Amphimeryx

Amphimeryx Temporal range: Late Eocene – Early Oligocene 37–32.5  Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Amphimeryx murinus holotype mandible, National Museum of Natural History, France
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Artiodactyla
Family:	† Amphimerycidae
Genus:	† Amphimeryx Pomel, 1848
Type species
†Amphimeryx murinus Cuvier, 1822
Other species
†A. collotarsus?Pomel, 1851 †A. riparius Aymard, 1855
Synonyms
Genus synonymy AmphimerixPomel, 1849 HyægulusPomel, 1851 PalæonAymard, 1855 Amphimoeryx Gervais, 1859 Xiphodontherium Filhol, 1877 Synonyms of A. murinus Anoplotherium murinumCuvier, 1822 Xiphodontherium primævumFilhol, 1877 Xiphodontherium secundariumFilhol, 1877 Synonyms of A. collotarsus Hyægulus collotarsusPomel, 1851 Hyægulus murinusPomel, 1851 Synonyms of A. riparius Palæon ripariumAymard, 1855

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 ἀμφί (near) and μήρυξ (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 1.511 kg (3.33 lb).

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

Mandibles of Pseudamphimeryx renevieri (left) and Amphimeryx murinus (right)

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. ^[1] The genus name Amphimeryx derives from the Ancient Greek words ἀμφί (near) and μήρυξ (ruminant) meaning "near ruminant". ^[2] 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. ^[3]

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, ^[4] he later reclassified the species to Amphimeryx. ^[5]

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. ^{[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. ^[8]

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. ^[9] In 1891, Swiss palaeontologist Ludwig Rütimeyer established three more species of Xiphodontherium: X. pygmaeum, X. obliquum, and X. schlosseri. ^[10] The British naturalist Richard Lydekker in 1885 synonymized Xiphodontherium with Xiphodon and transferred X. secundarium into the latter genus. ^[11]

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. ^[12]

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. ^{[13] [14]} On the other hand, some palaeontologists have continued using the name A. collotarsus, also spelled "A. collatarsus." ^{[7] [15]}

Classification

Because of some similar anatomical traits of the amphimerycids to those of ruminants (like the Java mouse-deer ( Tragulus javanicus), pictured), they were previously considered ruminants by biologists. Today, their evolutionary relationship to ruminants and other artiodactyls proves unclear.

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. ^[16] 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. ^[17] 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. ^[16]

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. ^{[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. 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. ^{[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. 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 used for the journal and another published work about the cainotherioids is outlined below: ^[22]

Eurodexis russelli Dichobune leporina Amphimeryx murinus Xiphodon castrense Tylopoda Paratoceras coatesi Eotylopus reedi Ruminantia Parvitragulus priscus Lophiomeryx chalaniati Archaeomeryx optatus Mixtotheriidae Mixtotherium cuspidatum Anoplotheriidae Anoplotherium latipes Dacrytherium ovinum Cainotherioidea Robiacinidae Robiacina lavergnesis Robiacina minuta Robiacina quercyi Cainotheriidae Palembertina deplasi Oxacroninae Paroxacron bergeri Paroxacron valdense Oxacron courtoisii Cainotheriinae Cainotherium laticurvatum Caenomeryx Caenomeryx filholi Caenomeryx procommunis Plesiomeryx 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. 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: ^[20]

Artiodactyla

Bunophorus Gunophorus Diacodexis Protodichobune Eurodexis Buxobune Mouillacitherium Meniscodon Hyperdichobune Dichobune robertiana Dichobune leporina Homacodon Gobiohyus Suina Khirtharia Entelodon Palaeocheorus Perchoerus Haplobunodontidae Haplobunodon Cuisitherium Lophiobunodon Mixtotherium Robiacina Dacrytherium Diplobune Xiphodon Paraxiphodon Cainotherium Paroxacron Archaeomeryx Amphimeryx Pseudamphimeryx Aumelasia Hallebune Amphirhagatherium Cebochoeridae 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. 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. ^[18]

Description

Skull

A. collotarsus mandible, Natural History Museum of Basel

The Amphimerycidae is defined in part as having an elongated snout and large orbits that are widened in their backs. ^[16] 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. ^[13] Pseudamphimeryx and Amphimeryx, both known by multiple skull specimens, have very similar forms but differ based on a few characteristics. ^[23] 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 . ^[24]

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. ^[23] 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. ^[24] 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. ^[23]

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 fossils, the frontonasal suture is implied to have formed a W shape on the skull's upper surface like that of Pseudamphimeryx. Both amphimerycid genera also have similar, although not identical, medial positions of the infraorbital foramen in the maxilla. The palatine bones of Amphimeryx and Pseudamphimeryx are narrower at their front than back ends. ^[23]

The mandible of Amphimeryx is straight at the lower edge of its horizontal branch, or the mandibular corpus, and has a large and slightly rounded angular border. It is unclear as to whether or not the coronoid process of the mandible is positioned high as in ruminants (a gap that Colette Dechaseaux recognized by drawing alternate reconstructions of the skull of A. murinus with different coronoid process positions, one originally by Stehlin and the other by her with a higher position). ^{[23] [16]}

Amphimeryx, or A. cf. murinus, is also known from a brain endocast, although the endocasts of it and Pseudamphimeryx were not as closely described in detail. Its neocortex was described by Dechaseaux as being of a primitive and simple type in the larger evolutionary scale of artiodactyls. ^{[23] [25]}

Dentition

The dental formula of the Amphimerycidae is 3.1.4.33.1.4.3 for a total of 44 teeth, consistent with the primitive dental formula for early-middle Palaeogene placental mammals. ^{[13] [26]} The canines (C/c) are incisiform (incisor (I/i) form) and therefore differ little with the incisors themselves. The premolars (P/p) are elongated and may generally be separated by diastemata (gaps between teeth). The lower premolars have three lobes, or developed areas on their crowns. The upper molars (M/m) are more developed in form and are generally subtriangular in shape, although some may be more rectangular. They have five crescent-shaped (selenodont) tubercles and sometimes a partial hypocone cusp that may be present in all species. ^{[13] [16]} Amphimerycids differ from ruminants, particularly the basal clade Tragulina, in the retentions of their first premolars and their high levels of specialization in their selenodonty and number of cusps in their molars. ^[27] Their dentitions more closely resemble those of xiphodonts or dacrytheriines than of ruminants. ^[16]

Amphimeryx specifically is diagnosed in part as having flat canines and premolars that are compressed on their transverse side. P₂ is separated from P₁ and P₃ by very large diastemata between it. The molars each have five crescent-shaped cusps, the protocone cusp being connected to the parastyle cusp in the upper molars. The upper molars lack a middle cingulum and have W-shaped ectolophs (crests or ridges of upper molar teeth). The labial cuspids of the lower molars are strongly crescent-shaped while the lingual cuspids are subconical. The peaks of the crescent shapes on the metaconid and entoconid form very acute angles, a diagnostic trait differing the molars of Amphimeryx from those of Pseudamphimeryx. ^{[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 canines are similar to incisors but differ by their somewhat asymmetrical shapes. ^[12] P¹ and P² have both been described as narrow and elongated, but the former tooth is larger than the latter. ^[23] In Amphimeryx, the upper molar row slightly increases in size from M¹ to M³. ^[13] The overall selenodonty and brachyodonty (low-crowned teeth) of amphimerycids suggest that they were adapted towards folivorous (leaf-eating) dietary habits. ^[28]

