Mixtotherium (Latin: (mixed) + Ancient Greek: (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 to the latest mixtotheriid M. cuspidatum with an estimated weight of .
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.
Taxonomy Early history
In 1880, the French naturalist Henri Filhol described fossils from the deposits of the French commune of Caylus, Tarn-et-Garonne (formerly "Caylux"). For one species (the other species he described now belong to Metriotherium and Dacrytherium), he designated the binomial name Mixtotherium cuspidatum to a small "pachyderm" with a continuous series of teeth. The specimen had a strong upper canine and upper molars with five sharp points (three in the front area). He also noticed that it had a proportionally enormous sagittal crest. The genus name Mixtotherium derives from both Latin for (mixed) and Ancient Greek for (beast or wild animal), meaning "mixed beast".
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. According to Filhol, the upper incisors were missing and the strong canines stuck out beyond the premolars similar to Mixtotherium. The naturalist stated the premolars were similar to those of Mixtotherium but that it had specific dental differences from it. As a result, he designated another binomial name Adrotherium depressum. Adrotherium derives from the Ancient Greek words ("stout" or "large") plus meaning "stout beast".
Henri Filhol provided more detailed descriptions of M. cuspidatum in 1882, confirming he made the genus name Mixtotherium known in 1880. He also provided an image for the skull of the species. Likewise, he reaffirmed the validity of A. depressum in 1884, reproducing an image of the skull cast that he previously described.
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 concluded that it must have had affinities with anoplotheriids like Anoplotherium and Diplobune based on dentition and gave another binomial name Uphelognatos quercyi.
In 1891, Swiss palaeontologist Ludwig Rütimeyer erected the species M. gresslyi based on some upper jaw fossils from the Swiss municipality of Egerkingen that were previously classified as "Hyopotamus (= Bothriodon) gresslyi". He recognized that the species name would have drawn attention to the taxonomic confusion resulting from another taxon "H. gresslyi", which in 1908 was synonymized with Haplobunodon lydekkeri by the Swiss palaeontologist Hans Georg Stehlin.
The same year that Rütimeyer erected M. gresslyi, German palaeontologist Karl Alfred von Zittel synonymized Mixtotherium with Diplobune and Adrotherium with Dacrytherium, synonymizing A. depressum with D. cayluxense (= D. ovinum). He did not indicate the status of the species M. cuspidatum. In 1896, palaeontologist Charles Earle objected to von Zittel's synonymy of Mixtotherium with Diplobune, considering it to be a valid genus entirely distinct from both Diplobune and Anoplotherium. 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. He considered that mixtotheres were intermediate between cebochoerids and anoplotheriids and arose from a common ancestral group of it and merycoidodonts.
Stehlin synonymized both Uphelnognatos and Adrotherium with the revalidated Mixtotherium in 1908, transferring both individual species of the junior synonyms to the senior synonym as M. queryci and M. depressum. He erected two additional species of mixtotheriids. The first was M. priscum from Egerkingen, which he stated was somewhat larger than M. gresslyi. The second was M. Leenhardti from the Quercy phosphorites deposits. In 1910, he erected M. infans from other fossils from Egerkingen, stating that it was a small-sized species.
In 1913, German palaeontologist Martin Schmidt erected the species M. mezi from the Jebel Qatrani Formation of Egypt, making it the first species classified as Mixtotherium from outside Europe. The species was eventually synonymized with Bothriogenys sp. by Patricia A. Holroyd et al. in 2010.
During 1927, British palaeontologist Helga Sharpe Pearson established the family Mixtotheriidae of which Mixtotherium is the only member. 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. In 1945, American palaeontologist George Gaylord Simpson demoted the Mixtotheriidae to subfamily rank within the Cebochoeridae as Mixtotheriinae, for which the other listed subfamily was Cebochoerinae.
Later taxonomic interpretations
British palaeontologist Jerry J. Hooker in 1986 recognized the validity of the Mixtotheriidae, with Mixtotherium as the only genus classified in the family. He stated that M. cuspidatum was the type species and that the other species included are M. depressum, M. gresslyi, M. quercyi, M. leenhardti, and M. infans as valid species. Hooker also synonymized M. priscum with M. gresslyi on the basis that the two species were difficult to separate from each other. However, he also argued that only M. cuspidatum, M. gresslyi, and M. infans were well-characterized whereas M. quercyi and M. depressum very closely resemble M. cuspidatum. Hooker recognized the possibility of subspecies for Mixtotherium based on Egerkingen material. He stated that a complete revision of the genus would be ideal. 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.
In 2000, Hooker and Marc Weidmann referenced the 1986 synonymization of M. priscum with M. gresslyi, hence not listing the former as a valid species. They also transferred the species Robiacina lavergnensis, previously erected by Sudre in 1977, to Mixtotherium as M. lavergnense. They also synonymized R. weidmanni, previously named by Sudre in 1978, with M. lavergnense. Damien Becker et al. in 2013 adopted the reclassification of M. lavergnense, but in 2020, Romain Weppe et al. chose to retain in Robiacina the species R. lavergnensis, going contrary to the previous reclassification by Hooker and Weidmann. 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.
Classification
Mixtotherium is the type and only genus of the artiodactyl family Mixtotheriidae. The genus was endemic to western Europe and lived from the Middle to Late Eocene (~44.9 to 37 Ma). Originally, it was classified as a member of the superfamily Cainotherioidea with the Cainotheriidae by Hooker and Weidmann in 2000. Since 2020, however, the Mixtotheriidae is no longer classified within the superfamily, although it is considered to be a sister group to it. 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. The evolutionary origin of the Mixtotheriidae is unknown as its sudden appearance by MP13 could not be linked to any prior taxon.
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. 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. Other researchers consider them more closely related to ruminants than tylopods based on dental morphology. Different phylogenetic analyses have produced different results for the "derived" (or of new evolutionary traits) selenodont Eocene European artiodactyl families, making it uncertain whether they were closer to the Tylopoda or Ruminantia.
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. The results retrieved that the superfamily was closely related to the Mixtotheriidae and Anoplotheriidae. 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. The phylogenetic tree published in the article and another work about the cainotherioids is outlined below:
In 2020, Vincent Luccisano et al. created a phylogenetic tree of the basal artiodactyls, a majority endemic to western Europe, from the Palaeogene. In one clade, the "bunoselenodont endemic European" Mixtotheriidae, Anoplotheriidae, Xiphodontidae, Amphimerycidae, Cainotheriidae, and Robiacinidae are grouped together with the Ruminantia. The phylogenetic tree as produced by the authors is shown below: