The research history of Palaeotherium is complicated given its extensive fossil record and lengthy taxonomic history, with the earliest record of its fossils dating back to 1782 when the French physicist Robert de Lamanon described the skull of what the naturalist Georges Cuvier described as belonging to P. medium in 1804. Cuvier initially recognized its affinities to tapirs and rhinoceroses and classified fossil material to three different species based on size. From 1805 to 1824, he established additional species based on the morphologies of postcranial remains and drew a reconstructed skeleton of P. magnum in 1824. The fossil mammal genus was the fourth to have been recognized with undisputed taxonomic authority. Palaeotherium had since been a subject of significant attention by many other palaeontologists, and it was gradually revised to be recognized as taxonomically distinct from its other perissodactyl relatives.
Since 1804, many species names, valid or invalid, were erected by European and North American naturalists. Palaeotherium was not known by any complete skeleton until when two different skeletons of P. magnum were uncovered in 1873 and the early 20th century, respectively, within France. The most significant taxonomic revisions were conducted by German palaeontologist Jens Lorenz Franzen, then a graduate student, in 1968 when he synonymized many species names and recognized subspecies within the genus based on morphological variations from different species. Although not as strongly influential in modern culture, it is a well-known taxon within European palaeontology and has been acknowledged as an important find in the history of palaeontology.
Early research history First documentations
The earliest documentation of fossils belonging to Palaeotherium dates back to March 1782 in a scientific journal by the French physicist Robert de Lamanon. The scientist recorded that there were fossils located in the quarries of Montmartre, a large hill near Paris, France. He noted that the fossils were products of animals preserved in deposits that no longer existed. Among the fossils was a skull with both the upper and lower jaws that belonged to the nobleman Philippe-Laurent de Joubert. Lamanon recognized that it had dentition consisting of molars and incisors that were more akin to ruminants than carnivores. Because of the presence of incisors in the fossil animal unlike ruminants and their conical shapes, he hypothesized that the animal had aquatic adaptations but that there are no modern analogues for any extant aquatic animal in terms of dentition. As a result, he proposed the idea that the now-extinct animal was amphibious and fed on both herbs and fish based on the shape of its teeth.
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 very fragmentary fossil materials stored at the National Museum of Natural History, France, his palaeontological insight remained limited early on. The fossils of Montmartre were credited with great importance to the field of palaeontology, as unlike surface-level taxa such as Mammut and Megatherium, those found near Paris were embedded in deeper and harder sediments, falling between the Pleistocene-aged mammals and the Cretaceous-aged reptiles such as Mosasaurus. In 1798, he documented fossils from Montmartre, amongst various other notable fossils worldwide, the former of which he thought initially could have belonged to the canid genus Canis based on its dentition. Not long after in the same year, he changed his mind and thought that the fossil mammal instead would have been within the order of pachyderms, theorizing that it would have been closest to tapirs and that it would have had trunks like them. He also figured out that the animals of Montmartre were of multiple species with different sizes and numbers of toes.
Cuvier's early taxonomy
In 1804, Georges Cuvier described the sets of fossils from the gypsum quarries of the outskirts of Paris (known as the Paris Basin). These fossils were known by Cuvier as early as 1800 but were not formally described yet. He recognized that dentition was critical in ascertaining the dietary and phylogenetic affinities of a given fossil animal. Cuvier pointed out that the fossil molars suggested closer affinities of the fossil mammal to rhinoceroses or hyraxes than ruminants. The naturalist than observed that the fossil dentition came in several different sizes based on his measurements of their circumferences, therefore determining that the jaws in fact belonged to two different species of the same genus. The mammals, according to Cuvier, had complete sets of 28 molars, 4 canines, and 12 incisors. He knew that the mammals would have been herbivorous in nature and thought that the complete dentition would have made it closer in affinity with tapirs but that the molar shapes were similar to those of rhinoceroses. The zoologist therefore established the genus name Palaeotherium and established the first species name Palaeotherium medium. The genus name means "ancient beast," for which the etymology is a compound of the Greek prefix () meaning 'old' or 'ancient' and the suffix () meaning 'beast' or 'wild animal'.
Cuvier then stated that Palaeotherium must have been a pachyderm intermediate in evolutionary affinity between tapirs and rhinoceroses. He asked a series of questions about the biology of the extinct genus and presented the idea that examining the morphology of its head would help to answer them. The naturalist wrote that there was an important fossil that was first described in a 1782 paper by the late physicist Robert de Lamanon; the fossil in question was of a skull stored in a collection of Marquis de Drée. Previously, Lamanon thought that the skull belonged to an omnivorous amphibian, which Cuvier rejected. Cuvier proposed the idea that Palaeotherium, based on its nasal bones, had trunks more similar to tapirs than elephants. He said that the head of P. medium must have been on average about long, that its body size would have been somewhat smaller than those of tapirs but roughly matching up with regular pigs.
