User:PrimalMustelid/sandbox

PrimalMustelid/sandbox Temporal range: Late Eocene – Early Oligocene
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 Amphimerix Pomel, 1849 Hyægulus Pomel, 1851 Palæon Aymard, 1855 Amphimoeryx Gervais, 1859 Xiphodontherium Filhol, 1877 Synonyms of A. murinus Anoplotherium murinum Cuvier, 1822 Xiphodontherium primævum Filhol, 1877 Xiphodontherium secundarium Filhol, 1877 Synonyms of A. collotarsus Hyægulus collotarsus Pomel, 1851 Hyægulus murinus Pomel, 1851 Synonyms of A. riparius Palæon riparium Aymard, 1855

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. The genus name Amphimeryx derives from the Ancient Greek words ἀμφί (near) and μήρυξ (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.

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

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. 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:

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 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

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. 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 and lack of preorbital fossa 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 slight 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. 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.

The mandible of Amphimeryx is straight at the lower edge of its horizontal branch, or the mandibular corpus, and has a large 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).

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.