Mixtotherium

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Mixtotherium
Temporal range: Middle – Late Eocene 45–37  Ma
Mixtotherium priscum skull.jpg
Mixtotherium priscum cranium, Natural History Museum of Basel
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Family:† Mixtotheriidae
Pearson, 1927
Genus:† Mixtotherium
Filhol, 1880
Type species
†Mixtotherium cuspidatum
Filhol, 1880
Other species
  • †M. depressumFilhol, 1883
  • †M. quercyiFilhol, 1888
  • †M. gresslyi Rütimeyer, 1891
  • †M. priscum? Stehlin, 1908
  • †M. leenhardtiStehlin, 1908
  • †M. infansStehlin, 1910
Synonyms
Genus synonymy
  • AdrotheriumFilhol, 1883
  • UphelognatosFilhol, 1888
  • AdiotheriumEarle, 1896
Synonyms of M. depressum
  • Adrotherium depressumFilhol, 1883
Synonyms of M. quercyi
  • Uphelognatos quercyiFilhol, 1888

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.

Contents

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.

Taxonomy

Early history

Mixtotherium cuspidatum skull 1882.png
Mixtotherium Adrotherium depressum skull 1884.png
Skull material of Mixtotherium cuspidatum from 1882 (left) and "Adrotherium" (= Mixtotherium) depressum from 1884 (right)

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. [1] The genus name Mixtotherium derives from both Latin for mixtus (mixed) and Ancient Greek for θήρ (beast or wild animal), meaning "mixed beast". [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. 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. [3] Adrotherium derives from the Ancient Greek words αδρος ("stout" or "large") plus θήρ meaning "stout beast". [2]

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. [4] Likewise, he reaffirmed the validity of A. depressum in 1884, reproducing an image of the skull cast that he previously described. [5]

Sketch of the mandible of M. leenhardti with an M1-M3 dental series, 1908 Mixtotherium leenhardti mandible 1908.png
Sketch of the mandible of M. leenhardti with an M1-M3 dental series, 1908

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. [6]

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", [7] which in 1908 was synonymized with Haplobunodon lydekkeri by the Swiss palaeontologist Hans Georg Stehlin. [8]

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. [9] 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. [10]

Stehlin synonymized both Uphelnognatos and Adrotherium with the revalidated Mixtotherium in 1908, tranferring 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. [8] In 1910, he erected M. infans from other fossils from Egerkingen, stating that it was a small-sized species. [11]

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. [12] [13] The species was eventually synonymized with Bothriogenys sp. by Patricia A. Holroyd et al. in 2010. [14]

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. [15] 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. [16]

Later taxonomic interpretations

Mandible of M. cuspidatum with a reconstructed complete dental set, 1908 Reconstructed dentition Mixtotherium.png
Mandible of M. cuspidatum with a reconstructed complete dental set, 1908

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. [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. [18]

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. [19] Damien Becker et al. in 2013 adopted the reclassification of M. lavergnense, [20] 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. [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. [22]

Classification

The Mixtotheriidae is thought to be closely related to the Cainotheriidae, including Cainotherium (skeleton at Natural History Museum of Basel) Cainotherium restoration.jpg
The Mixtotheriidae is thought to be closely related to the Cainotheriidae, 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. [19] [23] Since 2020, however, the Mixtotheriidae is no longer classified within the superfamily, although it is considered to be a sister group to it. [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. The evolutionary origin of the Mixtotheriidae is unknown as its sudden appearance by MP13 could not be linked to any prior taxon. [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. [27] 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. [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. 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. [21] The phylogenetic tree published in the article and another work about the cainotherioids is outlined below: [29]

In 2022, Weppe conducted a phylogenetic analysis in his academic thesis regarding Palaeogene artiodactyl lineages, focusing most specifically on the endemic European families. He found that the Anoplotheriidae, Mixtotheriidae, and Cainotherioidea form a clade based on synapomorphic dental traits (traits thought to have originated from their most recent common ancestor). 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. He determined that the Mixtotheriidae forms a clade with the Cainotherioidea based on close anatomical traits, therefore contradicting previous results supporting the Cainotherioidea being more closely related to the Anoplotheriidae than the Mixtotheriidae. [26]

Description

Skull

Drawning of a cranium and incomplete mandible of M. cuspidatum by Hans Georg Stehlin, 1908. Colette Dechaseaux used the drawings and redrew the outlines of the incomplete mandible portions to resemble those of hyraxes in 1974. Mixtotherium cuspidatum cranium mandible 1908.png
Drawning of a cranium and incomplete mandible of M. cuspidatum by Hans Georg Stehlin, 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. [30] [23] The zygomatic arches are also wide. These traits are also generally observed in large adapine primates like Leptadapis and Magnadapis in an instance of convergent evolution. [22]

Similar to anthracotheres, the lengthy ear canal and the tympanic bone's neck that covers it in the skull of Mixtotherium are compressed by the postglenoid and post-tympanic processes (or projections) of the squamosal bone. The tympanic region of mixtotheres is less specialized than those of anthracotheres, with a less extreme compression and lengthening of the tympanic neck, no sideway shift of the glenoid (or shallow) surface, and a short ear canal. [15] The skull of Mixtotherium can superficially resemble that of Cebochoerus due to the low skull as well as the compositions of the parietal bone and jugal bone at the skull's front as well as the occipital bone at the back. The mixtothere differs from the cebochoerid by having a swollen as opposed to flattened tympanic bone. [30]

The horizontal back portion of the mandible, or the mandibular corpus, is noticeably large, reminiscent of those of hyracoids including extant hyraxes. [22] The horizontal ramus of M. leenhardti is slightly deeper compared to other species, but it could have been the result of an individual's old age. [17] Mandibular fossil evidence is incomplete, leading to the outline not being completely known. Palaeoneurologist Colette Dechaseaux used the drawings of the skull and mandible of M. cuspidatum by Stehlin to adjust them to appear articulated with each other. She reconstructed the mandibular condyle and coronoid process of the mandible, both back portions of the mandible, of Mixtotherium as being similar to 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. [31]

Endocast anatomy

Rock hyrax (Procavia capensis) skull, National Museum of Natural History. The mandibular rami of hyraxes are wide similar to those of Mixtotherium. Procavia capensis 5zz.jpg
Rock hyrax (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. [22] The original virtual plaster model as studied by Oliac et al. in 2021 has a volume of 16,500 mm (650 in)3 while the second and more complete model with filled missing volumes measures 19,503 mm (767.8 in)3, which they noted was still an underestimation. The olfactory bulbs measure 2,145 mm (84.4 in)3, filling in 13% of the original endocast volume. [22] According to Dechaseaux, the endocast of Mixtotherium differs from other contemporary artiodactyls such as the dichobunids, amphimerycids, cebochoerids, and cainotheres in the elongated brain resulting in pointed olfactory peduncles and olfactory bulbs. The olfactory bulbs are proportionally large and slightly straight although not to the upper surface of the neocortex. [32] 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. [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". The maximum height of the forebrain is little more than half its length. Unlike other endemic European artiodactyl endocasts she studied, the cerebrum has a low height but is at a much higher position than the neocortex. Three furrows can be observed on the neocortex: a rectilinear suprasylvia that extends over the back region of the cerebral hemisphere up to the middle of the olfactory part of the brain, a narrow gyrus between the olfactory region and the suprasylvia, and another furrow that is not entirely visible. The suprasylvia fissure is located on the right cerebral hemisphere and appears parallel to the sagittal axis of the endocast. The neocortex itself does not cover the olfactory bulbs or the cerebellum. The frontal lobe is narrow while the temporal lobe is enlarged. The cerebellar vermis is enlarged and rounded, its area in front of the rimary fissure of the cerebellum being wider and more swollen. The cerebellar hemispheres are small and not extensive. The characteristics of the hemispheres and vermis led Dechaseaux to conclude that the paleocerebellum was more developed than the neocerebellum. [32]

In Mixtotherium, the positions of the furrows and the circular vermis that protrudes back are similar to what is observed in Diplobune. These traits are derived compared to various other basal artiodactyls with known endocasts and support the close affinities of the Mixtotheriidae and Anoplotheriidae. It differs from Diacodexis and Dichobune by the more square-shaped outlines and divergence of the olfactory bulbs chambers. Similar to the Raoellidae, the location of the brain lies in the back area relative to the orbits, the cerebrum being in the back position to the postorbital process. The brain's location may have to do with the wide zygomatic arches and large sagittal crest, which are evident for large muscles of mastication for grinding food. [22]

