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''Pseudamphimeryx'' is an extinct genus of Paleogene, 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 orbit (anatomy), 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.
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 molar (tooth), 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 orbit (anatomy), 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 chevrotain, 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.
''Pseudamphimeryx'' belongs to the Amphimerycidae, a Paleogene, 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 Paleogene zones, 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 Tylopoda, tylopods (i.e. camelids and merycoidodonts) or ruminants. Different phylogenetic analyses have produced different results for the "apomorphy and synapomorphy, 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 (phylogenetics), 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.
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 skull, 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 Dichobunidae, dichobunids ''Dichobune'' and ''Mouillacitherium''.
The frontal bones of both amphimerycid genera are large and flat, being particularly sizeable in their supraorbital portions; this trait is more pronounced in ''Amphimeryx''. The frontal bones 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'' (underside of the orbit) 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. 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 by research 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 ''Pseudamphimeryx'' is undulated at the lower edge of its horizontal branch, or the mandibular corpus, and, like in ''Amphimeryx'', has a large and slightly rounded angular border. The coronoid process of the mandible is positioned lower in relation to the overall skull unlike in ruminants, and the temporomandibular joint is also lower than in ''Amphimeryx''.
''Pseudamphimeryx'' is also known from a brain endocast, although the endocasts of it and ''Amphimeryx'' were not well-studied. Its neocortex was described by Dechaseaux as being of a primitive and simple type in the larger evolutionary scale of artiodactyls.
The dental formula of the Amphimerycidae is for a total of 44 teeth, consistent with the primitive dental formula for early-middle Palaeogene placental mammals. The canine tooth, canines (C/c) are incisiform (incisor (I/i) form) and therefore differ little from 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. 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. Their dentitions more closely resemble those of xiphodonts or dacrytheriines than of ruminants.
''Pseudamphimeryx'' specifically is diagnosed in part as having moderate diastemata between P1 and P2. P1 is similarly separated from P2 by a diastema but is closer to the lower canine. Its upper molars have five tubercles along with a single front cingulum (tooth), cingulum each. In the lower molars, the labial cuspids are crescent-shaped whereas the lingual cuspids are subconical. The peak of the crescent formed by the metaconid and entoconid cusps is rounder in the molars of ''Pseudamphimeryx'' than in those of ''Amphimeryx'', a diagnostic trait separating the two genera.
In terms of non-diagnostic features of the amphimerycids, both genera have somewhat asymmetrical incisors that are shovel-shaped and have sharp edges on their crowns. The canines are similar to incisors but differ by their slightly more asymmetrical shapes. P1 and P2 have both been described as narrow and elongated, but the former tooth is larger than the latter. The overall selenodonty and brachyodonty (low-crowned teeth) of amphimerycids suggest that they were adapted towards folivorous (leaf-eating) dietary habits.
The Amphimerycidae consists only of small-sized species within ''Amphimeryx'' and ''Pseudamphimeryx''. According to Blondel, amphimerycid species would have ranged from to total. Similarly, Hooker et al. stated that ''Pseudamphimeryx'' was a tiny artiodactyl genus that weighed less than total. In comparison, ''Amphimeryx'', while still small-sized compared to most other Palaeogene artiodactyls, was larger with estimated weights of based on its M1 and based on the astragalus.
Some species of ''Pseudamphimeryx'' had been differentiated based on size in addition to morphology, with Sudre differentiating ''P. pavloviae'' from ''P. renevieri'' (which coexisted) by the former being larger in size and having more elongated premolars. Hooker and Weidmann suggested that Sudre's measures for proportional sizes between the two species could be potentially challenged by the variations in dental morphology and gaps in quantitative analyses. Therefore, they argued that additional statistical analyses need to be conducted that identify and separate different molar types (first to third molars) to better represent species size differences.
For much of the Eocene, a greenhouse effect, hothouse 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.
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. 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. Therefore, the European mammals of the Late Eocene (MP17–MP20 of the Mammal Palaeogene zones) were mostly descendants of endemic Middle Eocene groups.
