Dichodon is an extinct genus of Palaeogene artiodactyls belonging to the family Xiphodontidae. It was endemic to Western Europe and lived from the middle Eocene up to the earliest Oligocene. The genus was first erected by the British naturalist Richard Owen in 1848 based on dental remains from the fossil beds in Hordle, England. He noticed similar dentitions to contemporary artiodactyls like those of the Anoplotheriidae and Dichobunidae and references the name of the genus Dichobune. Eventually, it was found to be more closely related to Xiphodon and now includes 11 species, although one of them may be synonymous.
Dichodon had brachyodont (low-crowned) dentition, its premolars being elongated similar to other xiphodonts. However, it differs from them by the generally stronger but varied degrees of elongation of the premolars and "molarization" of the fourth premolars, in which the earliest species had triangular top fourth premolars while later species had quadrangular ones. Its snout is also shorter and narrower compared to that of Xiphodon. The different morphologies of the two genera suggest different dietary specializations of folivory (leaf-eating), but the postcranial morphology of Dichodon remains poorly known compared to that of Xiphodon.
Dichodon lived in western Europe when it was an archipelago that was isolated from the rest of Eurasia, meaning that it lived in a tropical-subtropical environment with various other animals that also evolved with strong levels of endemism. The genus was speciose, composed of many small-sized species as well as medium-sized ones. D. cuspidatum and D. stehlini were especially large but are known only from single fossil localities. The small-sized D. frohnstettensis and the medium-sized D. cervinum, in comparison, frequently occur in many localities dating from the late middle to late Eocene.
It and other xiphodont genera went extinct by the Grande Coupure extinction/faunal turnover event, coinciding with shifts towards further glaciation and seasonality plus dispersals of Asian immigrant faunas into western Europe. The causes of its extinction are attributed to negative interactions with immigrant faunas (resource competition, predation), environmental turnover from climate change, or some combination of the two.
Taxonomy Research history Early history
In 1848, after having recognized ungulates as a taxonomic group defined by the Artiodactyla and Perissodactyla, British naturalist Richard Owen erected the genus Dichodon based on its "peculiar" dentition, classifying it as a member of the former. The dental and cranial fossils of Dichodon were uncovered were from the Eocene beds of Hordle, England by Alexander Pytts Falconer. Owen said that the dentition of Dichodon resembled those of both Merycopotamus and Dichobune because of the similar upper and lower jaws but also argued that the molars resembled those of anoplotheriids. Deriving it from the quantity and sharpness of the cusps of the teeth, he erected the binomial name Dichodon cuspidatus. The etymology of the genus name Dichodon is derived from the Ancient Greek words (two) and (tooth) in reference to the genus Dichobune, due to having similar molar mounds. Owen in 1857 then recorded that the fossils of Dichodon that he previously described from 1857-1858 were from an immature individual with milk teeth for a total of 32 teeth while the adult dentition based on fossils collected near Alum Bay in the Isle of Wight by a "Dr. Wright" had a complete dental set of 44 teeth.
In 1852, German palaeontologist Christian Erich Hermann von Meyer, writing to his colleague Heinrich Georg Bronn, told of fossils of Dichodon from the locality of Frohnstetten whose dentition did not resemble that of the species D. cuspidatus. He determined based on its molars that it was therefore a new species, which he named D. Frohnstettensis.
In an 1874 monograph published in 1876, Russian palaeontologist Vladimir Kovalevsky recognized three valid species of Dichodon: D. cuspidatus, D. Valdense, and D. Frohnstettense. Kovalevsky apparently did not specify the attributed fossils and etymology of D. Valdense, but Swiss palaeontologist Hans Georg Stehlin in 1910 suggested that Kovalevsky based the species on fossils previously described, but not named, by François Jules Pictet de la Rive. He stated that there was a small-sized species from the Swiss locality of Egerkingen, that it was smaller than D. Frohnstettense and that it would have been roughly the size of Cainotherium. Deciding not to establish a new genus because of incomplete material, he assigned to Dichodon the species D. simplex based on the simplicity of the premolars. The same year, British naturalist William Henry Flower expressed doubt regarding whether Dichodon was distinct enough from Xiphodon based on the different last premolar morphologies.
In 1885, British naturalist Richard Lydekker taxonomically reviewed Dichodon and other artiodactyls. He confirmed that it had a complete dental formula, selenodont molars, and elongated premolars like Xiphodon but also noted that its limb anatomy was unknown. He referenced two species but did not give mention to the others: D. cuspidatus and D. cervinus, the latter of which was previously erected and classified to the genus Dichobune by Owen in 1841. German palaeontologist Max Schlosser established the binomial name Tetraselenodon Kowalevskii based on fossils from the French department of Tarn-et-Garonne in 1886. He justified the genus by arguing that Pictet incorrectly referred its fossil material to Dichodon due to the dentition being simple-looking in form.