Dentition

M. priscum mandible, Natural History Museum of Basel Mixtotherium priscum.jpg
M. priscum mandible, Natural History Museum of Basel

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. [8] [33] 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. [8] The incisors overall are weak, and the I3 and canine are separated by a small diastema. The canines are prominent, robust, and have a slightly premolariform shape. P1 is narrow and adjacent to the upper canine. P1 is seemingly separated from the lower canine by a small diastema as well. P2 and P3 have well-developed lingual cusps while P2 and P3 are elongated and have 3 cusps aligned with each other. P4 is triangular in outline and molariform in shape but lacks the paraconule cusp. [30] [23]

The upper molars are both brachyodont (low-crowned) and bunoselodont, or bunodont (having rounded cusps) and selenodont, in form. They also appear roughly triangular or roughly trapezoidal in outline at an upper view of the teeth. Their conical paraconule cusps are reduced and are part of the preprotocrista crest (an enamel ridge). The protoconule cusps are weak, and the parastyle cusps and mesostyle cusps appear labially round. M1–M2 usually exhibit back-sided cingulids (or lower tooth ridges) that are round lingually and extend to the back of the entoconid cusp. The transverse third lobe of the M3 is compressed. [30] [23]

Postcranial skeleton

In terms of postcranial anatomy, mixtotheriids are known only from astragali and a calcaneus, both foot bones forming the tarsus, from the locality of La Défense in France, otherwise having no other documented postcranial bone. [23] Sudre and Ginsburg listed the postcranial remains as belonging to M. cf. gresslyi (cf. meaning uncertain species identification) because of the common appearance of the species in the locality. Based on one astragalus, the front trochlea (or pulley) is asymmetrical with a high elevation of the inner lip compared to the back trochlea. The astragali characteristics of Mixtotherium appear in all other contemporary primitive artiodactyls with the exception of Dacrytherium, which as an anoplotheriid has specialized morphologies. A calcaneum that may possibly belong to Mixtotherium was described as having a massive beak with a sloping navicular facet. [18] [23]

Body mass

Estimated size comparison of M. cuspidatum with a speculated body build Mixtotherium size comparison chart.png
Estimated size comparison of M. cuspidatum with a speculated body build

Helder Gomes Rodrigues et al. estimated the body mass of M. cf. gresslyi on astragali from La Défense, yielding the result of 3 kg (6.6 lb). 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. [34] [35] 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 2.8 kg (6.2 lb) in the case of M. gresslyi to 11 kg (24 lb) regarding M. cuspidatum. They did not offer body mass estimates for other valid species of mixtotheres. [22]

According to Hooker in 1986, M. gresslyi is a medium-sized species; it is smaller than M. cuspidatum but larger than M. infans. [17] Sudre et al. in 1990 stated that M. priscum was smaller than both M. leenhardti and M. cuspidatum, the latter of which they said was much larger. They also reported that M. cf. gresslyi from Quercy deposits was smaller than the Laprade fauna (MP14) species M. priscum. [36] Sudre and Ginsburg in 1993 argued that M. gresslyi as known from Lissieu is smaller than M. priscum from the Laprade fauna. [18]

Palaeobiology

Reconstruction of the head of M. cuspidatum Portrait of Mixtotherium.png
Reconstruction of the head of M. cuspidatum

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. [37] 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. [38] 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. [17]

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. [23] [22] One hypothesis was that despite the similar dental morphologies to indriid lemurs, mixtotheriids may have simply been ground-dwelling folivores. [17] Notably, mixtotheriids share similar cranial morphologies with two different arboreal mammal groups, namely the extinct adapine primates and the extant hyraxes. [22] 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. [26]

Palaeoecology

Palaeogeography of Europe and Asia during the middle Eocene with possible artiodactyl and perissodactyl dispersal routes. Middle Eocene Paleogeography Tethys Dispersals.jpg
Palaeogeography 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. [39] [40]