The Amphimerycidae, and by extent the older genus ''Pseudamphimeryx'', is first recorded by the appearance of ''P. schlosseri'' in the Swiss locality of Egerkingen α + β, dating back to MP14. Both families would have coexisted with perissodactyls (Palaeotheriidae, Lophiodontidae, and Hyrachyidae), non-endemic artiodactyls (Dichobunidae and Tapirulidae), endemic European artiodactyls (Choeropotamidae, Cebochoeridae, and Anoplotheriidae), and primates (Adapidae). The stratigraphic ranges of the early species of ''Amphimeryx'' also overlapped with metatherians (Herpetotheriidae), cimolestans (Pantolestidae, Paroxyclaenidae), rodents (Ischyromyidae, Theridomyoidea, Gliridae), eulipotyphlans, bats, apatotherians, carnivoraformes (Miacidae), and hyaenodonts (Hyainailourinae, Proviverrinae). Other MP13-MP14 sites have also yielded fossils of turtles and Crocodylomorpha, crocodylomorphs, and MP13 sites are stratigraphically the latest to have yielded remains of the bird clades Gastornithidae and Palaeognathae.
In addition to ''P. schlosseri'', other mammals that appeared in Egerkingen α + β include the herpetotheriid ''Amphiperatherium'', ischyromyids ''Ailuravus'' and ''Plesiarctomys'', pseudosciurid ''Treposciurus'', omomyid ''Necrolemur'', adapid ''Leptadapis'', proviverrine ''Proviverra'', palaeotheres (''Propalaeotherium'', ''Anchilophus'', ''Lophiotherium'', ''Plagiolophus (mammal), Plagiolophus'', ''Palaeotherium''), hyrachyid ''Chasmotherium'', lophiodont ''Lophiodon'', dichobunids ''Hyperdichobune'' and ''Mouillacitherium'', choeropotamid ''Rhagatherium'', anoplotheriid ''Catodontherium'', cebochoerid ''Cebochoerus'', tapirulid ''Tapirulus'', mixtotheriid ''Mixtotherium'', and the xiphodonts ''Dichodon (mammal), Dichodon'' and ''Haplomeryx''.
The unit MP16 records the appearances of ''P. renevieri'' and ''P. pavloviae'', both of which are known from the MP16 French locality of Robiac. Other mammal genera that cooccur in the site include the herpetotheriids ''Amphiperatherium'' and ''Peratherium'', apatemyid ''Heterohyus'', nyctitheriid ''Saturninia'', rodents (''Glamys'', ''Elfomys'', ''Plesiarctomys'', ''Ailuravus'', ''Remys''), omomyids ''Pseudoloris'' and ''Necrolemur'', adapid ''Adapis'', hyaenodonts ''Paroxyaena'' and ''Cynohyaenodon'', carnivoraformes ''Paramiacis'' and ''Quercygale'', palaeotheres (''Propalaeotherium'', ''Anchilophus'', ''Plagiolophus (mammal), Plagiolophus'', ''Pachynolophus'', ''Palaeotherium''), lophiodont ''Lophiodon'', hyrachyid ''Chasmotherium'', cebochoerids ''Acotherulum'' and ''Cebochoerus'', choeropotamid ''Choeropotamus'', tapirulid ''Tapirulus'', anoplotheriids (''Dacrytherium'', ''Catodontherium'', ''Robiatherium''), robiacinid ''Robiacina'', and xiphodonts (''Xiphodon'', ''Dichodon'', ''Haplomeryx'').
After MP16, a faunal turnover occurred, marking the disappearances of the lophiodonts and European hyrachyids as well as the extinctions of all European crocodylomorphs except for the alligatoroid ''Diplocynodon''. The causes of the faunal turnover have been attributed to 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. However, the environments were still subhumid and covered by subtropical evergreen forests. The Palaeotheriidae was the sole remaining European perissodactyl group, and frugivorous-folivorous or purely folivorous artiodactyls became the dominant group in western Europe.
