Amphimeryx is an extinct genus of small mammals with slightly deer-like forms belonging to the Amphimerycidae, classified within the order Artiodactyla. It was endemic to western Europe and lived from the Late Eocene to the Early Oligocene, about 37 to 32.5 million years ago. The taxonomic history of the artiodactyl genus extends back to 1804 when dental fossils of the type species A. murinus were first recognized by the French palaeontologist Georges Cuvier, who gave the species its current scientific name in 1822. It was first assigned by early naturalists to two other artiodactyl genera, Anoplotherium and Dichobune. In 1848, the French palaeontologist Auguste Pomel recognized that the dentition of A. murinus was roughly similar to those of ruminants and therefore designated to its own genus Amphimeryx, the name meaning "near ruminant". Two other species named in later studies, A. collotarsus and A. riparius, are purported, although their validities are uncertain.
Amphimeryx is best known for its fused cuboid bone and navicular bone in its hind legs. 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 (walking on its two middle toes). Despite its selenodont (crescent-like ridges) dentition being similar to ruminants, it differs from them in part by retaining its first premolars instead of losing them evolutionarily. Its molars have specialized levels of selenodonty, meaning that they 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. The dentition of Amphimeryx suggests that it may have been a leaf-eater. In comparison to contemporary artiodactyls, it was quite small, weighing as little as .
Amphimeryx inhabited the central region of western Europe back when it was an archipelago that was isolated from the rest of Eurasia, living in a tropical-subtropical environment with various other ungulates. It went extinct shortly after the Grande Coupure extinction event (thought to have occurred within a 33.9 to 33.4 Ma time range), 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 it was the last representative of the Amphimerycidae.
Taxonomy Research history
Amphimeryx has had a somewhat complex taxonomic history in which the type species A. murinus was studied and formally named as a species classified to earlier-named genera long before it was reassigned to its own genus. In addition, multiple species named by earlier taxonomists were synonymized (or made into invalid names for the same taxon) with A. murinus by later taxonomists while multiple genera were later made synonyms of Amphimeryx, resulting in transfers of species originally named for the formerly valid genera into the currently valid Amphimeryx.
Fossil evidence of Amphimeryx from Montmartre fossils in Paris, France was first described in 1804 by the French naturalist Georges Cuvier, who assigned them to the fossil artiodactyl genus Anoplotherium and created the species name Anoplotherium minimum; he noted that unlike with other species classified to Anoplotherium, A. minimum was not known by any postcranial fossil evidence (non-skull bones). In 1822, Cuvier renamed A. minimum to A. murinum and reassigned it, along with A. leporina and A. obliquum, into the newly named Anoplotherium subgenus Dichobune. In 1848, the French palaeontologist Auguste Pomel, recognizing Dichobune as a separate genus from Anoplotherium, 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". This reclassification was followed by the French palaeontologist Paul Gervais in 1848–1852, who described additional dental fossil evidence of A. murinus from the limestone marls of a location named "Barthélemy" in the French commune of Saint-Saturnin-lès-Apt. Gervais also noted that its dentition was similar to those of extant ruminants because of the double crescent shapes on the molars' crowns.
In 1851, Pomel created the genus Hyægulus, arguing that it was related to another artiodactyl Cainotherium and is known from tooth and foot fossils. The first species he named was H. collotarsus, which he said was the size of C. laticurvatum. The second that Pomel named was H. murinus (not to be confused with A. murinus), which according to Pomel was smaller and more gracile (or slender-built). 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 session at a science conference named "Congrès Scientifiques de France," 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 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, the 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 1906, Swiss palaeontologist Hans Georg Stehlin wrote that the dentition of Hyaegulus collotarsus very closely resembled that of Amphimeryx to the point that he made Hyaegulus a synonym of Amphimeryx. He then synonymized H. murinus with another artiodactyl species Oxacron courtoisi. In 1910, Stehlin reconfirmed his synonymization of Hyaegulus with Amphimeryx, of which he considered "H. collotarsus" a valid species still. 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, X. primaevum and X. secundarium, 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 Amphimerycidae, a family of artiodactyls 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 (43.5 to 41.2 Ma) of the Mammal Palaeogene zones, making them the first artiodactyls with selenodont dentition (crescent-like ridges on cheek teeth) 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 (43.5 to 37 Ma). Amphimeryx made its first appearance in MP18 (37 to 35 Ma) as the only other known amphimerycid genus and lasted up to MP21 (33.9 to 32.5 Ma), after the Grande Coupure faunal turnover event.
Because of its similar anatomical traits with ruminants, some palaeontologists had originally included the Amphimerycidae 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 found 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 (or evolutionarily early) artiodactyls, including 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