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 fourteen 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 (or three-toed) forelimbs and hindlimbs, small post- canine diastemata (gaps between teeth), 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 (or adapted for running). 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 (competition and/or predation), 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 () meaning 'old' or 'ancient' and the suffix () 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'' (''P. medium'', ''P. magnum'', ''P. crassum'', ''P. curtum''), six were valid but were eventually reclassified to different genera by different palaeontologists (''P. minus'', ''P. tapiroïdes'', ''P. buxovillanum'', ''P. aurelianense'', ''P. occitanicum'', and ''P. isselanum''), and three were considered invalid (''P. giganteum'', ''P. latum'', and ''P. indeterminatum'').

In 1812, Cuvier defined ''Palaeotherium'' as containing only tridactyl (or three-toed) 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.

Three sculptures representing ''Palaeotherium magnum'', ''Palaeotherium medium'' and "''Plagiolophus minus''" (= '' Plagiolophus'') 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 (it was replaced by a new republicated model in 2023); 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.

''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.

Later 19th century taxonomy

In the 19th century, several of Cuvier's ''Palaeotherium'' species have been reclassified under different genera. "''P.''" ''aurelianense'' was reclassified as its own genus '' Anchitherium'' by the German palaeontologist Hermann von Meyer in 1844. In an 1839–1864 osteography, the French naturalist Henri Marie Ducrotay de Blainville relisted "''P.''" ''tapiroides'', "''P.''" ''buxovillanum'' and "''P.''" ''occitanicum'' as species belonging to '' Lophiodon'', but the latter two were eventually moved to '' Paralophiodon'' and '' Lophiaspis'', respectively, in the 20th century. In 1862, Swiss zoologist Ludwig Ruetimeyer considered the previously recognised genera ''Plagiolophus'' and '' Propalaeotherium'' as distinct from ''Palaeotherium''; these contain the species ''P. minor'' and ''P. isselanum'', respectively.

The 19th century also saw the erection of several new ''Palaeotherium'' species. In 1853, French palaeontologist Auguste Pomel erected the species ''P. duvali'' based on limb bones that he thought were less stocky than those of ''P. curtum''. In his 1839–1864 osteography, Blainville erected ''P. girondicum'', pointing out that its fossils were from the Gironde Basin and that Cuvier only briefly referenced it in an 1825 publication. In 1863, the French naturalist Jean-Baptiste Noulet created the species ''P. castrense'' based on an incomplete mandible that was uncovered from the commune of Viviers-lès-Montagnes that was placed in a fossil collection from Castres. In 1869, Swiss palaeontologists Pictet and Humbert erected the species ''Plagiolophus siderolithicus'' based on molars that are similar to those of ''P. minor'' but were smaller in size. The same year, German palaeontologist Oscar Fraas erected ''P. suevicum'' based on teeth that he thought had distinct enamel. The French naturalist Paul Gervais, in 1875, described fossil bones and teeth from the French commune of Dampleux, noting that they belonged to a species smaller than other ''Palaeotherium'' species and with dental dimensions similar to those of ''Plagiolophus minor''. He assigned the fossils to the newly erected species ''P. eocaenum''.

''Palaeotherium'' skeletons

In 1873, the French geologist Gaston Casimir Vasseur uncovered the first complete skeleton of ''Palaotherium'', attributed to ''P. magnum'', from a gypsum quarry in the commune of Vitry-sur-Seine. The quarry was owned by the civil engineer Fuchs, who donated the skeleton to the National Museum of Natural History, France. The skeleton was described by Gervais in the same year, who noted that the neck was longer than expected and that the build was less stocky than that of tapirs and rhinoceroses. The skull of the specimen measures long. The naturalist said that the excavation of the specimen was difficult but completed by multiple skillful workers. Since its description, it has been displayed at the Gallery of Paleontology and Comparative Anatomy of the museum as an important and famous component.

During the 20th century, a second complete skeleton of ''P. magnum'' was excavated from the plasters in the French commune of Mormoiron. It was sent to the geological department of the University of Lyon and described after preparation by the Austrian geologist Frédéric Roman in 1922. Roman published a reconstruction of the skeleton in his 1922 monography. According to Austrian palaeontologist Othenio Abel in 1924, it was the most complete skeleton of ''Palaeotherium'' and amongst the most complete of any early Cenozoic mammal known at the time, missing only a few ribs and the left femur.