Postcranial skeleton

Restoration of A. murinus based on known fossil material

Amphimeryx is known from postcranial fossil evidence from A. collotarsus in La Debruge and A. murinus in Escamps at France. ^{[16] [7]} Its most well-known trait is within the tarsus of its hind feet, in which the cuboid is fused to the navicular as a single bone, a trait convergent with those of ruminants. ^[7] The fused "cubonavicular" is also known in Pseudamphimeryx. The metatarsal digits III and IV are elongated and partially fused to each other while the side digits II and V are greatly reduced to small but needlelike forms. In terms of the Escamps fossil, digit III measures 50 mm (2.0 in) long while digit II is no more than 14 mm (0.55 in) long. ^{[16] [13]} These traits are similarly recorded in derived ruminants, which have tetradactyl (four-toed) feet, absent digit I, reduced digits II and V, and fused digits III and IV that make up the cannon bone (the now-extinct primitive ruminants had pentadactyl (five-toed) feet, unreduced digits II and V, and unfused digits III and IV). ^{[27] [29]} Like other artiodactyls with only two elongated digits in each foot (digits III and IV), ^[30] Amphimeryx was functionally didactyl, meaning that it walked only on its two elongated toes per foot. ^[31] The metatarsal digits of Amphimeryx are smaller in length than those of the basal ruminant Archaeomeryx but larger than those of another basal ruminant Hypertragulus . ^[13]

The hind leg is known from complete fossil evidence from the locality of Escamps. The body of the femur has a slender and elongated shape with a slight arch. The femoral head is reduced in form while the greater trochanter is positioned as high as the head but is compressed in its mediolateral area. The trochanteric fossa of the greater trochanter is triangular, narrow, and deep in shape, being contained by two ridges. The tibia is long and slender, its tibial fossa being deep. The overall morphology of the tibia suggests that the fibula would have been greatly reduced. ^[13] Similarly, derived ruminants have reduced fibulas that fuse with the tibia (in the front feet, the ulna fuses with the radius). ^[27] The calcaneum is long and slender, its talar shelf being small and reduced. The astragalus is elongated, moreso than that of Oxacron ; its tubercle aligns with that of the calcaneum being poorly developed. ^[13] The primitive state of the astragalus sets Amphimeryx apart from ruminants; the approximately equal sizes of its trochleas and more rounded edge of its sustentacular facet also sets the genus apart from the Cainotheriidae. ^[31]

Size

Estimated size comparisons of A. murinus and A. riparius based on known fossil remains

The Amphimerycidae consists only of small-sized species within Amphimeryx and Pseudamphimeryx. ^[16] This has been demonstrated in the former in 1995 when Jean-Noël Martinez and Sudre made weight estimates of Palaeogene artiodactyls based on the dimensions of their astragali and M₁ teeth. 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. The two weight estimates of A. murinus from Escamps has yielded different results, the M₁ giving the body mass estimate of 1.846 kg (4.07 lb) and the astragalus yielding 1.511 kg (3.33 lb). The estimated body masses of Amphimeryx are small compared to most other Palaeogene artiodactyls in the study, although the researchers pointed out that the M₁ measurements could be overestimated compared to the astragalus estimate. ^[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. The researcher used linear measurements and their products with adjusted correction factors. The recalculations resulted in somewhat lower estimates compared to the 1995 results (with the exception of Diplobune minor, which as a shorter astragalus proportion than most other artiodactyls), displayed in the below graph: ^[32]

Estimated body masses (kg) of Palaeogene artiodactyls based on recalculated trochlear widths (Li1) in comparison to estimates from Martinez and Sudre (1995)

Palaeoecology

Further information: Mammal Palaeogene zones

Early pre–Grande Coupure Europe

Palaeogeography of Europe and Asia during the middle Eocene with possible artiodactyl and perissodactyl dispersal routes.