The Swiss palaeontologist Ludwig Ruetimeyer in 1891 described another species from Egerkingen whose fossil remains were smaller than those of D. cuspidatus. He stated that the upper jaw molar row of the newer species measured to in length while its lower jaw molar row length measured , in contrast to D. cuspidatus with an upper molar row length of and a lower molar row length of . Ruetimeyer assigned it the species name D. Cartieri.
Later revisions
For his 1910 monograph on artiodactyls, Stehlin, as part of his work in reaffirming Catodontherium as a valid genus, said that D. valdense, despite being an older name than C. robiacense, may not have been clearly defined. He also supported the validities of the other species D. cuspidatum, D. cervinum, D. frohnstettense, D. simplex, and D. cartieri. The Swiss palaeontologist additionally erected two species of Dichodon: the first was D. subtile from the Swiss locality of Mormont, which he said was a small species differing from others by the elongation and narrowing of the premolars. The second that he recognized was D. Rütimeyeri from Egerkingen, which he said was about the same size as D. Cartieri. He also synonymized Tetraselenodon with Dichodon and invalidated T. Kowalevskyi because of the dentition's similarity to that of D. cervinum.
In 1972, the French palaeontologist Jean Sudre erected D. lugdunensis, another small-sized species, based on dentition from the French locality of Lissieu. He said that the new species would have been part of a different lineage from that of D. cartieri plus that it was larger than D. simplex. He also confirmed that T. kowalevski is a synonym of D. cervinum. The next year, Sudre named another species D. stehlini from a large-sized molar originally from the locality of La Débruge in France. All species of Dichodon previously recognized as valid since Stehlin's 1910 revisions were listed by Jerry J. Hooker in 1986, although he emended D. subtile to D. subtilis and D. frohnstettense to D. frohnstettensis out of correcting naming incongruencies.
In 1988, Sudre established another species named D. vidalenci based on isolated teeth from Le Bretou in France, which he noted had very elongated premolars, and listed Dichodon sp. based on isolated short premolars. Hooker and Marc Weidmann in 2000 listed D. vidalenci as a possible synonym of D. subtilis but otherwise listed all other species except for D. stehlini. In addition, they erected the medium-sized species D. biroi from the Swiss municipality of Éclépens, establishing that they named the species after Philippe Biro because he collected the dental holotype specimens in 1946.
Classification
Dichodon belongs to the Xiphodontidae, a Palaeogene artiodactyl family endemic to western Europe that lived from the middle Eocene to the early Oligocene (~44 Ma to 33 Ma). It was suggested to have been a monotypic member of its own family, the Dichodontidae, by the American palaeontologist Edward Drinker Cope in 1889, although this is not accepted by modern authors. Like the other contemporary endemic artiodactyl families of western Europe, the evolutionary origins of the Xiphodontidae are poorly known. The Xiphodontidae is generally thought to have first appeared by MP14 faunal unit of the Mammal Palaeogene zones, making them the first representatives of artiodactyls with selenodont dentition to have appeared in the landmass along with the Amphimerycidae. More specifically, the first xiphodonts to appear were the genera Dichodon and Haplomeryx by MP14. Dichodon and Haplomeryx continued to persist into the late Eocene while Xiphodon made its first appearance by MP16. Another xiphodont, Paraxiphodon, is known to have occurred only in MP17a localities. The former three genera lived up to the early Oligocene where they have been recorded to have all gone extinct as a result of the Grande Coupure faunal turnover event.
The phylogenetic relations of the Xiphodontidae as well as the Anoplotheriidae, Mixtotheriidae and Cainotheriidae have been elusive due to the selenodont morphologies (or having crescent-shaped ridges) of the molars, which were convergent with tylopods or ruminants. Some researchers considered the selenodont families Anoplotheriidae, Xiphodontidae, and Cainotheriidae to be within Tylopoda due to postcranial features that were similar to the tylopods from North America in the Palaeogene. Other researchers consider them more closely related to ruminants than tylopods based on dental morphology. Different phylogenetic analyses have produced different results for the "derived" (or of new evolutionary traits) selenodont Eocene European artiodactyl families, making it uncertain whether they were closer to the Tylopoda or Ruminantia. Possibly, the Xiphodontidae may have arisen from an unknown dichobunoid group, thus making its resemblance to tylopods an instance of convergent evolution.
In an article published in 2019, Romain Weppe et al. conducted a phylogenetic analysis on the Cainotherioidea within the Artiodactyla based on mandibular and dental characteristics, specifically in terms of relationships with artiodactyls of the Palaeogene. The results retrieved that the superfamily was closely related to the Mixtotheriidae and Anoplotheriidae. They determined that the Cainotheriidae, Robiacinidae, Anoplotheriidae, and Mixtotheriidae formed a clade that was the sister group to the Ruminantia while Tylopoda, along with the Amphimerycidae and Xiphodontidae split earlier in the tree (the latter family is represented only by Xiphodon in the cladogram). The phylogenetic tree published in the article and another work about the cainotherioids is outlined below: