Lophiodon is an extinct genus of perissodactyls and the type genus of the Lophiodontidae, one of two major clades of the extinct suborder Ancylopoda. It, like the rest of the family, was endemic to western Europe and lived from the Early to Middle Eocene. Fossils of Lophiodon were first studied in 1804 when the French palaeontologist Georges Cuvier thought that they belonged to tapirs. After initially classifying the formally recognized species to Palaeotherium in 1812, Cuvier named the genus in 1822 based on the hill-like cusps on its molars. Many fossil species were named and assigned to Lophiodon throughout the 19th century but were reassigned to various other mammals including other members of the Lophiodontidae. There are fifteen species currently assigned to Lophiodon, one of questionable taxonomic status and one other pending a possible reassignment to another genus.
The skull of Lophiodon differed from Eocene perissodactyls of other families based on a consistently narrow rostrum and a tall and domed braincase but otherwise shared a lot of morphological traits with them. Lophiodon is noted for the losses of its first upper and lower premolars, strong canines, lengthy postcanine diastemata (gaps between teeth), and gradual molarization (or increased molar-like forms) of the premolars among other various specific dental traits (the latter of which is an evolutionary trend likely in response to changes in vegetation and faunal competition). Lophiodon has bilophodont dentition, meaning that its cheek teeth (mainly the molars) has two transverse ridges for folivory (leaf-eating diets). Postcranial fossil evidence of Lophiodon suggests that it was robust but non-cursorial like extant rhinoceroses and hippopotamuses, using its muscular forelimbs to support its heavy body. Lophiodon had four toes on its forelimbs and three toes on its hindlimbs, a primitive anatomical trait among perissodactyls like earlier species of palaeotheres and extant tapirs. The postcranial morphology of Lophiodon may have been shaped by how dry and terrestrial the environment around it was, and at least some may have possibly had semi-aquatic behaviours.
Lophiodon, relative to its family, ranged from medium to large in size and were usually the largest animals during the Middle Eocene of western Europe. Its various species had a massive range in body mass, from about minimum for the basal L. baroensis and L. corsaensis to more than in the cases of the later species L. rhinocerodes and L. lautricense. The large range in body mass of the various Lophiodon species is a reflection of an evolutionary trend in which the genus gradually grew larger in response to major climatic and ecological shifts, with L. lautricense being one of the latest species and arguably the largest species of its family with no size competition in the landmass it lived in.
Multiple species of Lophiodon appeared as early as the late Early Eocene, when western Europe had largely been separated from the North American and eastern Eurasian landmasses as an archipelago. Lophiodon represented an endemic group of perissodactyls unlike the contemporaneous palaeotheres and hyrachyids, although they and other mammals present in the archipelago were shaped by large degrees of endemism as well. Lophiodon species generally lived in hot and forested environments, with humidity and seasonality varying to some extent. Lophiodon, despite having initially existed with other lophiodont genera, eventually became the last representative of its family and became extinct by the end of the Middle Eocene. While the specific causes of its extinction are unclear, it was part of a major faunal turnover that also drove European hyrachyids and sebecian crocodylomorphs to extinction. The extinction causes of Lophiodon may have been due to environmental shifts from climate change and increased competition from the diversifying palaeotheres.
Taxonomy Research history
Cuvier's documentations and early 19th century taxonomy
Fossils of Lophiodon have first been studied in 1804 by the French palaeontologist Georges Cuvier, who originally referred to some of them as belonging to a small-sized tapir (with some close relatives also being studied by him before 1822). In 1812, he erected within the extinct genus Palaeotherium the species P. giganteum, P. tapiroïdes, P. buxovillanum, and P. occitanicum, describing them as being the size of rhinoceroses, oxes, pigs, and sheep, respectively. As early as 1817, French zoologist Henri Marie Ducrotay de Blainville suggested that some species classified to Palaeotherium by Cuvier like P. tapiroïdes should be classified under the new genus Tapirotherium. He suggested that it would have been intermediate in morphology between Tapirus and Palaeotherium. He said that he did not know of any upper jaw belonging to Tapirotherium but suggested that it would be similar to that of Palaeotherium. He noted a lower half of a jaw from a collection belonging to an individual named Mr. de Drée, which he said contained three incisors, a strong canine, and six molars, the last three of the latter of which he said were similar to those of tapirs and Palaeotherium.
