Extinct genus of endemic Palaeogene European artiodactyls
Amphimeryx is an extinct genus of Palaeogene artiodactyls belonging to the Amphimerycidae that was endemic to the central region of western Europe and lived from the Late Eocene to the Early Oligocene. It was erected in 1848 by the French palaeontologist Auguste Pomel, who argued that its dentition was roughly similar to those of ruminants. Hence, the etymology of the genus name means "near ruminant," of which it derives from the ancient Greek words (near) and (ruminant). The type species A. murinus was previously recognized as a species of "Dichobune" by the French palaeontologist Georges Cuvier in 1822 before its eventual reclassification to its own genus. Two other species A. collotarsus and A. riparius are recognized also today although the former may be synonymous with "A. murinus" while the latter is known solely by a now-lost fossil specimen.
It is best known for its fused cuboid bone and navicular bone of its hind legs, which make up a single bone. The fused "cubonavicular bone" is also recorded in derived ruminants including extant ones in an instance of parallel evolution. Additional traits shared by "Amphimeryx" with modern ruminants include its middle two digits being fused and its two side digits being greatly reduced, making it functionally didactyl (or walking on its two middle toes). Despite its selenodont (crescent-like ridges) dentition being similar to ruminants, it differs from them by retaining its first premolars instead of losing them evolutionarily and its specialized level of selenodonty in its molars that have five total cusps on them. The skull of "Amphimeryx" is elongated and has a sloped form in its front, large orbits, and a long snout. It is very similar to the preceding "Pseudamphimeryx" but differs by its well-developed occipital crests. The dentition of "Amphimeryx" suggests that it may have had a preference for leaf-eating diets. Especially in comparison to contemporary artiodactyls, it was tiny-sized, weighing as little as .
"Amphimeryx" was a small-sized artiodactyl temporally occurring after "Pseudamphimeryx" that inhabited western Europe back when it was an archipelago that was isolated from the rest of Eurasia, meaning that it lived in a tropical-subtropical environment with various other faunas that also evolved with strong levels of endemism. This meant that it coexisted with a wide variety of other artiodactyls and perissodactyls. It last occurred just shortly after the Grande Coupure extinction/faunal turnover event, coinciding with shifts towards further glaciation and seasonality plus dispersals of Asian immigrant faunas into western Europe. The extinction causes of "Amphimeryx" are unclear, but "A. riparius" was the last representative of the Amphimerycidae.
Taxonomy.
In 1848, the French palaeontologist Auguste Pomel, without further specifications of the specimens, reclassified ""Dichobune obliqua" and "D. murina"" to the newly named genus "Amphimeryx", also stating that it would have been close in affinity to ruminants. The genus name "Amphimeryx" derives from the Ancient Greek words (near) and (ruminant) meaning "near ruminant". "Amphimeryx murinus" was previously erected for "Anoplotherium", more specifically "Dichobune" when it was first considered an "Anoplotherium" subgenus, as "A. murinum" by the French naturalist Georges Cuvier in 1822 based on Montmartre fossils from Paris in France.
In 1848–1852, French palaeontologist Paul Gervais described what he determined to be an unknown species of ruminant, basing it off an upper molar and a portion of another one from the limestone marls of a location named "Barthélemy" in the French commune of Saint-Saturnin-lès-Apt. He confirmed that it was from the Eocene and assigned the material to "Amphimeryx". Gervais stated that its dentition was similar to those of extant ruminants because of the double crescent shapes on the molar crowns. While he did list one species "Dichobune murinum", he later reclassified the species to "Amphimeryx".
In 1851, Pomel erected the genus "Hyægulus", arguing that it was related to "Cainotherium" and is known from dental and foot fossils. The first species he named was "H. collotarsus", which he said was the size of "C. laticurvatum". The second named species was "H. murinus", which according to Pomel was smaller and more gracile. The palaeontologist described "Hyægulus" as having both a cuboid bone that is fused to the navicular bone and metatarsal bones that are not fused together. In 1855 during a science conference, the French palaeontologist Auguste Aymard read a report for a fossil collection belonging to Pichot-Dumazel, listing "Palæon riparium" among the taxa represented in it.
The French palaeontologist Henri Filhol in 1877 created the genus "Xiphodontherium" and recognized its two species. The first named species was "X. primævum", which Filhol wrote was related to "Xiphodon" and was known from a lower jaw in the French locality of Mouillac in the department of Tarn-et-Garonne. The second species that he named was "X. secundarium", also from Mouillac. He also observed that both species had complete dentitions for a total of 44 teeth. In 1891, Swiss palaeontologist Ludwig Rütimeyer established three more species of "Xiphodontherium": "X. pygmaeum", "X. obliquum", and "X. schlosseri". The British naturalist Richard Lydekker in 1885 synonymized "Xiphodontherium" with "Xiphodon" and transferred "X. secundarium" into the latter genus.
In 1910, Swiss palaeontologist Hans Georg Stehlin synonymized multiple genera with "Amphimeryx". In his synonymization of "Hyaegulus", he invalidated "H. murinus" but considered ""H. collotarsus" a valid species of "Amphimeryx". He also synonymized "Palaeon" but retained validity of "P. riparium" as a species of "Amphimeryx" ("A. riparius"). Stehlin additionally invalidated "Xiphodontherium" and made its two species synonyms of "A. murinus". He also reclassified "X." "schlosseri" to the new genus "Pseudamphimeryx" and synonymized both "X. pygmaeum" and "X. obliquum" with it. Stehlin then tentatively reclassified "Anoplotherium obliquum"" to "Haplomeryx" instead of "Dichobune" or "Amphimeryx".