The Late Eocene unit MP17 records as many as four species of ''Pseudamphimeryx'': ''P. renevieri'', ''P. havloviae'', ''P. salesmei'', and ''P. hantonensis''. MP17a confirms the continued occurrence of ''P. renevieri'' in the French locality of Fons 4; MP17b is the latest unit that ''Pseudamphimeryx'' occurs and records both ''P. renevieri'' and ''P. pavloviae'' from another French locality of Perrière. Starting at MP18, ''Amphimeryx'' makes its first appearance and therefore succeeds ''Pseudamphimeryx''. In Perrière, ''Pseudamphimeryx'' fossils have been found along with those of the herpetotheriids ''Peratherium'' and ''Amphiperatherium'', pseudorhyncocyonid ''Pseudorhyncocyon'', apatemyid ''Heterohyus'', nyctitheriid ''Saturninia'', various rodents and bats, omomyids ''Pseudoloris'' and ''Microchoerus'', adapid ''Leptadapis'', hyaenodontid ''Hyenodon'', miacid ''Quercygale'', palaeotheres (''Lophiotherium'', ''Palaeotherium'', and ''Plagiolophus''), dichobunid ''Mouillacitherium'', cebochoerid ''Acotherulum'', mixtothere ''Mixtotherium'', anoplotheriid ''Dacrytherium'', tapirulid ''Tapirulus'', and the xiphodonts ''Dichodon'' and ''Haplomeryx''.
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 molar (tooth), 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 orbit (anatomy), 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 chevrotain, 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 Paleogene, 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 Paleogene zones, 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 Tylopoda, tylopods (i.e. camelids and merycoidodonts) or ruminants. Different phylogenetic analyses have produced different results for the "apomorphy and synapomorphy, 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 (phylogenetics), 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 skull, 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 Dichobunidae, dichobunids ''Dichobune'' and ''Mouillacitherium''.
The frontal bones of both amphimerycid genera are large and flat, being particularly sizeable in their supraorbital portions; this trait is more pronounced in ''Amphimeryx''. The frontal bones 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'' (underside of the orbit) 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. 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 by research 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 ''Pseudamphimeryx'' is undulated at the lower edge of its horizontal branch, or the mandibular corpus, and, like in ''Amphimeryx'', has a large and slightly rounded angular border. The coronoid process of the mandible is positioned lower in relation to the overall skull unlike in ruminants, and the temporomandibular joint is also lower than in ''Amphimeryx''.
''Pseudamphimeryx'' is also known from a brain endocast, although the endocasts of it and ''Amphimeryx'' were not well-studied. Its neocortex was described by Dechaseaux as being of a primitive and simple type in the larger evolutionary scale of artiodactyls.
Dentition
The dental formula of the Amphimerycidae is for a total of 44 teeth, consistent with the primitive dental formula for early-middle Palaeogene placental mammals. The canine tooth, canines (C/c) are incisiform (incisor (I/i) form) and therefore differ little from 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. 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. Their dentitions more closely resemble those of xiphodonts or dacrytheriines than of ruminants.
''Pseudamphimeryx'' specifically is diagnosed in part as having moderate diastemata between P1 and P2. P1 is similarly separated from P2 by a diastema but is closer to the lower canine. Its upper molars have five tubercles along with a single front cingulum (tooth), cingulum each. In the lower molars, the labial cuspids are crescent-shaped whereas the lingual cuspids are subconical. The peak of the crescent formed by the metaconid and entoconid cusps is rounder in the molars of ''Pseudamphimeryx'' than in those of ''Amphimeryx'', a diagnostic trait separating the two genera.
In terms of non-diagnostic features of the amphimerycids, both genera have somewhat asymmetrical incisors that are shovel-shaped and have sharp edges on their crowns. The canines are similar to incisors but differ by their slightly more asymmetrical shapes. P1 and P2 have both been described as narrow and elongated, but the former tooth is larger than the latter. The overall selenodonty and brachyodonty (low-crowned teeth) of amphimerycids suggest that they were adapted towards folivorous (leaf-eating) dietary habits.
