Xiphodon (from Ancient Greek ξίφος (xíphos), meaning "sword", and ὀδούς (odoús), meaning "tooth") is the type genus of the extinct Palaeogene artiodactyl family Xiphodontidae. It, like other xiphodonts, was endemic to Western Europe and lived from the Middle Eocene up to the earliest Oligocene. Fossils from Montmartre in Paris, France that belonged to X. gracilis were first described by the French naturalist Georges Cuvier in 1804. Although he assigned the species to Anoplotherium, he recognized that it differed from A. commune by its dentition and limb bones, later moving it to its own subgenus in 1822. Xiphodon was promoted to genus rank by other naturalists in later decades. It is today defined by the type species X. gracilis and two other species, X. castrensis and X. intermedium. Xiphodon had specialized bladelike selenodont dentition, with its brachyodont (low-crowned) incisors, canines, and premolars having sharp edges for cutting through higher vegetation such as leaves and shrubs. It also retained primitive molars compared to its relative Dichodon, indicating different dietary specializations. Xiphodon is also the only xiphodontid to be known from postcranial fossils. Its skull morphology, combined with slender and elongated limbs, suggest similar behaviours to North American Palaeogene camelids such as Poebrotherium, including cursoriality (running adaptations). However, the full extent of its behaviour and evolutionary relationships remain uncertain, and its resemblances to camelids are probably an instance of convergent evolution. Xiphodon 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 endemic faunas. The xiphodont made its first appearance in the Middle Eocene shortly before a shift towards drier but still subhumid conditions, which led to increasingly abrasive plants. Species of Xiphodon were relatively small with the second-appearing species X. intermedium having an estimated weight of 4.6 kg (10 lb). X. gracilis was the last and largest species within the genus in an evolutionary size increase trend. 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 1804, the French naturalist Georges Cuvier established multiple fossil species as belonging to the genus Anoplotherium other than A. commune. One of the species he named was A. medium, which he said had slender, elongated, and didactyl (two-toed) feet. He thought that Anoplotherium had didactyl hooves instead of tridactyl (three-toed) hooves, which would have separated it from the other "pachyderm" Palaeotherium. Based on the hooves and dentition, he concluded that Anoplotherium was similar to ruminants or camelids. In 1807, Cuvier gave further elaboration to his thoughts on the limb bones, suggesting that it superficially resembles those of llamas. He explained that the third phalanx of A. medium differed from those of llamas by its slightly larger proportions. He put forward his argument that because its third phalanx more closely resembled those of ruminants, it was more closely related to the mammal group than A. commune was to them. Cuvier also said that other postcranial morphologies of the femoral head and tibia more closely resembled those of ruminants than those of camels. He attributed damaged lumbar vertebrae to A. medium in 1808.
Cuvier published his drawings of skeletal reconstructions of two species of Anoplotherium in 1812 based on known fossil remains including A. medium. He noted that he had no evidence for torso or tail bones of A. medium but that he had fossils of its skull, neck, tibia, and tarsus bone, adding to the hind foot evidence that he described years prior. He stated that in contrast to the more robust A. commune, A. medium was more gracile in form and therefore would have been built for cursoriality similar to extant ungulates such as gazelles or roe deer. He hypothesized, therefore, that unlike A. commune which he thought had semi-aquatic habits, A. medium could not have lived in marshes or ponds. Instead, he said, it would have grazed on herbs and shrubs on dry lands and had more "timid" behaviours not unlike gracile ruminants. Cuvier also proposed that it probably did not have a long tail unlike A. commune and that it had mobile ears like deer for hearing danger in advance. A. medium, according to the naturalist, had short fur and probably did not ruminate. In 1822, Cuvier established the subgenus Xiphodon for the genus Anoplotherium and changed the species name Anoplotherium medium to Xiphodon gracile because he felt that it was a more fitting species name. He argued that the species has a head roughly the shape plus shape of the "corinne" (an archaic term for the dorcas gazelle) with sharp snouts and differs from A. commune on the basis of long and sharp molars. However, he also suggested that the two species do not differ on the genus level. It alongside other Paris Basin fossil species were depicted in 1822 drawings by the French palaeontologist Charles Léopold Laurillard under the direction of Cuvier, although the restorations were not as detailed as Cuvier's. The genus name Xiphodon means "sword tooth" and is a compound of the Ancient Greek words ξίφος (xiphos, 'sword') and ὀδούς (odoús, 'tooth').
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