20th century revisions

In 1904, Swiss palaeontologist Hans Georg Stehlin created the species ''P. lautricense'' based on an upper jaw stored in the Muséum de Toulouse that originated from sandstone deposits at Castres. He also assigned two somewhat crushed skulls to this species. In his monography on palaeotheres, published the same year, Stehlin considered most species of ''Palaeotherium'' as potentially valid, but noted that most taxonomists were reluctant to invalidate species erected by Cuvier. Stehlin considered ''P. girondicum'' to be a form of ''P. magnum'', and described two forms of ''P. curtum'' from jaw fragments from La Débruge. He also named three new species – ''P. Mühlbergi'', based on dental material from the Swiss municipality of Obergösgen; ''P. Renevieri'', based on new finds from Mormont and a mandible identified by Pictet in 1869; and ''P. Rütimeyeri'', from the municipality of Egerkingen, which he described as having primitive premolars. In 1917, French palaeontologist Charles Depéret recognized two additional species of ''Palaeotherium'' – ''P. Euzetense'' and ''P. Stehlini''.

In 1968, upcoming German palaeontologist Jens Lorenz Franzen, then a graduate student, made major revisions of ''Palaeotherium'' in his dissertation. He invalidated several species as dubious names (''P. giganteum'' (considered to have been a rhinocerotid instead), ''P. gracile'', ''P. parvulum'', ''P. commune'', ''P. primaevum'', and ''P. gervaisii'') and synonymized many others with ''P. magnum'' (''P. aniciense'', ''P. subgracile''), ''P. medium'' (''P. brivatense'', ''P. moeschi''), ''P. crassum'' (''P. indeterminatum''), ''P. curtum'' (''P. latum'' and ''P. buseri''), ''P. duvali'' (''P. kleini''), and ''P. muehlbergi'' (''P. velaunum''). He additionally invalidated many species that had been erected throughout the 19th and early 20th centuries. He also erected ''P. pomeli'' based on fossils from a locality in Castres and reclassified "''Plagiolophus''" ''siderolithicum'' as a species of ''Palaeotherium''. Furthermore, Franzen converted some species into subspecies (''P. magnum girondicum'', ''P. magnum stehlini'', ''P. medium suevicum'', and ''P. medium euzetense'') and named six additional subspecies.

In 1975, Spanish palaeontologist María Lourdes Casanovas-Cladellas erected the species ''P. crusafonti'' from a left maxilla with dentition from the Spanish site of Roc de Santa. In 1980, both she and José-Vicente Santafé Llopis established a second Iberian species, ''P. franzeni'', from the Spanish municipality of Sossís based on differences in dentition. In 1985, the French palaeontologist Jean-Albert Remy named a new subspecies, ''P. muehlbergi thaleri'', in honor of fellow palaeontologist Louis Thaler; these fossils, consisting of two skulls with mandibles, were from the commune of Saint-Étienne-de-l'Olm.

In 1991, Casanovas-Cladellas and Santafé Llopis erected ''P. llamaquiquense'' from partial jaw material from the Spanish locality of Llamaquique in the city of Oviedo, where the name derived from. The next year in 1992, Remy proposed the creation of two subgenera of ''Palaeotherium'' based on cranial characteristics: ''Palaeotherium'' and ''Franzenitherium''. In 1993, the Spanish palaeontologist Miguel Ángel Cuesta Ruiz-Colmenares established the species ''P. giganteum'' based on teeth from the Mazaterón site in the Duero Basin, considering it to be the largest species of ''Palaeotherium'' known. In 1998, Casanovas-Cladellas et al. erected the subspecies ''P. crassum sossissense'' from a fragmented right maxilla with dentition from Sossís in Spain. They also invalidated the previously named ''P. franzeni'' and reassigned the material to ''P. magnum stehlini''.

