Palaeotherium
Palaeotherium is an extinct genus of equoid that lived in Europe and possibly the Middle East from the Middle Eocene to the Early Oligocene. It is the type genus of the Palaeotheriidae, a group exclusive to the Palaeogene that was closest in relation to the Equidae, which contains horses plus their closest relatives and ancestors. Fossils of Palaeotherium were first described in 1782 by the French naturalist Robert de Lamanon and then closely studied by another French naturalist, Georges Cuvier, after 1798. Cuvier erected the genus in 1804 and recognized multiple species based on overall fossil sizes and forms. As one of the first fossil genera to be recognized with official taxonomic authority, it is recognized as an important milestone within the field of palaeontology. The research by early naturalists on Palaeotherium contributed to the developing ideas of evolution, extinction, and succession and demonstrating the morphological diversity of different species within one genus.
Since Cuvier's descriptions, many other naturalists from Europe and the Americas recognized many species of Palaeotherium, some valid, some reclassified to different genera afterward, and others being eventually rendered invalid. The German palaeontologist Jens Lorenz Franzen modernized its taxonomy due to his recognition of many subspecies as part of his dissertation in 1968, which were subsequently accepted by other palaeontologists. Today, there are sixteen known species recognized, many of which have multiple subspecies. In 1992, the French palaeontologist Jean-Albert Remy recognized two subgenera that most species are classified to based on cranial anatomies: the specialized Palaeotherium and the more generalized Franzenitherium.
Palaeotherium is an evolutionarily derived member of its family with tridactyl forelimbs and hindlimbs, small post-canine diastemata, and premolars that are usually developed into molar-like forms. It shares many similar anatomical traits with other perissodactyls and has a large diversity in anatomical traits by species, with some species like P. magnum, P. curtum, and P. crassum being stockier in build and P. medium being more cursorial. The genus ranges in size from the small species P. lautricense, with an estimated weight of, to the massive P. giganteum, thought to have been capable of weighing over. P. magnum, known by two mostly complete skeletons from France, could have reached approximately in shoulder height and in length. The large-sized species were therefore amongst the largest mammals in the Eocene of Europe. Palaeotherium may have lived in herds and, as demonstrated by its dentition, was able to actively niche partition with another palaeothere Plagiolophus by specializing on softer leaves and fruit, although both were mostly leaf-eating.
Palaeotherium and other genera of the subfamily Palaeotheriinae likely descended from the earlier subfamily Pachynolophinae, which lived in both Europe and Asia as opposed to North America unlike undisputed members of the Equidae. By the time that the first species P. eocaenum appeared in the middle Eocene, western Europe was an archipelago that was isolated from the rest of Eurasia, meaning that it and subsequent species lived in an environment with various other faunas that also evolved with strong levels of endemism. The Iberian Peninsula had its own level of endemism with several species that are only known within the region, although they were replaced by more widespread species from central Europe by the late Eocene. Within both the middle and late Eocene, Palaeotherium consistently maintained a high species diversity and endured major environmental changes leading to a faunal turnover that occurred by the beginning of the late Eocene.
By the early Oligocene, most of its species went extinct along with many genera of western European mammals as part of the Grande Coupure extinction and faunal turnover event, the causes of the extinctions being attributed mainly to environmental changes from increased glaciation and seasonality, negative interactions with immigrant faunas from Asia, or some combination of the two. P. medium survived past the Grande Coupure probably due to its cursorial nature that allowed it to travel across open lands more efficiently and escape immigrant carnivores; it was the last species of its genus and went extinct not long after the faunal turnover event.
Taxonomy
Research history
First descriptions
In 1782, the French naturalist Robert de Lamanon described a fossil skull including the upper and lower jaws that was collected from the quarries of Montmartre, a hill near Paris that belonged to the nobleman Philippe-Laurent de Joubert. He recognized that the molars and incisors were roughly similar to those of ruminants but noted that the dentition lacked modern analogues. Consequently, he hypothesized that the animal was extinct, had an amphibious lifestyle, and fed on both plants and fish.Since 1796, the French naturalist Georges Cuvier innovated the idea of vanished worlds of extinct animals, but as his observations of fossils were mostly limited to drawings and fragmentary fossils stored at the National Museum of Natural History, France, his palaeontological insight was limited early on. In 1798, he documented fossils from Montmartre, suggesting initially that they could have belonged to the canid genus Canis based on dental morphology. Later in the same year, he instead suggested that the fossils belonged to a pachyderm that was most closely related to tapirs and had trunks like them. He also figured out that the animals of Montmartre were of multiple species with different sizes and numbers of toes. The fossils of Montmartre were credited with great importance to the field of palaeontology, as they were embedded in deeper and harder sediments than other fossil mammals such as Megatherium. The science historian Bruno Belhoste argued that Cuvier's study of Palaeotherium in 1798 "marks the true birth of paleontology".