For much of the Eocene, a hothouse climate 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 mya), most of the ungulate form dentitions shifted from bunodont (or rounded) cusps to cutting ridges (i.e. lophs) for folivorous diets. ^{[33] [34]}

Land connections between western Europe and North America were interrupted around 53 Ma. From the early Eocene up until the Grande Coupure extinction event (56–33.9 mya), western Eurasia was separated into three landmasses: western Europe (an archipelago), Balkanatolia (in-between the Paratethys Sea of the north and the Neotethys Ocean of the south), and eastern Eurasia. ^[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. ^[34] Therefore, the European mammals of the late Eocene (MP17–MP20 of the Mammal Palaeogene zones) were mostly descendants of endemic middle Eocene groups. ^[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. The surviving herbivorous faunas shifted their dentitions and dietary strategies accordingly to adapt to abrasive and seasonal vegetation. ^{[37] [38]} 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. ^{[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. ^{[40] [41] [42]}

Late Eocene

Restoration of Cebochoerus , which coexisted with Amphimeryx in western Europe

After the latest occurrence of Pseudamphimeryx in MP17b, Amphimeryx made its first temporal appearance in MP18, in which both A. murinus and A. collotarsus (if the latter is valid) cooccur. ^{[43] [7]} It was exclusive to the western European archipelago and is known only from the Central European region, more specifically what is now France and Switzerland. Of note is that whereas Pseudamphimeryx is recorded from the United Kingdom, Amphimeryx is not. ^[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. ^{[16] [20] [15] [44]} It also coexisted with palaeotheriids, the sole perissodactyl group of the late Eocene of western Europe. ^[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. ^{[51] [52]} In addition to snakes, frogs, and salamandrids, rich assemblage of lizards are known in western Europe as well from MP16-MP20, representing the Iguanidae, Lacertidae, Gekkonidae, Agamidae, Scincidae, Helodermatidae, and Varanoidea, most of which were able to thrive in the warm temperatures of western Europe. ^[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. ^[54]

Extinction

A panorama of the Headon Hill Formation in the Isle of Wight. The stratigraphy of it and the Bouldnor Formation led to better understandings of faunal chronologies from the Late Eocene up to the Grande Coupure.

The Grande Coupure event during the latest Eocene to earliest Oligocene (MP20-MP21) is one of the largest and most abrupt faunal turnovers in the Cenozoic of Western Europe and coincident with climate forcing events of cooler and more seasonal climates. ^[55] The event led to the extinction of 60% of western European mammalian lineages, which were subsequently replaced by Asian immigrants. ^{[56] [57] [58]} The Grande Coupure is often dated directly to the Eocene-Oligocene boundary at 33.9 Ma, although some estimate that the event began slightly later, at 33.6–33.4 mya. ^{[59] [60]} The event occurred during or after the Eocene-Oligocene transition, an abrupt shift from a hot greenhouse world that characterised much of the Palaeogene to a coolhouse/icehouse world from the early Oligocene onwards. The massive drop in temperatures results from the first major expansion of the Antarctic ice sheets that caused drastic pCO₂ decreases and an estimated drop of ~70 m (230 ft) in sea level. ^[61]

Many palaeontologists agree that glaciation and the resulting drops in sea level allowed for increased migrations between Balkanatolia and western Europe. 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. In 2022, Alexis Licht et al. suggested that the Grande Coupure could have possibly been synchronous with the Oi-1 glaciation (33.5 Ma), which records a decline in atmospheric CO₂, boosting the Antarctic glaciation that already started by the Eocene-Oligocene transition. ^{[35] [62]}

The Grande Coupure event marked a large faunal turnover marking the arrivals of later anthracotheres, entelodonts, ruminants (Gelocidae, Lophiomerycidae), rhinocerotoids (Rhinocerotidae, Amynodontidae, Eggysodontidae), carnivorans (later Amphicyonidae, Amphicynodontidae, Nimravidae, and Ursidae), eastern Eurasian rodents (Eomyidae, Cricetidae, and Castoridae), and eulipotyphlans (Erinaceidae). ^{[63] [64] [56] [65]}

MP20 marks the last known appearance of A. murinus, but the species A. riparius is apparently recorded solely from the MP21 French locality of Ronzon. Many other artiodactyl genera from western Europe disappeared as a result of the Grande Coupure extinction event, the Ronzon locality indicates that the Amphimerycidae may have survived past the event but went extinct not long after. ^{[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. ^{[59] [66]}

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