Classification

''Palaeotherium'' is the type genus of the Palaeotheriidae, largely considered to be one of two major hippomorph families in the superfamily Equoidea, the other being the Equidae. Alternatively, some authors have proposed that equids are more closely related to the Tapiromorpha than to the Palaeotheriidae. It is also usually thought to consist of two families, the Palaeotheriinae and Pachynolophinae; a few authors alternatively have argued that pachynolophines are more closely related to other perissodactyl groups than to palaeotheriines. Some authors have also considered the Plagiolophinae to be a separate subfamily, while others group its genera into the Palaeotheriinae. ''Palaeotherium'' has also been suggested to belong to the tribe Palaeotheriini, one of three proposed tribes within the Palaeotheriinae along with the Leptolophini and Plagiolophini. The Eurasian distribution of the palaeotheriids (or palaeotheres) were in contrast to equids, which are generally thought to have been an endemic radiation in North America. Some of the most basal equoids of the European landmass are of uncertain affinities, with some genera being thought to potentially belong to the Equidae. Palaeotheres are well-known for having lived in western Europe during much of the Palaeogene but were also present in eastern Europe, possibly the Middle East, and, in the case of pachynolophines (or pachynolophs), Asia.

The Perissodactyla makes its earliest known appearance in the European landmass in the MP7 faunal unit of the Mammal Palaeogene zones. During the temporal unit, many genera of basal equoids such as ''Hyracotherium'', '' Pliolophus'', '' Cymbalophus'', and '' Hallensia'' made their first appearances there. A majority of the genera persisted to the MP8-MP10 units, and pachynolophines such as ''Propalaeotherium'' and '' Orolophus'' arose by MP10. The MP13 unit saw the appearances of later pachynolophines such as '' Pachynolophus'' and '' Anchilophus'' along with definite records of the first palaeotheriines such as ''Palaeotherium'' and '' Paraplagiolophus''. The palaeotheriine ''Plagiolophus'' has been suggested to have potentially made an appearance by MP12. It was by MP14 that the subfamily proceeded to diversify, and the pachynolophines were generally replaced but still reached the late Eocene. In addition to more widespread palaeothere genera such as ''Plagiolophus'', ''Palaeotherium'', and '' Leptolophus'', some of their species reaching medium to large sizes, various other palaeothere genera that were endemic to the Iberian Peninsula, such as '' Cantabrotherium'', '' Franzenium'', and '' Iberolophus'', appeared by the middle Eocene.

The phylogenetic tree for several members of the family Palaeotheriidae, as well as three outgroups, as created by Remy in 2017 and followed by Remy et al. in 2019 is defined below:

As shown in the above phylogeny, the Palaeotheriidae is recovered as a monophyletic clade, meaning that it did not leave any derived descendant groups in its evolutionary history. ''Hyracotherium'' ''sensu stricto'' (in a strict sense) is defined as amongst the first offshoots of the family and a member of the Pachynolophinae. "''H.''" ''remyi'', formerly part of the now-invalid genus ''Propachynolophus'', is defined as a sister taxon to more derived palaeotheres. Both ''Pachynolophus'' and '' Lophiotherium'', defined as pachynolophines, are defined as monophyletic genera. The other pachynolophines '' Eurohippus'' and ''Propalaeotherium'' consistute a paraphyletic clade in relation to members of the derived and monophyletic subfamily Palaeotheriinae (''Leptolophus'', ''Plagiolophus'', and ''Palaeotherium''), thus making Pachynolophinae a paraphyletic subfamily clade.

Inner systematics

Since 1968, many species of ''Palaeotherium'' have multiple defined subspecies that are justified by various intraspecific variations. Later since 1992, two subgenera are officially recognized for ''Palaeotherium''. The first of these subgenera is ''Palaeotherium'', which includes the type species ''P. magnum'' along with ''P. medium'', ''P. crassum'', ''P. curtum'', ''P. castrense'', ''P. siderolithicum'', and ''P. muehlbergi''. The second subgenus is ''Franzenitherium'', which includes the type species ''P. lautricense'' as well as ''P. duvali'' and was named in honor of Franzen's review of ''Palaeotherium''. The subgenus ''Palaeotherium'' is distinct from another subgenus ''Franzenitherium'' based on specialized traits. For example, the orbit of ''Palaeotherium'' being aligned in front of the skull's midlength is a specialized trait compared to that of ''Franzenitherium'' being aligned more with the skull's midlength. Several ''Palaeotherium'' species are too fragmentary to be placed in any of the subgenera. The following table lists all valid species and subspecies of ''Palaeotherium'', the subgenus that each is classified to, the Mammal Palaeogene faunal units that they are recorded from based on fossil deposit appearances, the authors who named the taxa, and the year that they were formally named:

Description

Skull

The Palaeotheriidae are distinguished from other perissodactyls mostly based on features of the skull. For example, the orbits are generally wide open at the back and are located in the middle of the skull or slightly more frontwards. The nasal bones of palaeotheres are thick to very thick. ''Palaeotherium'' itself is characterized by several cranial traits that distinguish it from other palaeothere genera such as an elongated zygomatic process of the squamosal bone extending to the maxilla and the presence of an anastomosis (anatomical connection between two passageways) roughly at the sphenoid bone and prominent temporalis muscle developments. The calvaria ranges in base length from to depending on the species.

The height and weight proportions of the skull of ''Palaeotherium'' are roughly equivalent with those of other taxa within the Equoidea; members of the superfamily have relatively shortened front facial areas. The skull's top peaks at the far back area, although this is not observed in ''P. lautricense''. The sagittal crest can be prominent and depends on the age and sex of the individual for development. In comparison to other equoids where the skull's maximum width extends above the front root of the parallel zygomatic arches, those of ''Palaeotherium'' and most other palaeotheres (except ''Leptolophus'') extend back to the joint of the squamosal bone and mandible. The orbit on the skull of ''Palaeotherium'', unlike that of other equoids, is proportionally smaller and located somewhat in front of the skull's midlength, the latter trait of which may be further extended in the case of ''P. medium''. Similar to other Palaeogene equoids, the front edge of the orbit is aligned with M¹ or M² while the back area is wide. Unlike most other palaeotheres, its nasal opening stretches up to the P³ tooth at minimum or up to the front edge of the orbit above M³ in the case of ''P. magnum''. While the shapes and proportions of the nasal bones vary by species, they extend beyond P¹ in adults and sometimes even the canine like in equines. The temporal fossae are large but vary in proportion. The cranial vault is broad, domed, and wider than the overall skull.

The horizontal ramus of the mandible is overall thick plus tall and has an elongated mandibular symphysis, but the width and lower area morphology vary by species. It is wide in both the front and back areas and low compared to equines. The joint for the squamosal and mandible of ''Palaeotherium'' is low compared to those of ''Plagiolophus'' and ''Leptolophus''. The angular process, located above the angle of the mandible, is blocked from further expansion by the mandibular notch and is well-developed in its rear like in Palaeogene equids.

Dentition

Derived palaeotheres are generally diagnosed as having selenolophodont ( selenodont-lophodont ridge form) upper molars (M/m) and selenodont (crescent-shaped ridge form) lower molars that are mesodont, or medium-crowned, in height. The canines (C/c) strongly protrude and are separated from the premolars (P/p) by medium to long diastemata (gaps between two close teeth) and from the incisors (I/i) by short ones in both the upper and lower dentition. The other teeth are paired closely with each other in both the upper and lower rows. The dental formula of ''Palaeotherium'' is for a total of 44 teeth, consistent with the primitive dental formula for early-middle Palaeogene placental mammals.

The incisors are shovel-shaped and, like in modern horses, are used for chewing at right angles in relation to their longitudinal axes. They have no cutting functions but instead are used for grasping food akin to how tweezers grasp items. The canines are proportionally large and dagger-shaped. They were probably not used for cutting or chewing given how they are oriented, but may have been used in self-defence and conspecific fights. The decreased length of the postcanine diastema in ''Palaeotherium'' and the equid subfamily Anchitheriinae may be correlated with increases in body size. This trend may be due to the necessity to improve chewing performances through molarization and proportional size increases of the premolars. Postcanine diastemata are strongly reduced in early species such as ''P. castrense''; in later species, they vary from small (''P. crassum'', ''P. curtum'') to large (''P. medium'', ''P. magnum''). The separation of cheek teeth from the incisors and canines attests to their independent and specific chewing functions.