Early taxonomy and depictions
In 1804, Cuvier confirmed that the skull previously reported by de Lamanon belonged to a mammal. The skull preserves a complete set of 44 teeth that are similar to those of rhinoceroses and hyraxes. Cuvier recognized that the skull differs from other mammals and therefore established a new genus and species, Palaeotherium medium. The genus name Palaeotherium means "ancient beast", which is a compound of the Greek prefix παλαιός meaning 'old' or 'ancient' and the suffix θήρ meaning 'beast' or 'wild animal'. He debunked Lamanon's hypothesis that Palaeotherium was an omnivorous amphibian and suspected that it had trunks akin to those of tapirs.From 1804 up to 1824, Cuvier erected a total of 13 species of Palaeotherium based on skull, dental, and postcranial material. He erected the second of these species, P. magnum, in 1804, explaining that it had similar but larger-sized dentition than P. medium. In describing the third and small-sized species, P. minus, he began to focus on the study of postcranial material rather than just cranial and dental material. In 1805, Cuvier erected P. crassum based on the three-toed forefeet, which were similar to tapirs and rhinoceroses in the shape of the metacarpal bones. In 1812, he named another species, P. curtum, based on metacarpal bones that were slightly smaller than those of P. crassum. As of 1968, four of the Palaeotherium species named by Cuvier were considered valid and remained classified in Palaeotherium, six were valid but were eventually reclassified to different genera by different palaeontologists, and three were considered invalid.
In 1812, Cuvier defined Palaeotherium as containing only tridactyl species. He also speculated on life appearance and behaviour of several Palaeotherium species, but cautioned that such interpretations are limited by the fragmentary fossil material. He suggested that P. magnum would have resembled a horse-sized tapir with sparse hair. P. crassum and P. medium would also have had a tapir-like appearance, with proportionally longer legs and feet in the latter. Cuvier also published a speculative skeletal reconstruction of P. minus and hypothesized that it was smaller than a sheep and potentially cursorial given its slender legs and face. Finally, he theorized that P. curtum would have been the bulkiest species. In 1822, Cuvier published a reconstruction of the skeleton of P. magnum, outlining that it was the size of a Javan rhinoceros, was stocky in build, and had a massive head. The same year, Palaeotherium was also depicted in drawings by the French palaeontologist Charles Léopold Laurillard under the direction of Cuvier.
File:Palaeotherium Crystal Palace 2018.jpg|thumb|left|Sculpture of P. medium as part of the Crystal Palace Dinosaurs sculptures on the Tertiary Island of the Crystal Palace Park, United Kingdom
Three sculptures representing Palaeotherium magnum, Palaeotherium medium and "Plagiolophus minus" are part of the Crystal Palace Dinosaurs exhibition in the Crystal Palace Park in London, which has been open to the public since 1854 and was created by the English sculptor Benjamin Waterhouse Hawkins. Both the P. magnum sculpture, the largest of the three, and the medium-sized P. medium sculpture were posed in a standing position, whereas the smaller "P. minus" sculpture depicts a sitting animal. The resemblance of the models to tapirs reflects early perceptions of the life appearance of Palaeotherium. However, the sculptures differ from living tapirs in several ways, such as shorter and taller faces, higher eye positions, slimmer legs, longer tails, and the presence of three toes on the forelimbs unlike the four toes of tapirs.
Of the three sculptures, P. medium most closely resembles a tapir, and it has remained mostly intact. P. medium was depicted as having thick skin and a slender face and trunk, representing outdated perceptions that it was a slow animal. The original P. magnum sculpture was last known from a 1958 photograph before it was lost at some point afterward ; the photograph reveals that it was the largest of the three sculptures and had a robust and muscular build with large and deep eyes, a proportionally large head, and bulky legs. The model's trunk was wide and descended below the lower lip. The overall anatomy appears to be based on elephants. Modern research in comparison suggests that Palaeotherium never had a trunk unlike with earlier works.
Palaeotherium proved to be a significant find to the field of palaeontology in multiple other aspects. For one, both the skeletal reconstruction drawing and the life restoration in Cuvier's works were incorporated into textbooks and handbooks around the world up to the 20th century. The genus was also incorporated into old orthogenesis models of the evolution of the horse theory as early as 1851 by British biologist Richard Owen and followed by other 19th century European naturalists such as Jean Albert Gaudry and Vladimir Kovalevsky.