Mixtotherium

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". 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 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 , including Cainotherium (skeleton at Natural History Museum of Basel) 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. 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. 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. {{clade| style=font-size:85%; line-height:85% 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: == Description ==

Skull , 1908. Colette Dechaseaux used the drawings and redrew the outlines of the incomplete mandible portions to resemble those of hyraxes in 1974. Mixtotherium is characterized by a low skull roof with a prominent sagittal crest that extends toward the back of the occipital ridge at the skull's back. The front side's frontal bones of the Mixtotheriidae are enlarged. The orbits for the eyes are enlarged and directed forwards to the skull's face, while the postorbital bar did not connect. The mastoid part of the temporal bone between the side portions of the occipital bone (exoccipitals) and the squamosal bone of the skull's back is not exposed. The tympanic part of the temporal bone (an inner bone of the ear) is proportionally small and moderately compressed between the center of the mandibular fossa (a fissure in the tympanic bone) of the temporal bone and a thick tympanic process. The snout (or muzzle) is short and wide. Endocast anatomy (Procavia capensis) skull, National Museum of Natural History. The mandibular rami of hyraxes are wide similar to those of Mixtotherium. M. cuspidatum is known by a plaster brain endocast which was described first by Dechaseaux in 1973 and is today held by the Victor Brun Natural History Museum. In a newer and more complete endocast model, half of the olfactory bulbs meet with each other then diverge. The cribriform plate is located in the frontmost area of the olfactory bulb chamber with a small underside expansion. The bulbs are separated from the cerebrum of the brain by a short and circular fissure. The incisors are formatted in a semicircular arc and are separated from each other by small diastemata, or gaps between teeth. The thickness level decreases slightly from the third incisors to the first incisors. The body mass of two mixtotheriid species have been estimated by Orliac et al. in 2021 based on a formula using dental measurements. They stated that the mixtotheriids ranged from in the case of M. gresslyi to regarding M. cuspidatum. They did not offer body mass estimates for other valid species of mixtotheres. Sudre and Ginsburg in 1993 argued that M. gresslyi as known from Lissieu is smaller than M. priscum from the Laprade fauna. == Palaeobiology ==

The Mixtotheriidae is one of the bunoselenodont family artiodactyl groups within western Europe. As a result, it is thought to have had mixed frugivorous-folivorous diets. According to Sudre in 1972, the different dental morphologies of mixtothere species may suggest different ecological habits. He stated that M. priscum has bunodont molars reminiscent of Dacrytherium while M. gresslyi has more selenodont molars and therefore may have had different diets from M. priscum. Hooker in 1986 inferred that because of the dental similarities of Mixtotherium to Dacrytherium, they both therefore may have had folivorous diets similar to indriid lemurs. The postcranial morphology of Mixtotherium is poorly known because of the overall lack of evidence. On the other hand, it is thought to have shared similar palaeobiologies with hyracoids. One hypothesis was that despite the similar dental morphologies to indriid lemurs, mixtotheriids may have simply been ground-dwelling folivores. Notably, mixtotheriids share similar cranial morphologies with two different arboreal mammal groups, namely the extinct adapine primates and the extant hyraxes. Because of its facial convergence with adapines and mandibular similarities with hyraxes, Weppe in his 2022 thesis speculated that it may have possibly had arboreal habits similar to the other mammal groups. == Palaeoecology ==

of Europe and Asia during the Middle Eocene with possible artiodactyl and perissodactyl dispersal routes. For much of the Eocene, a hothouse climate with humid, tropical environments with consistently high precipitations prevailed. 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. The omnivorous forms mostly either switched to folivorous diets or went extinct by the Middle Eocene (47–37 Ma) along with the archaic "condylarths". By the Late Eocene (approx. 37–33 Ma), most of the ungulate form dentitions shifted from bunodont cusps to cutting ridges (i.e. lophs) for folivorous diets. Land-based connections to the north of the developing Atlantic Ocean were interrupted around 53 Ma, meaning that North America and Greenland were no longer well-connected to western Europe. From the Early Eocene up until the Grande Coupure extinction event (56 Ma - 33.9 Ma), the western Eurasian continent was separated into three landmasses, the former two of which were isolated by seaways: western Europe (an archipelago), Balkanatolia, and eastern Eurasia (Balkanatolia was in between the Paratethys Sea of the north and the Neotethys Ocean of the south). The Holarctic mammalian faunas of western Europe were therefore mostly isolated from other continents including Greenland, Africa, and eastern Eurasia, allowing for endemism to occur within western Europe. skeleton, Naturmuseum Senckenberg. Mixtotherium'' cooccurred with lophiodonts for a majority of its existence. M. cf. gresslyi is the earliest-known representative of its genus in the western European fossil record within the MP13 French locality La Défense. 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). and MP13 sites are stratigraphically the latest to have yielded remains of the bird clades Gastornithidae and Palaeognathae. In addition to M. cf. gresslyi, other mammals that made appearances in La Défense include dichobunids (Dichobune, Meniscodon, and Hyperdichobune), cebochoerids Cebochoerus and Gervachoerus, and the lophiodont Lophiodon. 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. The surviving herbivorous faunas shifted their dentitions and dietary strategies accordingly to adapt to abrasive and seasonal vegetation. The environments were still subhumid and full of subtropical evergreen forests, however. The Palaeotheriidae was the sole remaining European perissodactyl group, and frugivorous-folivorous or purely folivorous artiodactyls became the dominant group in western Europe. The largest species M. cuspidatum is known only from MP17b localities like the French site of Perrière. 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. Both the Mixtotheriidae and the Robiacinidae are monogeneric families that are last recorded by MP17b within western Europe, whereas many other European endemic families that had long coexisted with them persisted. The extinctions of the two families may be correlated with increasingly open and dry environments resulting from changes in climate and vegetation. == References ==