In 1822, Cuvier dedicated an entire chapter to "tapir-like" mammal fossils found in various European localities. In the chapter's first section, he described various tapir-like teeth from several localities that he did not assign any name to and referenced several other localities that he would then describe in the next section. Among them were dental material from the French city of Orléans that he assigned to the informal name "tapir gigantesques", or "gigantic tapirs." In the second section, Cuvier erected the genus name Lophiodon, stating that its name references the transverse hill-like cusps of the molars like in modern tapirs. He also stated that like tapirs, Lophiodon had six incisors and two canines on each side of its mouth (or three incisors and one canine in each jaw's side). Cuvier argued that it differed from tapirs in having more oblique cusps in each molar, there being one rather than two cusps in each of the first molars, and the bases of the molars being less rectangular (especially the last ones). Furthermore, he said that some species of Lophiodon had different tooth morphologies more reminiscent of hyraxes and rhinoceroses and thus form a diverse range of forms by species like Palaeotherium. The genus name Lophiodon means "crested tooth", which is a compound of the Greek prefix () meaning 'crest' and the suffix () meaning 'tooth'.
Within the same second volume book, Cuvier said that he recognized multiple species of Lophiodon of various sizes from various localities, many of which he did not assign their own species names. Consisting of dental, cranial, and dental material, the fossils described by Cuvier as within Lophiodon were found by various different individuals who themselves uncovered them in many French localities, namely the French commune of Issel within the department of Aude, commune of Argenton-sur-Creuse within Indre, the hill range called Vosges (more specifically the Bastberg Hill near the commune of Bouxwiller (or "Buchsweiler") within the department of Bas-Rhin, commune of Soissons in Aisne, the French village of Boutonnet near the city of Montpellier, and from the valley Valdarno (or the Arno Valley). Cuvier considered multiple species he named within Palaeotherium to be in Lophiodon, like L. tapiroïdes and L. buxovillanum from Bouxwiller. The same year, French zoologist Anselme Gaëtan Desmarest formalized the listings of species within Lophiodon and also considered Tapirotherium to be synonymous with it, deriving L. tapirotherium of Issel from the former genus name. In 1829, German naturalist Johann Baptist Fischer relisted Lophiodon species previously described by Cuvier but wrote of additional species names based on the fossils that Cuvier had already described without having assigned names to them: L. Isselense of Issel and L. medium, L. minutum, and L. minimum of Argenton.
Several species from Europe and North America have been classified to Lophiodon before being assigned to different genera (usually newly erected ones) throughout the 19th century while several other species have been synonymized with other species including those of Lophiodon. For instance, in 1833, German naturalist Johann Jakob Kaup established the name L. Goldfusii based on molars. In another source, he erected the genus Chalicotherium based on fossils from the Grand Duchy of Hesse, transferring "L." Goldfussii into the taxon as Chalicotherium Goldfussii. In 1845, British palaeontologist Richard Owen erected the Lophiodon subgenus Coryphodon, describing it as horse-sized and roughly contemporaneous with "true" species of Lophiodon. He said that Corphodon differed from other Lophiodon species by the second and third cusps of the last lower molar that are combined into an obtuse-angled ridge. In 1846, Owen created the species name C. eocænus of the species he first recognized the previous year for Coryphodon when it was named. It was later in 1847 that French palaeontologist Auguste Pomel confirmed Coryphodon to be a distinct genus from Lophiodon, although he erected another subgenus Pachynolophus. In 1850, German palaeontologist Heinrich Georg Bronn considered Pachynolophus to be a distinct genus, of which "L." duvali was its type species. Several other mammal genera from Europe and North America whose species were initially classified in Lophiodon were Hyrachyus, Chasmotherium, Helaletes, Tapiravus, Heptodon, Colodon, Moropus, and Isectolophus.
Late 19th century taxonomy
In an 1848–1852 work, French palaeontologist Paul Gervais wrote of the species L. parisiense, basing it on fossils from Nanterre, Passy, Vaugirard and potentially Provins that were previously described by other palaeontologists like Blainville in his osteology series of fossil animals. In 1851, French naturalist Jean-Baptiste Noulet wrote of a large-sized lower jawbone donated by Mr. Belhomme in 1845. The fossil was previously collected by Count de Foucaud at his estate in Braconac in the French commune of Lautrec; he gave it to Belhomme to be kept within the French city of Toulouse and presented to the local municipality. Having studied the fossil repeatedly, he presented it to fellow French palaeontologist Édouard Lartet, who agreed with him on the bones having belonged to a new species of Lophiodon. Noulet cited his inspiration of French palaeontologist Charles Léopold Laurillard and his pledge for Belhomme and later Lartet to specifically publish research on de Foucaud's fossil as the reason of the article on what he considered to be an "interesting discovery." He continued that the mandible in question consisted of three incisors, a canine, and six molars on each side. He felt that the dental formula and the dental anatomy clearly proved that the fossil belonged to the genus Lophiodon but was initially unsure of what to make out of the double crescent ridges of the back molars. He later observed Cuvier's descriptions and figures of Lophiodon and found it to be in common with Lophiodon rather than other mammals like rhinoceroses and Palaeotherium. He also described the fossil as being a third larger than L. tapiroides from Bouxwiller. He gave the fossil the species name L. Lautricense, also listing the name "Lautrec's Lophiodon" in reference to the locality that the lower jaw was found at.