Pseudamphimeryx 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 Middle to Late Eocene. It was first erected in 1910 by the Swiss palaeontologist Hans Georg Stehlin, who assigned to it multiple species and noted specific differences from another amphimerycid, Amphimeryx. As of present, it is known by six species, although the validity of P. valdensis has been questioned while the earliest-appearing species P. schosseri has been suggested to not be an amphimerycid.
Pseudamphimeryx is very similar to Amphimeryx with its selenodont (crescent-like ridges) dentition and its fused "cubonavicular bone" (cuboid bone and navicular bone) of its hind legs, both of which are recorded in ruminants in an instance of parallel evolution. Both amphimerycid genera had large orbits and long snouts. Pseudamphimeryx has very specific differences with Amphimeryx such as the occipital crest forms. Its selenodont dentition suggests that it had a preference for leaf-eating diets. Pseudamphimeryx was a tiny-sized artiodactyl that likely weighed less than .
It 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. Although it survived a major faunal turnover by the Middle Eocene, it was eventually replaced by Amphimeryx in the Late Eocene.
Taxonomy
In 1910, the Swiss palaeontologist Hans Georg Stehlin erected the genus Pseudamphimeryx, introducing it as an artiodactyl that differed from Amphimeryx by specific cranial and dental traits. He first recognized the species Pseudamphimeryx Renevieri, which was previously classified as Cainotherium then Xiphodontherium (the latter genus of which he synonymized with Amphimeryx); "C." renevieri was previously named by the naturalists François Jules Pictet de la Rive and Aloïs Humbert in 1869. The second species that he classified to Pseudamphimeryx was P. Schlosseri, first named as a species of Xiphodontherium by Ludwig Ruetimeyer in 1891. Stehlin then named the species P. valdensis based on lower fossil molars from the Swiss locality of Mormont, stating that it was smaller than P. renevieri in size. He also recognized two additional taxa from the Phosphorites du Quercy site: P. Renevieri var. Pavlowiae and P. decedens.
In 1928, English palaeontologist Clive Forster-Cooper erected P. hantonensis based on two upper molars from Hordle in England. In 1978, French palaeontologist Jean Sudre recognized P. pavloviae as a distinct species whose name he emended from P. pavlowiae and erected another species named P. salesmei based on a mandible from the French locality of Salesmes.
In 1974, the French palaeontologist Colette Dechaseaux noted that P. decedens had notably large orbits along with preorbital fossae in front of them like in deer. She cast doubt that the species actually belonged to Pseudamphimeryx because of the molar forms. Similarly in 1978, Sudre expressed that P. decedens actually belonged to Pseudamphimeryx, stating that the short premolars, large orbits, and preorbital fossae are not present in any other skull of other amphimerycids. In 1984, Sudre tentatively reassigned P. decedens to the tragulid genus Cryptomeryx as C? decedens, building on to Dechaseaux's study by confirming that the by then lost skull would have belonged to a small ruminant. In 1986, however, Geneviève Bouvrain, Denis Geraads and Sudre revised Cryptomeryx as a synonym of Lophiomeryx, but it was alternatively considered a synonym of Iberomeryx by Bastien Mennecart et al. in 2010. C? decedens had tentatively been placed in Iberomeryx in a 2012 PhD thesis by Mennecart.
In 2000, the palaeontologists Jerry J. Hooker and Marc Weidmann suggested that P. pavloviae be emended back to P. pavlowiae based on Stehlin's original spelling and argued that P. valdensis was either a nomen dubium or a junior synonym of P. renevieri. In 2007, palaeontologists Jörg Erfurt and Grégoire Métais listed P. valdensis as a valid species without referencing Hooker and Weidmann's invalidation of the species.
Classification
Pseudamphimeryx belongs to 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 artiodactyls with selenodont (crescent-shaped ridge form) dentition 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 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. The affinities of the Amphimerycidae, 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" (or evolutionarily recent) 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, including Pseudamphimeryx, 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 (group with no evolutionarily descendants) set consisted of the Hyperdichobuninae, Amphimerycidae, Xiphodontidae, and Cainotherioidea based on dental synapomorphies (exclusive anatomical traits shared from an ancestral group), 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 (group with descendants) 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. Pseudmphimeryx specifically is described as having a skull whose peak appears initially concave at the occipital crest's front, ascends slightly, and then finally slopes down. 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. Its skull additionally resembles those of both Dacrytherium and Tapirulus.
The overall skull of Pseudamphimeryx is very elongated especially in comparison to Mouillacitherium but falls short of that of Amphimeryx. 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 each divide 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 Pseudamphimeryx 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 orientation of the occipital crest differs by amphimerycid genus, with that of Pseudamphimeryx being straight and vertical. 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.