Later, he wrote about a species that he deemed to have similar dentition to P. medium to the extent that it belonged to Palaeotherium. Cuvier observed that the species had larger-sized dentition compared to the other species based on imprints that he was provided. Thus, he established the species Palaeotherium magnum. He also erected the genus Anoplotherium in recognition of its differing dentition from that of Palaeotherium. He also gave mentions to the postcranial fossils of the genus and listed a newly recognized species named Palaeotherium minus. In a later journal of the same year, Cuvier described a mostly complete skeleton from the French commune of Pantin that he determined to have belonged to P. minus.
Other Cuvierian species and reconstructions
In 1805, Cuvier described additional postcranial fossil bones of Palaeotherium. He noted that its forefeet consisted of three short toes and that no other animal had postcranial bones that closely resembled those of the extinct genus. Nonetheless, he also gave emphasis to some of the fossil foot bones resembling those of either tapirs or rhinoceroses. Based on the metacarpal bone shapes located on the front feet, he erected the species name P. crassum, mentioning that it was a distinct species from P. medium.
In 1812, he examined more metacarpal bone material that he classified as belonging to Palaeotherium. He stated that the newer material was nearly the size of those of P. crassum. Thus, he felt the need to establish another species P. curtum based on the fossils. He then made a review of the species he previously erected. According to Cuvier, P. magnum was the size of a horse. He then stated that both P. medium and P. crassum were both the size of a pig, the former having narrow and elongated feet and the having broader and shorter feet. P. curtum did not have any listed size analogue, but the naturalist said that it had the feet of a horse. Finally, he said that P. minus was the size of a sheep and had narrow feet. He also listed five new additional species from surrounding areas of France that he did not further elaborate on, such as the rhinoceros-sized P. giganteum, the ox-sized P. tapiroïdes, the pig-sized P. buxovillanum plus P. aurelianense, and the sheep-sized P. occitanicum.
The naturalist also suggested palaeobiologies of the four species of Palaeotherium that he described from the gypsum quarries. He acknowledged that P. magnum had skull and limb bone material but lacked vertebra and rib fossils. Regardless, he was able to speculate based on available material that P. magnum would have resembled a tapir the size of a horse with bare amounts of hair. He also hypothesized that P. crassum would have resembled a tapir and been the size of one, which in theory would have caused people to confuse the two. P. medium, he suggested, would have also resembled a tapir but differed by higher legs and longer feet. He was able to construct a speculative skeletal reconstruction drawing of P. minus because of a previously found skeleton and hypothesized that it was smaller than a sheep and could have been cursorial with its slender legs and face. Finally, he theorized that P. curtum would have been the bulkiest species with lower legs compared to P. minus that were stocky like those of P. crassum. Cuvier also suggested that Palaeotherium as in the entire genus was tridactyl (or three-toed).
In 1822, Cuvier recognized three additional species of Palaeotherium based on postcranial materials. The first species he named was P. latum based on a medium-sized hind foot of different proportions from those of other species. The second species he named was P. indeterminatum based on an incomplete foot. The third and final species was P. minimum based on a single middle metatarsal bone. He also erected the genus Lophiodon, although he did not specify any species names within it. Cuvier also depicted a drawn reconstruction of the skeleton of P. magnum, outlining that it was the size of a Javan rhinoceros, was stocky in body build, and had a massive head. Palaeotherium was also depicted in 1822 drawings by the French palaeontologist Charles Léopold Laurillard under the direction of Cuvier. In 1824, he listed most species of Palaeotherium that he previously named and described, namely P. magnum, P. medium, P. crassum, P. latum, P. curtum, P. minus, and P. aureliense. He also recognized an additional species P. isselanum, but he did not describe its fossils.
Crystal Palace Dinosaurs
Since 1854, the family Palaeotheriidae is represented in the Crystal Palace Dinosaurs assemblage of the Crystal Palace Park by three individual sculptures, constructed by the English sculptor Benjamin Waterhouse Hawkins, that each represent different species, although the original large-sized P. magnum sculpture was lost at some point after 1958 and was replaced by a new replicated model in 2023. The other two palaeothere statues in the park represent the medium-sized P. medium and the small-sized "P. minus" (= Plagiolophus minor). Both the large P. magnum and P. medium were posed in standing positions whereas the smallest sitting statue was made to represent "P. minus". The models' resemblances to tapirs reflected early perceptions that the palaeothere species resembled them in body plan appearances. Despite this, the sculptures differ from living tapirs in several ways, such as shorter plus taller faces, higher eye positions, slender legs, longer tails, and the presence of three toes on the forelimbs unlike the four toes of the forelimbs of tapirs.