Xiphodon

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'). '' fawn, of the "Tertiary Island" of the Crystal Palace Dinosaurs assemblage In 1848, 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. 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. 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 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 tie them as being 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. {{clade| style=font-size:85%; line-height:85% In 2020, Vincent Luccisano et al. created a phylogenetic tree of the basal artiodactyls, a majority endemic to western Europe, from the Palaeogene. In one clade, the "bunoselenodont endemic European" Mixtotheriidae, Anoplotheriidae, Xiphodontidae, Amphimerycidae, Cainotheriidae, and Robiacinidae are grouped together with the Ruminantia. The phylogenetic tree as produced by the authors is shown below: In 2022, Weppe created a phylogenetic analysis in his academic thesis regarding Palaeogene artiodactyl lineages, focusing most specifically on the endemic European families. He stated that his phylogeny was the first formal one to propose affinities of the Xiphodontidae and Anoplotheriidae. He found that the Anoplotheriidae, Mixtotheriidae, and Cainotherioidea form a clade based on synapomorphic dental traits (traits thought to have originated from their most recent common ancestor). The result, Weppe mentioned, matches up with previous phylogenetic analyses on the Cainotherioidea with other endemic European Palaeogene artiodactyls that support the families as a clade. As a result, he argued that the proposed superfamily Anoplotherioidea, composing of the Anoplotheriidae and Xiphodontidae as proposed by Alan W. Gentry and Hooker in 1988, is invalid due to the polyphyly of the lineages in the phylogenetic analysis. However, the Xiphodontidae was still found to compose part of a wider clade with the three other groups. Within the Xiphodontidae, Weppe's phylogeny tree classified Haplomeryx as a sister taxon to the clade consisting of Xiphodon plus Dichodon. == Description ==

Skull Xiphodon is diagnosed as having an elongated skull that is convex in the upwards area leading up to the orbit. The orbits themselves are wide open in their back areas. The muzzle (or snout) is elongated and has a rounded appearance. In the xiphodont genus also are large tympanic parts of the temporal bone and visible periotic bones. The palatine foramen are extensive in size from the I3 to P1 teeth. The maxilla constitutes the majority of the side areas of the skull while the premaxilla extends to the alveolar processes. The nasal bones are narrow and elongated, its passages barely extending over the openings of the external nostrils and forming with it a narrow bony strip. In the back view, the snout appears to have a U-shaped outline. The snout of Xiphodon is similar to that of Dichodon but differs from it by its elongation plus rounded appearance and the maxillae constituting part of the snout being less extensive in height. The snout of Dichodon in comparison is shorter and narrower. The hard palate for the upper mouth appears concave and has a visible premaxillary-maxillary suture extending from the outer edge of the jaw to the back. Both palatine foramen types of Xiphodon have similar proportions and positions to the palatine foramen of Dichodon, but those of Xiphodon are greater in length and have different morphologies to those of Dichodon. Dechaseaux later uncovered a large spherical flocculus of the cerebrum from the same endocast in 1967. The flocculus is separated from the cerebellar hemisphere and occupies space within the petrous part of the temporal bone within the periotic bone of the ear. It also gives off an enclosed appearance within its outer edges. consistent with the primitive placental mammal dental formula of for a total of 44 teeth. As members of the Xiphodontidae, they share both small incisors and the absences of distinct diastemata. However, the postcranial fossils were later reassigned to Leptotheridium while the astragalus originally assigned to Dichodon was reclassified to Xiphodon. == Palaeobiology ==

The Xiphodontidae is a selenodont artiodactyl group in western Europe, meaning that the family was likely adapted for folivorous (leaf-eating) dietary habits. This was especially the case with Xiphodon, which displayed specialized dentition made for feeding on leaves, tree shoots, and shrubs. Xiphodon retained the primitive trait of having molars with five cusps and shifted towards cutting dentition, while Dichodon had progressively molarized premolars for the function of grinding food, meaning that the two genera had different types of ecological specializations. Dechaseaux considered that the two xiphodontid genera may have been more derived than North American Palaeogene tylopods. The forelimbs of Xiphodon appear to be similar to those of Palaeogene camelids, which had adaptations towards cursoriality. Because of the dental and postcranial similarities, Xiphodon could have been a European ecological counterpart. However, whether Xiphodon had pacing locomotion like camelids cannot be proven. Due to the lack of postcranial evidence of other xiphodonts, it is not possible to prove that their postcranial morphologies are similar to those of Xiphodon. == Palaeoecology ==

