It is also thought to have shared with hyraxes proportionally wide mandibles in the horizontal area, likely supported by large muscles of mastication for chewing through food.

The genus occurred exclusively in western Europe due to it being a mostly isolated archipelago during much of the Eocene, coexisting with a wide variety of other artiodactyls and perissodactyls in subtropical-tropical environments.

[2] Filhol described another species from the phosphorite deposits of Quercy in 1883 based on a skull cast that palaeontologist Jean Albert Gaudry gave to him.

[5] In 1888, Filhol described another species from the Quercy lime deposits based on a partial mandible with the 4th premolar and the 3 molars, observing that the dentition was peculiar.

He recognized that the species name would have drawn attention to the taxonomic confusion resulting from another taxon "H. gresslyi",[7] which in 1908 was synonymized with Haplobunodon lydekkeri by the Swiss palaeontologist Hans Georg Stehlin.

He also disagreed with von Zittel's synonymy of Adrotherium with Dacrytherium, suggesting that the genus was instead based on the milk teeth of Mixtotherium.

Pearson argued that the genus does not form a natural group, or a clade indicating close evolutionary relations, with Cebochoerus or the Anthracotheriidae, although they do possess similar anatomical traits.

[17] Palaeontologists Jean Sudre and Léonard Ginsburg in 1993 supported retaining the Mixtotheriidae as a family but argued for the distinctions of both M. gresslyi and M. priscum, pointing out that the mixtothere species of different localities had significant variations in size.

[21] In 2021, Maëva Judith Orliac et al. suggested based on previous sources that M. priscum was probably synonymous with M. gresslyi and that R. lavergnensis is to be retained within Robiacina.

[21] Mixtotheres made their first appearance within western Europe by MP13 of the Mammal Palaeogene Zones along with several other artiodactyl families and ranged up to MP17b based on fossil localities.

[23][24][25] The phylogenetic relations of the Mixtotheriidae as well as the Anoplotheriidae, Xiphodontidae and Cainotheriidae have been elusive due to the selenodont morphologies (or having crescent-shaped ridges) of the molars, which were convergent with tylopods or ruminants.

[26] Some researchers considered the selenodont families Anoplotheriidae, Xiphodontidae, and Cainotheriidae to be within Tylopoda due to postcranial features that were similar to the tylopods from North America in the Palaeogene.

[28][21] In an article published in 2019, Romain Weppe et al. conducted a phylogenetic analysis of the Cainotherioidea within the Artiodactyla based on mandibular and dental characteristics, specifically in terms of relationships with artiodactyls of the Palaeogene.

They determined that the Cainotheriidae, Robiacinidae, Anoplotheriidae, and Mixtotheriidae formed a clade that was the sister group to the Ruminantia while Tylopoda, along with the Amphimerycidae and Xiphodontidae split earlier in the tree.

[21] The phylogenetic tree published in the article and another work about the cainotherioids is outlined below:[29] Eurodexis russelli Dichobune leporina Amphimeryx murinus Xiphodon castrense Paratoceras coatesi Eotylopus reedi Parvitragulus priscus Lophiomeryx chalaniati Archaeomeryx optatus Mixtotherium cuspidatum Anoplotherium latipes Dacrytherium ovinum Robiacina lavergnesis Robiacina minuta Robiacina quercyi Palembertina deplasi Paroxacron bergeri Paroxacron valdense Oxacron courtoisii Cainotherium laticurvatum Caenomeryx filholi Caenomeryx procommunis Plesiomeryx cadurcensis Plesiomeryx huerzeleri In 2020, Vincent Luccisano et al. created a phylogenetic tree of the basal artiodactyls, a majority endemic to western Europe, from the Palaeogene.

The phylogenetic tree as produced by the authors is shown below:[28] Bunophorus Gunophorus Diacodexis Protodichobune Eurodexis Buxobune Mouillacitherium Meniscodon Hyperdichobune Dichobune robertiana Dichobune leporina Homacodon Gobiohyus Khirtharia Entelodon Palaeocheorus Perchoerus Haplobunodon Cuisitherium Lophiobunodon Mixtotherium Robiacina Dacrytherium Diplobune Xiphodon Paraxiphodon Cainotherium Paroxacron Archaeomeryx Amphimeryx Pseudamphimeryx Aumelasia Hallebune Amphirhagatherium Cebochoerus Gervachoerus Choeropotamus Siamotherium In 2022, Weppe conducted a phylogenetic analysis in his academic thesis regarding Palaeogene artiodactyl lineages, focusing most specifically on the endemic European families.

The result, Weppe mentioned, matches up with previous phylogenetic analyses on the Cainotherioidea with other endemic European Palaeogene artiodactyls that support the families as a clade.

[30] The horizontal back portion of the mandible, or the mandibular corpus, is noticeably large, reminiscent of those of hyracoids including extant hyraxes.

Under these conditions, she said, the mandible would have fit perfectly with the skull, the upper and lower teeth being properly in occlusion (full contact) with each other.

[22] Dechaseaux noticed that there is a deep and wide space between the neocortex and the cerebellar vermis, therefore revealing an exposed and lowered midbrain, which she considered "remarkable".

[22] Mixtotherium has a complete set of 3 three incisors, 1 canine, 4 premolars, and 3 molars on each half of the upper and lower jaws, consistent with the primitive placental mammal dental formula of 3.1.4.33.1.4.3 for a total of 44 teeth.

The astragali characteristics of Mixtotherium appear in all other contemporary primitive artiodactyls with the exception of Dacrytherium, which as an anoplotheriid has specialized morphologies.

The body mass formula based on astragali was previously established by Jean-Noël Martinez and Sudre in 1995 for Palaeogene artiodactyls, although Mixtotherium was not included in the initial study.

[17] Notably, mixtotheriids share similar cranial morphologies with two different arboreal mammal groups, namely the extinct adapine primates and the extant hyraxes.

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.

37–33 Ma), most of the ungulate form dentitions shifted from bunodont cusps to cutting ridges (i.e. lophs) for folivorous diets.

[37][43][44] The stratigraphic ranges of the early species of Mixtotherium also overlapped with metatherians (Herpetotheriidae), cimolestans (Pantolestidae, Paroxyclaenidae), rodents (Ischyromyidae, Theridomyoidea, Gliridae), eulipotyphlans, bats, apatotherians, carnivoraformes (Miacidae), and hyaenodonts (Hyainailourinae, Proviverrinae).

gresslyi, other mammals that made appearances in La Défense include dichobunids (Dichobune, Meniscodon, and Hyperdichobune), cebochoerids Cebochoerus and Gervachoerus, and the lophiodont Lophiodon.

[43][45][47][48] 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.

[24][26] In Perrière, its fossils were found with those of the herpetotheriids Peratherium and Amphiperatherium, pseudorhyncocyonid Pseudorhyncocyon, apatemyid Heterohyus, nyctitheriid Saturninia, various bats, rodents (Gliridae, Theridomyidae), omomyids Pseudoloris and Microchoerus, adapid Leptadapis, hyaenodontid Hyenodon, miacid Quercygale, palaeotheres (Lophiotherium, Palaeotherium, and Plagiolophis), dichobunid Mouillacitherium, cebochoerid Acotherulum, anoplotheriid Dacrytherium, tapirulid Tapirulus, xiphodonts Dichodon and Haplomeryx, and the amphimerycid Pseudamphimeryx.