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). [41] 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. [40] The European mammals of the late Eocene (MP17 - MP20 of the Mammal Palaeogene zones) were mostly descendants of endemic middle Eocene groups as a result. [42]

Lophiodon skeleton, Naturmuseum Senckenberg. Mixtotherium cooccurred with lophiodonts for a majority of its existence. Lophiodon sp. - Naturmuseum Senckenberg - DSC02246.JPG
Lophiodon 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. [18] By then, it would have coexisted with perissodactyls (Palaeotheriidae, Lophiodontidae, and Hyrachyidae), non-endemic artiodactyls (Dichobunidae and Tapirulidae), endemic European artiodactyls (Choeropotamidae (possibly polyphyletic, however), Cebochoeridae, and Anoplotheriidae), and primates (Adapidae). Both the Amphimerycidae and Xiphodontidae made their first appearances by the level MP14. [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). [25] Other MP13-MP14 sites have also yielded fossils of turtles and crocodylomorphs, [45] and MP13 sites are stratigraphically the latest to have yielded remains of the bird clades Gastornithidae and Palaeognathae. [46]

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 . [18]

Not all mixtotheriid species are well-correlated with faunal ranges, as some like M. quercyi are not well-documented due to only being described from older fossil collections. [26] M. priscum, M. gresslyi and M. infans are known exclusively from MP14 deposits, and a species described as M. cf. gresslyi from Creechbarrow Limestone is dated to MP16. [36] [25]

After MP16, a faunal turnover occurred, marking the disappearances of the lophiodonts and European hyrachyids as well as the extinctions of all European crocodylomorphs except for the alligatoroid Diplocynodon . [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. The surviving herbivorous faunas shifted their dentitions and dietary strategies accordingly to adapt to abrasive and seasonal vegetation. [49] [50] 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. [51] [37]

The largest species M. cuspidatum is known only from MP17b localities like the French site of Perrière. [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 . [25]

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. [26]

Related Research Articles

<span class="mw-page-title-main">Artiodactyl</span> Order of mammals

Artiodactyls are placental mammals belonging to the order Artiodactyla. Typically, they are ungulates which bear weight equally on two of their five toes. The other three toes are either present, absent, vestigial, or pointing posteriorly. By contrast, most perissodactyls bear weight on an odd number of the five toes. Another difference between the two orders is that many artiodactyls digest plant cellulose in one or more stomach chambers rather than in their intestine. Molecular biology, along with new fossil discoveries, has found that cetaceans fall within this taxonomic branch, being most closely related to hippopotamuses. Some modern taxonomists thus apply the name Cetartiodactyla to this group, while others opt to include cetaceans within the existing name of Artiodactyla. Some researchers use "even-toed ungulates" to exclude cetaceans and only include terrestrial artiodactyls, making the term paraphyletic in nature.

<i>Palaeotherium</i> Extinct genus of mammals

Palaeotherium is an extinct genus of equoid that lived in Europe and possibly the Middle East from the Middle Eocene to the Early Oligocene. It is the type genus of the Palaeotheriidae, a group exclusive to the Palaeogene that was closest in relation to the Equidae, which contains horses plus their closest relatives and ancestors. Fossils of Palaeotherium were first described in 1782 by the French naturalist Robert de Lamanon and then closely studied by another French naturalist, Georges Cuvier, after 1798. Cuvier erected the genus in 1804 and recognized multiple species based on overall fossil sizes and forms. As one of the first fossil genera to be recognized with official taxonomic authority, it is recognized as an important milestone within the field of palaeontology. The research by early naturalists on Palaeotherium contributed to the developing ideas of evolution, extinction, and succession and demonstrating the morphological diversity of different species within one genus.

<i>Anoplotherium</i> Extinct genus of endemic Paleogene European artiodactyls

Anoplotherium is the type genus of the extinct Palaeogene artiodactyl family Anoplotheriidae, which was endemic to Western Europe. It lived from the late Eocene to the earliest Oligocene. It was the fifth fossil mammal genus to be described with official taxonomic authority, with a history extending back to 1804 when its fossils from Montmartre in Paris, France were first described by the French naturalist Georges Cuvier. Discoveries of incomplete skeletons of A. commune in 1807 led Cuvier to thoroughly describe unusual features for which there are no modern analogues. His drawn skeletal and muscle reconstructions of A. commune in 1812 were amongst the first instances of anatomical reconstructions based on fossil evidence. Cuvier's contributions to palaeontology based on his works on the genus were revolutionary for the field, not only proving the developing ideas of extinction and ecological succession but also paving the way for subfields such as palaeoneurology. Today, there are four known species.

