Xiphodon (from Ancient Greek (), meaning "sword", and (), 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 . 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 (, 'sword') and (, 'tooth').
In an 1848–1852 work, the French naturalist Paul Gervais affirmed that Xiphodon was a distinct genus from Anoplotherium. He similarly conveyed that X. gracile was slender like antelopes but was slightly smaller than dorcas gazelles. He erected the second species X. gelyense from the French commune of Saint-Gély-du-Fesc. He also reclassified Hyopotamus (= Bothriodon) crispus into Xiphodon. The validity of Xiphodon as a genus was also supported by the British naturalist Richard Owen the same year, who also erected Dichodon. Owen emended the species X. gracile and X. gelyense to X. gracilis and X. gelyensis, respectively in 1857.
X. gracilis was amongst the fossil taxa depicted in the Crystal Palace Dinosaurs assemblage in the Crystal Palace Park in the United Kingdom, open to the public since 1854 and constructed by English sculptor Benjamin Waterhouse Hawkins. Benjamin apparently either refused to acknowledge the genus name or was unaware of it, meaning that sculptures of the species were referred to as "A. gracile". The extant sculptures of A. commune were historically confused with "A. gracile", the result of both species having been listed in the earliest Crystal Palace guidebooks. An illustration of Hawkins' workshop reveals that four sculptures representing "A. gracile" were constructed by him, three of which vanished without any traces.
The fourth sculpture was mistaken as a Megaloceros giganteus fawn and was associated with the Megaloceros sculptures for an unknown amount of time. The sole surviving sculpture measures long from the snout to the tail and has a llama-like appearance given its long neck, small head, large eyes, robust body, camel-like nose, branched lips, and a narrow snout. The sculpture's appearance overall matches up with Cuvier's anatomical description of the species, the main inaccuracy being the reconstruction of additional small digits similar to A. commune. Its design and intended representation as a herd were likely inspired by South American llama appearances and behaviours. The illustration of Hawkins' workshop implies that the Xiphodon gracilis sculptures were intended to represent a relaxed herd.
Additional species and synonyms
In 1873, Vladimir Kovalevsky rejected Gervais' reclassification of Hyopotamus crispus (= Elomeryx crispus) into Xiphodon. In 1876, British naturalist William Henry Flower expressed being unsure whether Dichodon was distinct enough from Xiphodon. As he disliked the concept of having multiple closely related genera, he chose to place in Xiphodon the newly erected species X. platyceps. The same year, Kovalevsky erected a newly determined smaller species that he named X. castrense after the French commune of Castres. He also stated that its sharp premolars justified the genus etymology "sword tooth". Gervais erected another species that he tentatively assigned to Xiphodon the same year as well, naming it X? tragulinum. In 1884, the French naturalist Henri Filhol erected the species X. magnum based on a lower jaw fossil, arguing that the species was larger than X. gracilis.
The British naturalist Richard Lydekker reviewed the known species of Dichodon and Xiphodon in 1885, confirming that both are distinct genera. He also reaffirmed the validities of both X. gracilis and X. gelyensis then synonymized Xiphodontherium, erected previously by Filhol in 1877, with Xiphodon, thus reclassifying Xiphodontherium secundarius into Xiphodon. He also suggested that Xiphodon platyceps may be synonymous with Dacrytherium ovinum. He did not reference X. castrense in his catalogue. In 1886, the German palaeontologist Max Schlosser transferred "X. gelyense" into the newer genus Phaneromeryx.
In 1910, the Swiss palaeontologist Hans Georg Stehlin synonymized Xiphodontherium with Amphimeryx, also making X. primaevum and X. secundarium synonymous with A. murinus in the process. He stated that X. platyceps was most likely synonymous with Dichodon cuspidatum, considered X? tragulinum to be a dubious name, and expressed doubt that X. magnum if valid truly belongs to Xiphodon. He also created the species X. intermedium based on dental measurements intermediate between the smaller X. castrense and the larger X. gracile.
In 2000, Jerry J. Hooker and Marc Weidmann listed X. castrensis as an emended name for X. castrense. According to Jörg Erfurt and Grégoire Métais in 2007, X. castrensis and X. intermedium lack definite differential diagnoses other than dental sizes.
Classification
Xiphodon is the type genus of 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). Like the other contemporary endemic artiodactyl families of western Europe, the evolutionary origins of the Xiphodontidae are poorly known. While Xiphodon had been thought to have appeared as early as MP10 of the Mammal Palaeogene zones based on one locality, this allocation is based on very poor fossil material. Instead, the Xiphodontidae is generally thought to have first appeared by MP14, making them the first selenodont dentition artiodactyl representatives to have appeared in the landmass along with the Amphimerycidae. More specifically, the first xiphodont representatives to appear were the genera Dichodon and Haplomeryx. 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.