Lophiodon

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 62 kg (137 lb) minimum for the basal L. baroensis and L. corsaensis to more than 2,000 kg (4,400 lb) 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.