<span class="mw-page-title-main">Xiphodontidae</span> Extinct family of mammals

Xiphodontidae is an extinct family of herbivorous even-toed ungulates, endemic to Europe during the Eocene 40.4—33.9 million years ago, existing for about 7.5 million years. Paraxiphodon suggests that they survived into the Lower Oligocene, at least.

<span class="mw-page-title-main">Anoplotheriidae</span> Extinct family of mammals

Anoplotheriidae is an extinct family of artiodactyl ungulates. They were endemic to Europe during the Eocene and Oligocene epochs about 44—30 million years ago. Its name is derived from the Ancient Greek: ἂνοπλος ("unarmed") and θήριον ("beast"), translating as "unarmed beast".

<i>Pterodon</i> (mammal) Extinct genus of mammals

Pterodon is an extinct genus of hyaenodont in the family Hyainailouridae, containing five species. The type species Pterodon dasyuroides is known exclusively from the late Eocene to the earliest Oligocene of western Europe. The genus was first erected by the French zoologist Henri Marie Ducrotay de Blainville in 1839, who said that Georges Cuvier presented one of its fossils to a conference in 1828 but died before he could make a formal description of it. It was the second hyaenodont genus with taxonomic validity after Hyaenodon, but this resulted in taxonomic confusion over the validities of the two genera by other taxonomists. Although the taxonomic status of Pterodon was revised during the late 19th and early 20th centuries, it became a wastebasket taxon for other hyaenodont species found in Africa and Asia. Today, only the type species is recognized as belonging to the genus while four others are pending reassessment to other genera.

Duerotherium is an extinct genus of Palaeogene artiodactyls known only from the Iberian Peninsula during the Middle Eocene, which contains one species D. sudrei. It, like other members of the Anoplotheriidae, was endemic to Western Europe. The anoplotheriine was described from a left fragment of a maxilla from the Mazaterón Formation of the Duero Basin in 2009. Its dentition is mostly typical of the Anoplotheriinae but differs by an elongated plus triangular 3rd upper premolar and very specific traits of the molars. It is thought to have been part of an endemic faunal assemblage that evolved within the Iberian Peninsula by the Middle Eocene, where climates were subtropical.

<i>Xiphodon</i> Extinct genus of endemic Palaeogene European artiodactyls

Xiphodon is the type genus of the extinct Palaeogene artiodactyl family Xiphodontidae. It, like other xiphodonts, was endemic to Western Europe and lived from the middle Eocene up to the earliest Oligocene. Fossils from Montmartre in Paris, France that belonged to X. gracilis were first described by the French naturalist Georges Cuvier in 1804. Although he assigned the species to Anoplotherium, he recognized that it differed from A. commune by its dentition and limb bones, later moving it to its own subgenus in 1822. Xiphodon was promoted to genus rank by other naturalists in later decades. It is today defined by the type species X. gracilis and two other species, X. castrensis and X. intermedium.

<i>Plagiolophus</i> (mammal)

Plagiolophus is an extinct genus of equoids belonging to the family Palaeotheriidae. It lived in Europe from the middle Oligocene to the early Oligocene. 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 was classified to Palaeotherium the same year but was reclassified to the subgenus Plagiolophus, named by Auguste Pomel in 1847. Plagiolophus was promoted to genus rank by subsequent palaeontologists and today includes as many as seventeen species. As proposed by the French palaeontologist Jean A. Remy in 2004, it is defined by three subgenera: Plagiolophus, Paloplotherium, and Fraasiolophus.