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 million years ago) 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. '', which was endemic to western Europe during the Eocene Xiphodon made its earliest known appearance in MP16 based on the locality of Robiac in France as X. castrensis. The species is restricted to MP16 localities. By then, it would have coexisted with perissodactyls (Palaeotheriidae, Lophiodontidae, and Hyrachyidae), non-endemic artiodactyls (Dichobunidae and Tapirulidae), endemic European artiodactyls (Choeropotamidae (possibly polyphyletic, however), Cebochoeridae, Mixtotheriidae, Anoplotheriidae, Amphimerycidae, and other members of Xiphodontidae), and primates (Adapidae, Omomyidae). It also cooccurred with metatherians (Herpetotheriidae), rodents (Ischyromyidae, Theridomyoidea, Gliridae), eulipotyphlans, bats, apatotherians, carnivoraformes (Miacidae), and hyaenodonts (Hyainailourinae, Proviverrinae). 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. In addition, several migrant mammal groups had reached western Europe by MP17a-MP18, namely the Anthracotheriidae, Hyaenodontinae, and Amphicyonidae. X. gracilis is well-represented in localities of France, Spain, and the United Kingdom. It has the longest known fossil record range within its genus, lasting from MP18 to MP20. Based on the MP19 French locality of Escamps, it coexisted with the likes of the herpetotheriids Peratherium and Amphiperatherium, pseudorhyncocyonid Pseudorhyncocyon, nyctitheres Saturninia and Amphidozotherium, various bats and rodents, omomyid Microchoerus, adapid Palaeolemur, hyainailourine Pterodon, amphicyonid Cynodictis, palaeotheres Palaeotherium and Plagiolophus, dichobunid Dichobune, choeropotamid Choeropotamus, anoplotheriids Anoplotherium and Diplobune, cainothere Oxacron, amphimerycid Amphimeryx, and the other xiphodonts Dichodon and Haplomeryx. == Extinction ==

in the Isle of Wight. The stratigraphy of it and the Bouldnor Formation led to better understandings of faunal chronologies from the Late Eocene up to the Grande Coupure. The Grande Coupure event during the latest Eocene to earliest Oligocene (MP20-MP21) is one of the largest and most abrupt faunal turnovers in the Cenozoic of Western Europe and coincident with climate forcing events of cooler and more seasonal climates. The event led to the extinction of 60% of western European mammalian lineages, which were subsequently replaced by Asian immigrants. The Grande Coupure is often dated directly to the Eocene-Oligocene boundary at 33.9 Ma, although some estimate that the event began slightly later, at 33.6–33.4 mya. The event occurred during or after the Eocene-Oligocene transition, an abrupt shift from a hot greenhouse world that characterised much of the Palaeogene to a coolhouse/icehouse world from the Early Oligocene onwards. The massive drop in temperatures results from the first major expansion of the Antarctic ice sheets that caused drastic pCO2 decreases and an estimated drop of ~ in sea level. Many palaeontologists agree that glaciation and the resulting drops in sea level allowed for increased migrations between Balkanatolia and western Europe. The Turgai Strait, which once separated much of Europe from Asia, 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. In 2022, 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 Grande Coupure event also marked a large faunal turnover marking the arrivals of later anthracotheres, entelodonts, ruminants (Gelocidae, Lophiomerycidae), rhinocerotoids (Rhinocerotidae, Amynodontidae, Eggysodontidae), carnivorans (later Amphicyonidae, Amphicynodontidae, Nimravidae, and Ursidae), eastern Eurasian rodents (Eomyidae, Cricetidae, and Castoridae), and eulipotyphlans (Erinaceidae). All three representatives Xiphodon, Dichodon, and Haplomeryx are last recorded in MP20 localities. The disappearances of the three genera meant the complete extinction of the Xiphodontidae. Many other artiodactyl genera from western Europe disappeared also as a result of the Grande Coupure extinction event. The extinctions of Xiphodon and many other mammals have been attributed to negative interactions with immigrant faunas (competition, predations), environmental changes from cooling climates, or some combination of the two. It was that found the diversity of endemic artiodactyls increased with the diversity of immigrant artiodactyls. In addition, the diversity of immigrant artiodactyls was negatively correlated with the extinction rate of endemic artiodactyls. This suggests immigrant artiodactyls did not play a role in the extinction of endemic artiodactyls. ==References==