Amphimerycidae

murinum" (= Amphimeryx murinus''), 1822) The earliest taxonomic history of the Amphimerycidae was in 1804 when the French naturalist Georges Cuvier erected Anoplotherium minimum, stating that unlike with other species assigned to Anoplotherium (A. commune, A. medium, and A. minus), A. minimum lacked known postcranial fossil evidence. In 1822, he emended A. minimum to A. murinum, noting that the species was still only known from its skull unlike with other Anoplotherium species, and classified it to the subgenus Dichobune. In 1848, the French palaeontologist Auguste Pomel erected the genus Amphimeryx for the reclassified species A. murinus, arguing that it was close to ruminants in affinity. In a 1891–1893 palaeontology textbook, the German palaeontologist Karl Alfred von Zittel classified Amphimeryx to the artiodactyl family Xiphodontidae. Swiss palaeontologist Hans Georg Stehlin reclassified it and the newly recognized Pseudamphimeryx to their own family, the Amphimerycidae, in 1910. He also noted that while Amphimeryx was long thought to have been closely related to Xiphodon, the possibility that both the Amphimerycidae and Xiphodon independently acquired similar anatomical traits couldn't be eliminated. In 1961, The French palaeontologist Jean Viret provided a formal diagnosis of the Amphimerycidae, describing the anatomical traits that separated the family from its relatives. Classification s (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. For much of the taxonomy history of the amphimerycids, their placements within or outside the Ruminantia had been disputed and still remains so today. In 1941, the American palaeontologist Edwin H. Colbert wrote about evolutionary affinities of fossil and extant ruminants, comparing Archaeomeryx to other artiodactyl genera like Amphimeryx, Hypertragulus, Gelocus, and Tragulus. He said that fossil evidence of Amphimeryx was not complete during his time of study but suggested that it may have been a primitive member of clade Tragulina but that Archaeomeryx was overall more primitive than it. He then classified Amphimeryx to its own ruminant superfamily Amphimerycoidea, separating it from other traguline superfamilies like the Hypertraguloidea and Traguloidea. His classification was followed by another American palaeontologist George Gaylord Simpson in 1945. In 1961, Viret reclassified the Amphimerycidae and the Xiphodontidae into the artiodactyl clade Ancodonta, therefore removing the former from the Ruminantia. Similarly, American palaeontologists S. David Webb and Beryl E. Taylor in 1980 argued that the Amphimerycidae had historically been tied to the Ruminantia due to postcranial convergences but otherwise had more in common with xiphodonts than ruminants in terms of dentition. However, they chose to tentatively reclassify the Xiphodontidae and Amphimerycidae to the Tylopoda instead, although they did also suggest the possibility of them being a sister group to ruminants. On the other hand, in 1997, the American palaeontologists Malcolm McKenna and Susan K. Bell reclassified the Amphimerycidae into the Ruminantia. While amphimerycids have typically been excluded from the Ruminantia due to their dental characteristics, it does not eliminate the possibility of them being the 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 close 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. {{clade| style=font-size:85%; line-height:85% 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: In 2022, Weppe conducted a phylogenetic analysis of Palaeogene artiodactyls, focusing mostly on the endemic European families. 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 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 ==

, Natural History Museum of Basel The Amphimerycidae is a family of small-sized artiodactyls whose species ranged in weight from to total. In a 1995 study by Jean-Noël Martinez and Jean Sudre on several Paleogene artiodactyls found that only Messelobunodon is smaller than Amphimeryx. Amphimerycids are defined in part as having an elongated snout and large orbits (eye sockets) that are widened in their backs. The incisors (I/i) are shovel-shaped and have sharp edges on their crowns. Their dentitions more closely resemble those of xiphodonts or dacrytheriines than of ruminants. Amphimeryx is distinguished from Pseudamphimeryx in part by the more well-developed occipital crest. While the peak of the skull's top of Amphimeryx slopes down to its front area, that of Pseudamphimeryx appears initially concave at the occipital crest's front, ascends slightly, and then finally slopes down. This trait has also been recorded in ruminants, suggesting that the amphimerycids and ruminants independently acquired the trait in an instance of parallel evolution. In Amphimeryx, the middle 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. Digit III measures long while digit II is no more than long. Like other artiodactyls with only two elongated digits in each foot (digits III and IV), Amphimeryx was functionally didactyl, meaning that it walked only on its two elongated toes per foot despite having four total. == Palaeoecology ==

Middle Eocene of Europe and Asia during the Middle Eocene with possible artiodactyl and perissodactyl dispersal routes. For much of the Eocene, a hothouse climate with humid, tropical environments with consistently high precipitations prevailed. Modern mammalian orders including the Perissodactyla, Artiodactyla, and Primates already evolved 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 severed 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 mammals of western Europe were therefore mostly isolated from other landmasses including Greenland, Africa, and eastern Eurasia, allowing for endemism to develop. The Amphimerycidae, and by extent the first genus Pseudamphimeryx, is first recorded by the appearance of P. schlosseri in the Swiss locality of Egerkingen α + β, dating back to MP14 (43.5–41.2 mya). 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). and MP13 (44.9–43.5 mya) sites are stratigraphically the latest to have yielded remains of the bird clades Gastornithidae and Palaeognathae. The unit MP16 (40.0–37.8 mya) records the appearances of P. renevieri and P. pavloviae, both of which are recorded from the French locality of Robiac. 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 tough vegetation. The surviving herbivorous faunas shifted their dentitions and dietary strategies accordingly to adapt to the tough and seasonal vegetation. However, the environments were still somewhat 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. == Notes ==