de-Londres; both of these localities are dated between MP10 and MP12. The MP11 unit records the likes of other members of the artiodactyl families Dichobunidae and "Diacodexeidae" along with the Hyaenodonta (Hyaenodontidae), Tillodontia (Esthonychidae), Pholidota (Eurotamanduidae), and Perissodactyla (Palaeotheriidae, Hyrachyidae, and Lophiodontidae). Specific genera recorded from Aumelas along with Dichobune include the sebecosuchian Iberosuchus, testudine Landreatchelys, amphilemurid Macrocranion, bat Stehlinia, hyaenodontids Matthodon and Leonhardtina, dichobunid Aumelasia, lophiodont Lophiodon, palaeotheres Propalaeotherium and Pachynolophus, and the herpetotheriid Amphiperatherium. It was after MP12 that a faunal turnover occurred, marking shifts in new ungulate faunas amidst cooling global temperates that occurred after the Early Eocene Climatic Optimum. D. robertiana is recorded from multiple localities dating to MP13 and D. cf. robertiana at MP14. The causes of the faunal turnover have been attributed to a shift from humid and highly tropical environments to drier and more temperate forests with open areas and more abrasive vegetation. The surviving herbivorous faunas shifted their dentitions and dietary strategies accordingly to adapt to abrasive and seasonal vegetation. The environments were still subhumid and full of subtropical evergreen forests, however. The Palaeotheriidae was the sole remaining European perissodactyl group, and frugivorous-folivorous or purely folivorous artiodactyls became the dominant group in western Europe. Late Eocene The Late Eocene unit MP18 records the appearances of two Dichobune species: D. leporina and D. fraasi. Both species extend beyond the Late Eocene, which ends at MP20-MP21. In addition, several migrant mammal groups had reached western Europe by MP17a-MP18, namely the Anthracotheriidae, Hyaenodontinae, and Amphicyonidae. The MP19 French locality of Escamps indicates that D. leporina coexisted with a variety of other mammals including the herpetotheriids Peratherium and Amphiperatherium, pseudorhyncocyonid Pseudorhyncocyon, nyctitheres Saturninia and Amphidozotherium, bats (Hipposideros, Vaylatsia Stehlinia), rodents (Glamys, Sciuroides, Paradelomys, Blainvillimys, Theridomys, and Patriotheridomys), omomyid Microchoerus, adapid Palaeolemur, hyainailourine Pterodon, amphicyonid Cynodictis, palaeotheres Palaeotherium and Plagiolophus, choeropotamid Choeropotamus, anoplotheriids (Anoplotherium, Diplobune, and Dacrytherium), cainothere Oxacron, amphimerycid Amphimeryx, and xiphodonts (Xiphodon, Dichodon, and Haplomeryx. The event is coincident with climate forcing events of cooler and more seasonal climates, the result being a 60% extinction rate of western European mammalian lineages while Asian faunal immigrants replaced them. The Grande Coupure is often marked by palaeontologists as part of the Eocene-Oligocene boundary as a result at 33.9 Ma, although some estimate that the event began 33.6-33.4 Ma. The event correlates directly with or after the Eocene-Oligocene transition, an abrupt shift from a greenhouse world characterizing much of the Paleogene to a coolhouse/icehouse world of the early Oligocene onwards. The massive drop in temperatures stems from the first major expansion of the Antarctic ice sheets that caused drastic pCO2 decreases and an estimated drop of ~ in sea level. The seaway dynamics separating western Europe from other landmasses to strong extents but allowing for some levels of dispersals prior to the Grande Coupure are complicated and contentious, but many palaeontologists agreed that glaciation and the resulting drops in sea level played major roles in the drying of the seaways previously acting as major barriers to eastern migrants from Balkanatolia and western Europe. The Turgai Strait is often proposed as the main European seaway barrier prior to the Grande Coupure, but some researchers challenged this perception recently, arguing that it completely receded already 37 Ma, long before the Eocene-Oligocene transition. Alexis Licht et al. suggested that the Grande Coupure could have possibly been synchronous with the Oi-1 glaciation (33.5 Ma), which records a decline in atmospheric CO2, boosting the Antarctic glaciation that already started by the Eocene-Oligocene transition. The Oi-1 glaciation, similar to the first glaciation event, caused large drops in sea level and pushed the global climate towards a coolhouse/icehouse environment. The extinctions of a majority of endemic artiodactyls have been attributed to competition with immigrant faunas, environmental changes from cooling climates, or some combination of the two. The Grande Coupure saw the extinctions of many artiodactyl genera previously endemic of Europe, including Anoplotherium and all representatives of "choeropotamids" (Amphirhagatherium, Choeropotamus) and xiphodontids (Xiphodon, Dichodon). Several ungulate genera like Palaeotherium and Acotherulum survived the Grande Coupure but nonetheless went extinct by MP21. Both D. leporina and D. fraasi are recorded at MP21 indicating their survivals, the former at Aubrelong 1 in France and the latter at Hoogbutsel in Belgium; D. leporina is not recorded in subsequent units, however. Two species are recorded from the Early Oligocene unit MP22, which also marks the last occurrence of Dichobune: D. fraasi and D. jehennei. Whereas the latest temporal appearance of D. fraasi is at the French locality of Valbro, D. jehennei has been uncovered from multiple localities within MP22 like Villebramar and La Plante 2. Metriotherium minutum, thought to have descended from a species of Dichobune, is also known to have coexisted with D. jehennei at Villebramar. Among other fossil mammals found there include the cricetid Atavocricetodon, theridomyids Blainvillimys and Elfomys, hyaenodont Hyaenodon, nimravids (Eofelis, Nimravus, and Quercylurus), palaeothere Plagiolophus, eggysodontid Eggysodon, rhinocerotid Ronzotherium, entelodont Entelodon, anthracothere Anthracotherium, gelocid Gelocus, and the lophiomerycid Lophiomeryx. == Notes ==