| Mixtotherium Temporal range: Middle – Late Eocene
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| Mixtotherium priscum cranium, Natural History Museum of Basel | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Family: | †Mixtotheriidae Pearson, 1927 |
| Genus: | †Mixtotherium Filhol, 1880 |
| Type species | |
| †Mixtotherium cuspidatum Filhol, 1880
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| Other species | |
| Synonyms | |
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Genus synonymy
Synonyms of M. depressum
Synonyms of M. quercyi
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Mixtotherium (Latin: mixtus (mixed) + Ancient Greek: θήρ (beast or wild animal) meaning "mixed beast") is an extinct genus of Palaeogene artiodactyls belonging to the monotypic family Mixtotheriidae. Known informally as mixtotheriids or mixtotheres, these artiodactyls were endemic to western Europe and occurred from the middle to late Eocene. The genus and type species were both first established by the French naturalist Henri Filhol in 1880. Several species are well known by good skull fossils, which were informative enough to allow for classifications of the species to their own family. The Mixtotheriidae, first recognized by Helga Sharpe Pearson in 1927, is currently known by 7 valid species, although M. priscum is thought by several authors to be synonymous with M. gresslyi. The affinities of the Mixtotheriidae in relation to other artiodactyl families is uncertain, but it is currently thought to have been related to the Cainotherioidea and Anoplotheriidae.
Mixtotherium had various unusual skull morphologies that no other contemporary Palaeogene artiodactyl shared. In instances of convergent evolution, mixtotheriids shared with adapid primates large sagittal crests (ridges on the top of the skull), wide zygomatic arches (cheek bones), short but wide snouts, and enlarged orbits (eye sockets) that are situated toward the center of the skull's face. 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. It also had weak incisors, robust but premolariform canines, and low-crowned plus bunodont-selenodont upper molars. Comparatively, its postcranial skeleton is almost unknown because only two types of foot bone fossils are attributed to it, making its overall anatomy unknown. Mixtotherium varied in size from the earlier-appearing M. gresslyi with an estimated body mass of 2.8 kg (6.2 lb) to the latest mixtotheriid M. cuspidatum with an estimated weight of 11 kg (24 lb).
Mixtotherium is thought to have been purely folivorous or frugivorous plus folivorous. It may have been either a ground dweller or an arboreal locomotor like hyraxes and basal primates, but these behaviors are almost completely speculative due to the lack of complete postcranial material. 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. The Mixtotheriidae probably went extinct by the late Eocene, although why is unclear.