The type species P. minor was initially described by the French naturalist Georges Cuvier in 1804 based on postcranial material including a now-lost skeleton originally from the Paris Basin.

It has longer postcanine diastemata (gaps between teeth) compared to Palaeotherium and brachyodont (low-crowned) dentition that evolutionarily progressed towards hypsodonty (high-crowned) in response to climatic trends.

Western Europe, where Plagiolophus was largely present, was an archipelago that was isolated from the rest of Eurasia, meaning that it lived in an environment with various other faunas that also evolved with strong levels of endemism.

More specifically, P. minor over time was observed to have consumed less hard foods (fruits, seeds) and became more specialized but less selective towards tough, abrasive, and older leaves in response to environmental trends in the late Eocene to early Oligocene.

Plagiolophus was consistently diverse for much of its evolutionary history and survived far past the Grande Coupure extinction event, likely because some of its species were well-adapted towards major environmental trends as a result of their dietary changes and cursorial nature.

The French prefect Nicolas Frochot later acquired it and brought it to the National Museum of Natural History, France, where Cuvier was then able to observe that it must have been a skeleton of a Palaeotherium species.

[8] British palaeontologist Richard Owen in 1848 wrote about a nearly complete lower jaw with both deciduous and permanent dental sets that was uncovered from the Eocene beds of Hordle, England by Alexander Pytts Falconer, observing that it had one less premolar for a total of 3 of them than in other species of Palaeotherium and erecting the genus Paloplotherium based on the mandible.

In 1865 for example, French palaeontologist Jean Albert Gaudry recognized Paloplotherium as valid genus instead of Plagiolophus, grouping P. minor, P. ovinus, and P. annectens into it and erecting another species P.

[15] In 1869, Swiss palaeontologists François Jules Pictet de la Rive and Aloïs Humbert wrote that Palaeotherium, Paloplotherium, and Plagiolophus were all valid genera and erected two species for the latter: P. siderolithicus using fossil molars from a museum collection and P. valdensis based on a mandible that was smaller in proportion than that of P.

[21] In one of his monographies, written the same year, Stehlin erected Palaeotherium Rütimeyeri with official fossil descriptions to replace the previous species name and synonymized Paloplotherium javali with Plagiolophus fraasi.

[25] In 1989, palaeontologists Michel Brunet and Yves Jehenne considered Paloplotherium to be distinct from Palaeotherium and erected for the former genus two additional species: P. majus from the fossil collections of the Quercy Phosphorites Formation and P. ministri from the French commune of Villebramar.

[30] Remy, in 2004, followed up by erecting P. ringeadei, named after Ruch fossil deposit discoverer Michel Ringead and known by a skull of an adult female with cheek teeth, and P. mamertensis, which was assigned a left maxilla with partial dentition from Robiac for a holotype specimen.

[31][42] The subgenus Plagiolophus is defined by a shallow nasal notch that is always located in front of P2, the lack of any preorbital fossa and a thinner body of the mandible compared to that of Paloplotherium.

[31] The horizontal ramus of the manible is robust but varies in such based on factors pertaining to species morphology and sexual dimorphism, its underside being mostly convex but also straight at the front area.

The subgenus Plagiolophus differs from Paloplotherium by its longer postcanine diastemata and greater degree of hypsodonty, and the former has proportionally narrow and oblique lingual lophs in its upper cheek teeth compared to that of the latter.

[53] Both Palaeotherium and Plagiolophus have dentitions that are both capable of chewing through harder items such as fruits without wearing their teeth down quickly compared to their pachynolophine predecessors (i.e. Hyracotherium and Propalaeotherium).

[31] Leptolophus having a hypsodonty level similar to later Neogene equids suggests a distinct niche partitioning dietary strategy from contemporary palaeotheres, with Plagiolophus not showing a stricter preference towards abrasive plants based on dental evidence.

In a later Late Eocene German locality of Frohnstetten (MP20) in comparison, the cheek teeth of P. minor have similar amount of pits but has more rounded cusps and slightly less scratches, suggesting that it consumed less fruit and more abrasive leaves.

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.

[63][64] 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).

[67] The Geiseltal Obere Mittelkhole locality, dating to MP13, records fossils of P. cartieri along with the herpetotheriid Amphiperatherium, carnivoraforme Quercygale, hyaenodont Proviverra, amphilemurid Amphilemur, archaeonycterid Matthesia, paroxyclaenid Pugiodens, adapid Europolemur, omomyid Nannopithex, dichobunid Messelobunodon, choeropotamids Rhagatherium and Amphirhagatherium, lophiodonts Lophiodon and Paralophiodon, and the other palaeotheres Propalaeotherium and Lophiotherium.

[68] P. annectens, P. cartailhaci, and P. mamertensis are located in the MP16 French locality of Robiac 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 (Palaeotherium, Leptolophus, Anchilophus, Metanchilophus, Lophiotherium, Pachynolophus, Eurohippus).

[64][66][69][70] 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.

[76] The MP18 locality of La Débruge in France holds fossil records of both P. oweni and P. minor along with the herpetotheriid Peratherium, theridomyids Blainvillimys and Theridomys, ischyromyid Plesiarctomys, glirid Glamys, hyaenodonts Hyaenodon and Pterodon, amphicyonid Cynodictis, palaeotheres Palaeotherium 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.

[83] The seaway dynamics separating western Europe from other landmasses to strong extents but allowing for some levels of dispersals prior to the Grande Coupure are complicated and contentious, but many palaeontologists agreed that glaciation and the resulting drops in sea level played major roles in the drying of the seaways previously acting as major barriers to eastern migrants from Balkanatolia and western Europe.

The Turgai Strait 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.

[81] The earliest Oligocene marked the arrivals of later anthracotheres, entelodonts, ruminants (Gelocidae, Lophiomerycidae), rhinocerotoids (Rhinocerotidae, Amynodontidae, Eggysodontidae), carnivorans (later Amphicyonidae, Amphicynodontidae, Nimravidae, and Ursidae), eastern Eurasian rodents (Eomyidae, Cricetidae, and Castoridae), and eulipotyphlans (Erinaceidae).

[58] The climatic trends from the Grande Coupure event favored palaeothere species that had light body builds and were built for cursoriality such as P. minor, allowing them to transverse across more open lands and escape from newly arrived predators where shelter otherwise was scarce.

P. minor was found in Soumailles along with the theridomyids Blainvillimys and Theridomys, nimravid Eusmilus, entelodont Entelodon, cebochoerid Acotherulum, eggysodont Eggysodon, and the palaeothere Palaeotherium.

[91][92] In the MP25 French locality of Le Garouillas, the last surviving palaeothere species P. javali (the largest species of Plagiolophus) coexisted with the likes of the herpetotheriids Amphiperatherium and Peratherium, nyctithere Darbonetus, talpid Myxomygale, erinaceid Tetracus, bats (Vespertiliavus, Vaylatsia, Stehlinia), theridomyids (Blainvillimys, Issiodoromys, Theridomys), cricetid Eucricetodon, glirid Gliravus, nimravids (Quercylurus, Nimravus, Dinailurictis), amynodont Cadurcotherium, chalicothere Schizotherium, suoid Doliochoerus, dichobunid Metriotherium, cainotheres Plesiomeryx and Cainomeryx, lophiomerycid Lophiomeryx, and the bachithere Bachitherium.