<i>Ocepeia</i> Extinct Afrotherian mammal

Ocepeia is an extinct genus of afrotherian mammal that lived in present-day Morocco during the middle Paleocene epoch, approximately 60 million years ago. First named and described in 2001, the type species is O. daouiensis from the Selandian stage of Morocco's Ouled Abdoun Basin. A second, larger species, O. grandis, is known from the Thanetian, a slightly younger stage in the same area. In life, the two species are estimated to have weighed about 3.5 kg (7.7 lb) and 10 kg (22 lb), respectively, and are believed to have been specialized leaf-eaters. The fossil skulls of Ocepeia are the oldest known afrotherian skulls, and the best-known of any Paleocene mammal in Africa.

<span class="mw-page-title-main">Cainotheriidae</span> Extinct family of mammals

Cainotheriidae is an extinct family of artiodactyls known from the Late Eocene to Middle Miocene of Europe. They are mostly found preserved in karstic deposits.

The Quercy Phosphorites Formation is a geologic formation and lagerstätte in Occitanie, southern France. It preserves fossils dated to the Paleogene period, or MP16 to MP28 zones of the European land mammal age classification, ranging from approximately 38 to 25 Ma.

<i>Dichodon</i> (mammal) Extinct genus of endemic Palaeogene European artiodactyls

Dichodon is an extinct genus of Palaeogene artiodactyls belonging to the family Xiphodontidae. It was endemic to Western Europe and lived from the middle Eocene up to the earliest Oligocene. The genus was first erected by the British naturalist Richard Owen in 1848 based on dental remains from the fossil beds in Hordle, England. He noticed similar dentitions to contemporary artiodactyls like those of the Anoplotheriidae and Dichobunidae and references the name of the genus Dichobune. Eventually, it was found to be more closely related to Xiphodon and now includes 11 species, although one of them may be synonymous.

<i>Bachitherium</i> Extinct genus of mammals

Bachitherium is an extinct genus of Paleogene ruminants that lived in Europe from the late Eocene to the late Oligocene. The genus was erected in 1882 by Henri Filhol based on fossil remains found in the Quercy Phosphorites Formation. Bachitherium curtum was defined the type species, and another species called B. insigne; five more species have since been named although one, B. sardus, is currently pending reassessment. The genus name derives from "Bach", the French locality where its first fossils were found, and the Greek θήρ/therium meaning "beast". Bachitherium has historically been assigned to various families within the ruminant infrorder Tragulina, but was reclassified to its own monotypic family Bachitheriidae by Christine Janis in 1987.

<i>Diplobune</i> Extinct genus of endemic Palaeogene European artiodactyls

Diplobune is an extinct genus of Palaeogene artiodactyls belonging to the family Anoplotheriidae. It was endemic to Europe and lived from the late Eocene to the early Oligocene. The genus was first erected as a subgenus of Dichobune by Ludwig Rütimeyer in 1862 based on his hypothesis of the taxon being a transitional form between "Anoplotherium" secundaria, previously erected by Georges Cuvier in 1822, and Dichobune. He based the genus etymology off of the two-pointed pillarlike shapes of the lower molars, which had since been a diagnosis of it. However, in 1870, Diplobune was elevated to genus rank by Oscar Fraas, who recognized that Diplobune was a distinct genus related to Anoplotherium and not Dichobune. After several revisions of the anoplotheriids, there are currently four known species of which D. minor is the type species.

<i>Dacrytherium</i> Extinct genus of endemic Palaeogene European artiodactyls

Dacrytherium is an extinct genus of Palaeogene artiodactyls belonging to the family Anoplotheriidae. It occurred from the Middle to Late Eocene of Western Europe and is the type genus of the subfamily Dacrytheriinae, the older of the two anoplotheriid subfamilies. Dacrytherium was first erected in 1876 by the French palaeontologist Henri Filhol, who recognised in his studies that it had dentition similar to the anoplotheriids Anoplotherium and Diplobune but differed from them by a deep preorbital fossa and a lacrimal fossa, the latter of which is where the genus name derives from. D. ovinum, originally classified in Dichobune, is the type species of Dacrytherium. Henri Filhol named D. elegans in 1884, and Hans Georg Stehlin named the species D. priscum and D. saturnini in 1910.