The premolars and preceding deciduous teeth both tend to have molarized forms (meaning molar-like shapes) and have newly developing hypocone cusps on them. The forms of the deciduous premolars (dP) of juvenile ''Palaeotherium'' and other palaeotheriines distinguish them from the earlier pachynolophines, where the dP²-dP⁴ of juvenile ''P. renevieri'' and ''P. magnum'' are both molarized and four-cusped (although dP¹ is triangular). Late Eocene species of ''Palaeotherium'' tend to have more molariform premolars. The non-molarized premolars are composed of four to five cusps (one to two external, two intermediate, and one internal) while the molarized premolars and molars have six cusps (two external, two intermediate, and two internal). The upper molars are medium-crowned (shorter than those of modern equids) and have ectolophs (crests or ridges of upper molar teeth) that are about twice the height of the inner cusps and curve into a W shape. The W-shaped ectolophs themselves are made up of two articulated crescents. The lower molarized premolars and molars are about half as wide as their upper counterparts. The mesostyle cusp (a small cusp type) present in the molars thicken from M¹ to M³. The lingual lobes (or divisions) in the upper molars are closely aligned with the ectolophs.

Postcranial skeleton

The overall postcranial anatomy of ''Palaeotherium'' is best known from a skeleton of ''P. magnum'' uncovered from Mormoiron. The vertebral column is made up of seven large cervical vertebrae, seventeen thoracic vertebrae, six lumbar vertebrae, six sacral vertebrae, and fifteen caudal vertebrae. The cervical vertebrae, comprising the neck, measure long while the caudal vertebrae, comprising the tail, measure long. The sacrum is triangular and similar to that of the Equidae, but is slightly wider in its front area. ''P. magnum'' would have had a total of thirty-four ribs based on the total number of thoracic vertebrae. Like in equids, the front ribs are strong and flattened. The back portion of the thorax would have been wider than in horses and roughly comparable to those of tapirs and rhinoceroses but was not as long as that of the latter. The ribs are separated from the sternum, which is approximately the same size as the thorax.

''P. magnum'' has generally strong and stocky limb bones. The femora (upper thigh bones) of ''P. crassum'' and ''P. medium'', in comparison, are less robust. ''Palaeotherium'' has a straighter and less concave trochlea of the astragalus than ''Plagiolophus''. The calcaneum is semirectangular in shape but slightly wider on its rear end. The cuboid bone is high and narrow, similar to that of ''Anchitherium''.

Most species of ''Palaeotherium'' have tridactyl (three-toed) hindlimbs and forelimbs, in contrast to earlier palaeotheres that have tetradactyl (four-toed) forelimbs and tridactyl hindlimbs. ''P. eocaenum'' might have had a tetradactyl forelimb, as indicated by a manus that has been tentatively assigned to it. ''Palaeotherium'' differs from ''Plagiolophus'' in its long and narrow carpals and in its metacarpal bones, which are close in length to each other and develop into wide ungual phalanges. The tridactyl foot morphology with all three digits being functional suggests digitigrade locomotion.

''Palaeotherium'' shows an exceptional amount of variation in the shape of its third metacarpal and its manus dimensions. ''P. curtum'' has very robust forelimb bones including a short and stocky manus, which suggests that it was stocky in build. ''P. magnum'' and ''P. crassum'' resemble tapirs, especially the mountain tapir (''Tapirus pinchaque''), in the build of their forelimbs. ''P. magnum'' has less slender radii and metacarpals compared to those of ''P. crassum'' and is therefore comparable to those of modern tapirs. ''P. medium'', along with ''Plagiolophus'', appear to be the most cursorial palaeotheres due to their elongated and gracile metacarpals. ''P. medium'' has a more unique foot morphology compared to other ''Palaeotherium'' species due to narrower and higher feet and longer metapodial bones. The cursorial adaptations of ''P. medium'' is further supported by the morphology of the humerus. The middle metatarsal bone is larger and more robust than the others. The fourth toe of ''P. magnum'' appears slightly arched and is slightly longer than the second toe.

Footprints

Several types of tracks have been suggested to belong to ''Palaeotherium'', among them the ichnogenus '' Palaeotheriipus'' that was named by the palaeontologist Paul Ellenberger in 1980 based on tracks from lacustrine limestones in the department of Gard

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