Postcranial skeleton
According to Sudre and Cécile Blondel in 1995 and Erfurt and Métais in 2007, there are three reports of postcranial fossils assigned to ''Pseudamphimeryx'': a "cubonavicular" bone (fused cuboid bone and navicular bone of the hind legs) of ''P. pavloviae'' from La Bouffie plus ''P. salesmei'' from Salesmes and an talus bone, astragalus of ''P. renevieri'' from the French commune of Euzet. The latter fossil is suggested to prove the presence of a fused cubonavicular bone in ''P. renevieri''. The fused cubonavicular bone trait has also been reported in ''Amphimeryx'', which appeared after ''Pseudamphimeryx'' and is also generally a defining trait of ruminants in an instance of parallel evolution. The later relative ''Amphimeryx'', unlike ''Pseudamphimeryx'', is known by complete hind leg evidence including the cubonavicular bone. The overall morphology of the hind leg's bones are reminiscent of those of derived (including extant) ruminants such as in mouse deer, tragulids. More specifically, ''Amphimeryx'' was functionally didactyl (two-toed) because, as in derived ruminants, its two middle toes, making up a single cannon bone, are elongated while its two side digits are greatly reduced. On the other hand, ''Amphimeryx'' differs from ruminants by the primitive morphology of the astragalus.Size
The Amphimerycidae consists only of small-sized species within ''Amphimeryx'' and ''Pseudamphimeryx''. According to Blondel, amphimerycid species would have ranged from to total. Similarly, Hooker et al. stated that ''Pseudamphimeryx'' was a tiny artiodactyl genus that weighed less than total. In comparison, ''Amphimeryx'', while still small-sized compared to most other Palaeogene artiodactyls, was larger with estimated weights of based on its M1 and based on the astragalus.
Some species of ''Pseudamphimeryx'' had been differentiated based on size in addition to morphology, with Sudre differentiating ''P. pavloviae'' from ''P. renevieri'' (which coexisted) by the former being larger in size and having more elongated premolars. Hooker and Weidmann suggested that Sudre's measures for proportional sizes between the two species could be potentially challenged by the variations in dental morphology and gaps in quantitative analyses. Therefore, they argued that additional statistical analyses need to be conducted that identify and separate different molar types (first to third molars) to better represent species size differences.
Palaeoecology
For much of the Eocene, a greenhouse effect, hothouse 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.
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. 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. Therefore, the European mammals of the Late Eocene (MP17–MP20 of the Mammal Palaeogene zones) were mostly descendants of endemic Middle Eocene groups.
The Amphimerycidae, and by extent the older genus ''Pseudamphimeryx'', is first recorded by the appearance of ''P. schlosseri'' in the Swiss locality of Egerkingen α + β, dating back to MP14. Both families would have coexisted with perissodactyls (Palaeotheriidae, Lophiodontidae, and Hyrachyidae), non-endemic artiodactyls (Dichobunidae and Tapirulidae), endemic European artiodactyls (Choeropotamidae, Cebochoeridae, and Anoplotheriidae), and primates (Adapidae). The stratigraphic ranges of the early species of ''Amphimeryx'' also overlapped with metatherians (Herpetotheriidae), cimolestans (Pantolestidae, Paroxyclaenidae), rodents (Ischyromyidae, Theridomyoidea, Gliridae), eulipotyphlans, bats, apatotherians, carnivoraformes (Miacidae), and hyaenodonts (Hyainailourinae, Proviverrinae). Other MP13-MP14 sites have also yielded fossils of turtles and Crocodylomorpha, crocodylomorphs, and MP13 sites are stratigraphically the latest to have yielded remains of the bird clades Gastornithidae and Palaeognathae.