<i>Catodontherium</i> Extinct genus of Palaeogene artiodactyls

Catodontherium is an extinct genus of Palaeogene artiodactyls belonging to the family Anoplotheriidae. It was endemic to Western Europe and had a temporal range exclusive to the middle Eocene, although its earliest appearance depends on whether C. argentonicum is truly a species of Catodontherium. It was first named Catodus by the French palaeontologist Charles Depéret in 1906, who created two species for the genus and later changed the genus name to Catodontherium in 1908. The Swiss palaeontologist Hans Georg Stehlin renamed one species and classified two other newly erected species to Catodontherium in 1910. Today, there are four known species, although two remain questionable in genus placement.

Ephelcomenus is an extinct genus of Palaeogene artiodactyls endemic to Western Europe. It contains one species E. filholi, which was first described by Richard Lydekker in 1889 but eventually classified to its own genus by the Swiss palaeontologist Johannes Hürzeler in 1938. It has an uncertain stratigraphic range, but some sources suggest that it was present in the Oligocene after the Grande Coupure turnover event of western Europe.

Robiatherium is an extinct genus of Palaeogene artiodactyls containing one species R. cournovense. The genus name derives from the locality of Robiac in France where some of its fossil were described plus the Greek θήρ/therium meaning "beast" or "wild animal". It was known only from the middle Eocene and, like other anoplotheriids, was endemic to Western Europe. The genus was erected by Jean Sudre in 1988 for a species originally attributed to the xiphodont genus Paraxiphodon in 1978. Robiatherium had dentitions typical of the subfamily Anoplotheriinae, differing from other genera by specific differences in the molars. It is one of the earliest-appearing anoplotheriine species in the fossil record as well as the earliest to have appeared in Central Europe.

Haplomeryx is an extinct genus of Palaeogene artiodactyls belonging to the family Xiphodontidae. It was endemic to Western Europe and lived from the middle Eocene up to the earliest Oligocene. Haplomeryx was first established as a genus by the German naturalist Max Schlosser in 1886 based on a molar tooth set from Quercy Phosphorites deposits. Three additional species were erected and classified to the xiphodontid genus while one other species, first recognized in 1822, was tentatively classified to it and remains unresolved in affinity.

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  46. ↑ Buffetaut, Eric; Angst, Delphine (2014). Stratigraphic Distribution of Large Flightless Birds in the Palaeogene of Europe. STRATI 2013: First International Congress on Stratigraphy At the Cutting Edge of Stratigraphy. doi:10.1007/978-3-319-04364-7_190.
  47. ↑ Martin, Jeremy E. (2015). "A sebecosuchian in a middle Eocene karst with comments on the dorsal shield in Crocodylomorpha". Acta Palaeontologica Polonica. 60 (3): 673–680. doi: 10.4202/app.00072.2014 . S2CID   54002673.
  48. ↑ Antunes, Miguel Telles (2003). "Lower Paleogene Crocodilians from Silveirinha, Portugal". Palaeovertebrata. 32: 1–26.
  49. ↑ Robinet, Céline; Remy, Jean Albert; Laurent, Yves; Danilo, Laure; Lihoreau, Fabrice (2015). "A new genus of Lophiodontidae (Perissodactyla, Mammalia) from the early Eocene of La Borie (Southern France) and the origin of the genus Lophiodon Cuvier, 1822". Geobios. 48 (1): 25–38. Bibcode:2015Geobi..48...25R. doi:10.1016/j.geobios.2014.11.003.
  50. ↑ Perales-Gogenola, Leire; Badiola, Ainara; Gómez-Olivencia, Asier; Pereda-Suberbiola, Xabier (2022). "A remarkable new paleotheriid (Mammalia) in the endemic Iberian Eocene perissodactyl fauna". Journal of Vertebrate Paleontology. 42 (4). Bibcode:2022JVPal..42E9447P. doi:10.1080/02724634.2023.2189447. S2CID   258663753.
  51. ↑ Solé, Floréal; Fischer, Valentin; Le Verger, Kévin; Mennecart, Bastien; Speijer, Robert P.; Peigné, Stéphane; Smith, Thierry (2022). "Evolution of European carnivorous mammal assemblages through the Paleogene". Biological Journal of the Linnean Society. 135 (4): 734–753. doi:10.1093/biolinnean/blac002.
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