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.

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.

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.

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.

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.

[6][7] 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.

"[7][15] 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).

[16][18] 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.

[19] 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.

[18][20][21] 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.

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 used for the journal and another published work about the cainotherioids is outlined below:[22] Eurodexis russelli Dichobune leporina Amphimeryx murinus Xiphodon castrense Paratoceras coatesi Eotylopus reedi Parvitragulus priscus Lophiomeryx chalaniati Archaeomeryx optatus Mixtotherium cuspidatum Anoplotherium latipes Dacrytherium ovinum Robiacina lavergnesis Robiacina minuta Robiacina quercyi Palembertina deplasi Paroxacron bergeri Paroxacron valdense Oxacron courtoisii Cainotherium laticurvatum Caenomeryx filholi Caenomeryx procommunis Plesiomeryx cadurcensis Plesiomeryx huerzeleri In 2020, Vincent Luccisano et al. created a phylogenetic tree of the basal artiodactyls, a majority endemic to western Europe, from the Palaeogene.

The phylogenetic tree as produced by the authors is shown below:[20] Bunophorus Gunophorus Diacodexis Protodichobune Eurodexis Buxobune Mouillacitherium Meniscodon Hyperdichobune Dichobune robertiana Dichobune leporina Homacodon Gobiohyus Khirtharia Entelodon Palaeocheorus Perchoerus Haplobunodon Cuisitherium Lophiobunodon Mixtotherium Robiacina Dacrytherium Diplobune Xiphodon Paraxiphodon Cainotherium Paroxacron Archaeomeryx Amphimeryx Pseudamphimeryx Aumelasia Hallebune Amphirhagatherium Cebochoerus Gervachoerus Choeropotamus Siamotherium 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.

[13][16] In terms of non-diagnostic features of the amphimerycids, both genera have incisors that are shovel-shaped, have sharp edges on their crowns, and have horizontal positions in relation to the dental row.

The astragali are common bones in fossil assemblages due to their reduced vulnerability to fragmentation as a result of their stocky shape and compact structure, explaining their choice for using it.

[31] In 2014, Takehisa Tsubamoto reexamined the relationship between astragalus size and estimated body mass based on extensive studies of extant terrestrial mammals, reapplying the methods to Palaeogene artiodactyls previously tested by Sudre and Martinez.

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.

[35] The Holarctic mammalian faunas of western Europe were therefore mostly isolated from other landmasses including Greenland, Africa, and eastern Eurasia, allowing for endemism to develop.

[36] The first appearance of Amphimeryx by MP18 occurred long after the extinction of the endemic European perissodactyl family Lophiodontidae in MP16, including the largest lophiodont Lophiodon lautricense, which weighed over 2,000 kg (4,400 lb).

The extinction of the Lophiodontidae was part of a faunal turnover, which likely was the result of a shift from humid and highly tropical environments to drier and more temperate forests with open areas and more abrasive vegetation.

[39][28] MP16 also marked the last appearances of most European crocodylomorphs, of which the aligatoroid Diplocynodon was the only survivor due to seemingly adapting to the general decline of tropical climates of the Late Eocene.

[16] Amphimeryx coexisted with a wide diversity of artiodactyls in western Europe by MP18, ranging from the more widespread Dichobunidae, Tapirulidae, and Anthracotheriidae to many other endemic families consisting of the Xiphodontidae, Choeropotamidae, Cebochoeridae, Amphimerycidae, and Cainotheriidae.

[36] Late Eocene European groups of the clade Ferae represented predominantly the Hyaenodonta (Hyaenodontinae, Hyainailourinae, and Proviverrinae) but also contained Carnivoramorpha (Miacidae) and Carnivora (small-sized Amphicyonidae).

[39] Other mammal groups present in the Late Eocene of western Europe represented the leptictidans (Pseudorhyncocyonidae),[45] primates (Adapoidea and Omomyoidea),[46] eulipotyphlans (Nyctitheriidae),[47] chiropterans,[34] herpetotheriids,[48] apatotherians,[49] and endemic rodents (Pseudosciuridae, Theridomyidae, and Gliridae).

[50] The alligatoroid Diplocynodon, present only in Europe since the upper Paleocene, coexisted with pre-Grande Coupure faunas as well, likely consuming insects, fish, frogs, and eggs due to prey partitioning previously with other crocodylomorphs that had since died out by the Late Eocene.

[53] The MP18 locality of La Débruge of France indicates that A. murinus coexisted with a wide variety of mammals, namely the herpetotheriid Peratherium, rodents (Blainvillimys, Theridomys, Plesiarctomys, Glamys), hyaenodonts (Hyaenodon and Pterodon), amphicyonid Cynodictis, palaeotheres (Plagiolophus, Anchilophus, Palaeotherium), dichobunid Dichobune, choeropotamid Choeropotamus, cebochoerids Cebochoerus and Acotherulum, anoplotheriids (Anoplotherium, Diplobune, Dacrytherium), tapirulid Tapirulus, xiphodonts Xiphodon and Dichodon, cainothere Oxacron, and the anthracothere Elomeryx.

The MP19 locality of Escamps has similar faunas but also includes the herpetotheriid Amphiperatherium, pseudorhyncocyonid Pseudorhyncocyon, bats (Hipposideros, Vaylatsia, Vespertiliavus, Stehlinia), primates (Microchoerus, Palaeolemur), cainothere Paroxacron, and the xiphodont Haplomeryx.

The Turgai Strait, which once separated much of Europe from Asia, is often proposed as the main European seaway barrier prior to the Grande Coupure, but some researchers challenged this perception recently, arguing that it completely receded already 37 Ma, long before the Eocene-Oligocene transition.

[16][28] The causes of the extinctions of many other mammals in western Europe have been attributed to negative interactions with immigrant faunas (competition, predations), environmental changes from cooling climates, or some combination of the two.