In 1978, the French palaeontologist Jean Sudre synonymized "A. collotarsus" with "A. murinus" because he did not think that size differences alone were adequate enough to justify species distinctness. He additionally noted that "A. riparius", diagnosed solely as being large-sized, is only known from a type specimen originally from Ronzon that had since been lost. On the other hand, some palaeontologists have continued using the name "A. collotarsus", also spelled ""A. collatarsus"."
Classification.
"Amphimeryx" is the type genus of the Amphimerycidae, a Palaeogene artiodactyl family endemic to western Europe that lived from the middle to the earliest Oligocene (~44 to 33 Ma). Like the other contemporary endemic artiodactyl families of western Europe, the evolutionary origins of the Amphimerycidae are poorly known. The family is generally thought to have made its first appearance by the unit MP14 of the Mammal Palaeogene zones, making them the first selenodont dentition artiodactyl representatives to have appeared in the landmass along with the Xiphodontidae. The first representative of the Amphimerycidae to have appeared was "Pseudamphimeryx", lasting from MP14 to MP17. "Amphimeryx" made its first appearance in MP18 as the only other known amphimerycid genus and lasted up to MP21, after the Grande Coupure faunal turnover event. 
Because of its similar anatomical traits with ruminants, some palaeontologists had originally included it within the suborder Ruminantia while some others rejected the placement. Today, its similarities with ruminants is thought to have been an instance of parallel evolution, in which amphimerycids and ruminants independently gained similar traits. While amphimerycids have typically been excluded from the Ruminantia due to dental characteristics, it does not eliminate the possibility of them being sister taxa to ruminants by the latter independently gaining longer legs and more selenodont (crescent-shaped) dentition. Its affinities, along with those of other endemic European artiodactyls, are unclear; the Amphimerycidae, Anoplotheriidae, Xiphodontidae, Mixtotheriidae, and Cainotheriidae have been determined to be closer to either tylopods (i.e. camelids and merycoidodonts) or ruminants. Different phylogenetic analyses have produced different results for the "derived" 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 on 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 used for the journal and another published 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:
In 2022, Weppe conducted a phylogenetic analysis in his academic thesis regarding Palaeogene artiodactyl lineages, focusing most specifically on the endemic European families. One large monophyletic set consisted of the Hyperdichobuninae, Amphimerycidae, Xiphodontidae, and Cainotherioidea based on dental synapomorphies, of which the hyperdichobunines are paraphyletic in relation to the other clades. In terms of the amphimerycids, while the clade consisting of "P. renevieri" and "A. murinus" was recovered as a sister group to the other endemic artiodactyl clades, the placement of "P. schlosseri" has rendered the Amphimerycidae paraphyletic in relation to the derived amphimerycid species and other families. He argued that the Amphimerycidae thus needs a systemic revision for which "P. schlosseri" would be assigned to a new genus and removed from the Amphimerycidae.
Description.
Skull.
The Amphimerycidae is defined in part as having an elongated snout and large orbits that are widened in their backs. "Amphimeryx" specifically is described as having a skull whose peak at its top area rapidly slopes down to the skull's front. The skull is also diagnosed as having strong body orifices in its basicranium and projecting occipital crests. "Pseudamphimeryx" and "Amphimeryx", both known by multiple skull specimens, have very similar forms but differ based on a few characteristics. "Amphimeryx" is also distinguished from "Pseudamphimeryx" by the more well-developed occipital crest present on the snout of the latter. Its skull additionally resembles those of both "Dacrytherium" and "Tapirulus". 
The overall skull of "Amphimeryx" is very elongated compared to even those of both "Pseudamphimeryx" and "Mouillacitherium". The parietal bone and squamosal bone both make up a prominent portion of the cranial cavity's wall. Both amphimerycid genera have especially prominent occipital and sagittal crests, the latter of which divides into two less prominent branches behind the fronto-parietal suture that extend up to the supraorbital foramen. This is unlike "Mouillacitherium" where the crest's extension only goes up to the foramen's back. The glenoid surface of "Amphimeryx" is positioned slightly above the overall base of the skull and has a slightly convex form as opposed to a flat one like in primitive ruminants. The glenoid region of the skull also has a deep concavity above it like in ruminants but unlike in anoplotheriids. The zygomatic arch, or cheek bone, is thin. The back of the skull of "Amphimeryx" is similar that of "Tapirulus" but is even narrower and has lesser-developed occipital crests. The orientation of the occipital crest differs by amphimerycid genus, with that of "Amphimeryx" being tilted backwards. Amphimerycids have primitive "mastoid" forms (in which the periotic bone of the ear is exposed to the skull's surface) akin to those of the dichobunids "Dichobune" and "Mouillacitherium".
The frontal bones of both amphimerycid genera are large plus flat, being particularly sizeable in their supraorbital portions; this trait is more pronounced in "Amphimeryx". That of "Amphimeryx" is close to the orbits' upper edges and is more prominent in position between the two orbits than that of "Pseudamphimeryx". The supraorbital foramen of "Amphimeryx" is wider than it is long and is proportionally larger than that of "Pseudamphimeryx". It is also more perpendicular to the sagittal plane in its back edge, which is not oriented backwards like in "Pseudamphimeryx". The lacrimal bone of both amphimerycids, but especially in "Amphimeryx", has an extensive "pars facialis" and is quadrangular in shape, narrowing at its front. The orbit is large, is positioned back in relation to the overall skull, is wide at its back area, and is more curved at its upper compared to lower edge. There is no difference between both amphimerycids in terms of the orbits, suggesting based on their morphologies that the snouts of both genera are elongated. The optic foramen, located in the sphenoid bone, extends more forward in "Amphimeryx" than in "Pseudamphimeryx". While the nasal bone is not as well-preserved in "Amphimeryx"