In addition to ''P. schlosseri'', other mammals that appeared in Egerkingen α + β include the herpetotheriid ''Amphiperatherium'', ischyromyids ''Ailuravus'' and ''Plesiarctomys'', pseudosciurid ''Treposciurus'', omomyid ''Necrolemur'', adapid ''Leptadapis'', proviverrine ''Proviverra'', palaeotheres (''Propalaeotherium'', ''Anchilophus'', ''Lophiotherium'', ''Plagiolophus (mammal), Plagiolophus'', ''Palaeotherium''), hyrachyid ''Chasmotherium'', lophiodont ''Lophiodon'', dichobunids ''Hyperdichobune'' and ''Mouillacitherium'', choeropotamid ''Rhagatherium'', anoplotheriid ''Catodontherium'', cebochoerid ''Cebochoerus'', tapirulid ''Tapirulus'', mixtotheriid ''Mixtotherium'', and the xiphodonts ''Dichodon (mammal), Dichodon'' and ''Haplomeryx''.
The unit MP16 records the appearances of ''P. renevieri'' and ''P. pavloviae'', both of which are known from the MP16 French locality of Robiac. Other mammal genera that cooccur in the site include the herpetotheriids ''Amphiperatherium'' and ''Peratherium'', apatemyid ''Heterohyus'', nyctitheriid ''Saturninia'', rodents (''Glamys'', ''Elfomys'', ''Plesiarctomys'', ''Ailuravus'', ''Remys''), omomyids ''Pseudoloris'' and ''Necrolemur'', adapid ''Adapis'', hyaenodonts ''Paroxyaena'' and ''Cynohyaenodon'', carnivoraformes ''Paramiacis'' and ''Quercygale'', palaeotheres (''Propalaeotherium'', ''Anchilophus'', ''Plagiolophus (mammal), Plagiolophus'', ''Pachynolophus'', ''Palaeotherium''), lophiodont ''Lophiodon'', hyrachyid ''Chasmotherium'', cebochoerids ''Acotherulum'' and ''Cebochoerus'', choeropotamid ''Choeropotamus'', tapirulid ''Tapirulus'', anoplotheriids (''Dacrytherium'', ''Catodontherium'', ''Robiatherium''), robiacinid ''Robiacina'', and xiphodonts (''Xiphodon'', ''Dichodon'', ''Haplomeryx'').
After MP16, a faunal turnover occurred, marking the disappearances of the lophiodonts and European hyrachyids as well as the extinctions of all European crocodylomorphs except for the alligatoroid ''Diplocynodon''. The causes of the faunal turnover have been attributed to 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. However, the environments were still subhumid and covered by subtropical evergreen forests. The Palaeotheriidae was the sole remaining European perissodactyl group, and frugivorous-folivorous or purely folivorous artiodactyls became the dominant group in western Europe.
The Late Eocene unit MP17 records as many as four species of ''Pseudamphimeryx'': ''P. renevieri'', ''P. havloviae'', ''P. salesmei'', and ''P. hantonensis''. MP17a confirms the continued occurrence of ''P. renevieri'' in the French locality of Fons 4; MP17b is the latest unit that ''Pseudamphimeryx'' occurs and records both ''P. renevieri'' and ''P. pavloviae'' from another French locality of Perrière. Starting at MP18, ''Amphimeryx'' makes its first appearance and therefore succeeds ''Pseudamphimeryx''. In Perrière, ''Pseudamphimeryx'' fossils have been found along with those of the herpetotheriids ''Peratherium'' and ''Amphiperatherium'', pseudorhyncocyonid ''Pseudorhyncocyon'', apatemyid ''Heterohyus'', nyctitheriid ''Saturninia'', various rodents and bats, omomyids ''Pseudoloris'' and ''Microchoerus'', adapid ''Leptadapis'', hyaenodontid ''Hyenodon'', miacid ''Quercygale'', palaeotheres (''Lophiotherium'', ''Palaeotherium'', and ''Plagiolophus''), dichobunid ''Mouillacitherium'', cebochoerid ''Acotherulum'', mixtothere ''Mixtotherium'', anoplotheriid ''Dacrytherium'', tapirulid ''Tapirulus'', and the xiphodonts ''Dichodon'' and ''Haplomeryx''.
Notes
References
{{Taxonbar, from=Q131437903 Eocene Artiodactyla Eocene mammals of Europe Prehistoric Artiodactyla genera Fossil taxa described in 1910