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.

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.

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.

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.

[20] 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.

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,[22] but the latter two were eventually moved to Paralophiodon and Lophiaspis, respectively, in the 20th century.

[23][24] 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.

[22] 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.

[31] 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.

[33][20] 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 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).

[41] 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.

[14][43] 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 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.

[68][71] A second ichnospecies, P. sarjeanti, was described from eastern Iran and opens the possibility that palaeotheres could have extended in geographical range to the region by the middle to late Eocene.

The evolutionary history of palaeotheres might have emphasized the sense of smell rather than sight or hearing, evident by the smaller orbits and the apparent lack of a derived auditory system.

[72] Palaeotherium was amongst the largest mammals to inhabit Europe during the middle to late Eocene, with only a few contemporary mammalian groups such as lophiodonts, anoplotheriids, and other palaeotheres reaching similar or larger body sizes.

[72][55][62] According to Sandra Engels in a conference paper, both Palaeotherium and Plagiolophus have dentitions capable of processing harder items such as hard fruits, while their predecessors, such as Hyracotherium and Propalaeotherium, were adapted to softer food.

Modern mammalian orders including the Perissodactyla, Artiodactyla, and Primates (or the suborder Euprimates) appeared already by the early Eocene, diversifying rapidly and developing dentitions specialized for folivory.

[77] The Holarctic mammalian faunas of western Europe were therefore mostly isolated from other landmasses including Greenland, Africa, and eastern Eurasia, allowing for endemism to develop.

[79][78] The stratigraphic ranges of the early species of Palaeotherium also overlapped with metatherians (Herpetotheriidae), cimolestans (Pantolestidae, Paroxyclaenidae), rodents (Ischyromyidae, Theridomyoidea, Gliridae), eulipotyphlans, bats, apatotherians, carnivoraformes (Miacidae), and hyaenodonts (Hyainailourinae, Proviverrinae).

Other mammal genera recorded within the locality include the herpetotheriid Amphiperatherium, ischyromyids Ailuravus and Plesiarctomys, pseudosciurid Treposciurus, omomyid Necrolemur, adapid Leptadapis, proviverrine Proviverra, palaeotheres (Propalaeotherium, Anchilophus, Lophiotherium, Plagiolophus), hyrachyid Chasmotherium, lophiodont Lophiodon, dichobunids Hyperdichobune and Mouillacitherium, choeropotamid Rhagatherium, anoplotheriid Catodontherium, amphimerycid Pseudamphimeryx, cebochoerid Cebochoerus, tapirulid Tapirulus, mixtotheriid Mixtotherium, and the xiphodonts Dichodon and Haplomeryx.

ruetimeyeri and all the aforementioned species from the region in MP16 along with the herpetotheriids Amphiperatherium and Peratherium, apatemyid Heterohyus, nyctithere Saturninia, omomyids (Necrolemur, Pseudoloris, and Microchoerus), adapid Adapis, ischyromyid Ailuravus, glirid Glamys, pseudosciurid Sciuroides, theridomyids Elfomys and Pseudoltinomys, hyaenodonts (Paracynohyaenodon, Paroxyaena, and Cynohyaenodon), carnivoraformes (Simamphicyon, Quercygale, and Paramiacis), cebochoerids Cebochoerus and Acotherulum, choeropotamids Choeropotamus and Haplobunodon, tapirulid Tapirulus, anoplotheriids (Dacrytherium, Catodontherium, and Robiatherium, dichobunid Mouillacitherium, robiacinid Robiacina, xiphodonts (Xiphodon, Dichodon, Haplomeryx), amphimerycid Pseudamphimeryx, lophiodont Lophiodon, hyrachyid Chasmotherium, and other palaeotheres (Plagiolophus, Leptolophus, Anchilophus, Metanchilophus, Lophiotherium, Pachynolophus, Eurohippus).

[51] P. giganteum is recorded from the Spanish locality of Mazaterón along with the testudines Hadrianus and Neochelys, alligatoroid Diplocynodon, baurusuchid Iberosuchus, adapoid Mazateronodon, omomyid Pseudoloris, pseudosciurid Sciuroides, theridomyids Pseudoltinomys and Remys, hyaenodont Proviverra, anoplotheriids (Duerotherium and cf.

[78][80][84][85] The causes of the faunal turnover have been attributed to a shift from humid and highly tropical environments to drier and more temperate forests with open areas and more abrasive vegetation.

[55] By the late Eocene, the latest species of Palaeotherium were widespread throughout western Europe, including what is now Portugal,[54] Spain,[89][90] France,[56] Switzerland,[91] Germany,[92] and the United Kingdom.

[56][43] The locality indicates that the multiple subspecies of Palaeotherium coexisted with the herpetotheriid Peratherium, theridomyids Blainvillimys and Theridomys, ischyromyid Plesiarctomys, glirid Glamys, hyaenodonts Hyaenodon and Pterodon, amphicyonid Cynodictis, palaeotheres Plagiolophus and Anchilophus, dichobunid Dichobune, choeropotamid Choeropotamus, cebochoerids Cebochoerus and Acotherulum, anoplotheriids (Anoplotherium, Diplobune, and Dacrytherium), tapirulid Tapirulus, xiphodonts Xiphodon and Dichodon, cainothere Oxacron, amphimerycid Amphimeryx, and the anthracothere Elomeryx.

The Turgai Strait, which once separated much of Europe from Asia, is often proposed as the main European seaway barrier prior to the Grande Coupure, but some researchers challenged this perception recently, arguing that it completely receded already 37 Ma, long before the Eocene-Oligocene transition.