Haplomeryx is an extinct genus of Palaeogene artiodactyls belonging to the family Xiphodontidae. It was endemic to Western Europe and lived from the Middle Eocene up to the earliest Oligocene. Haplomeryx was first established as a genus by the German naturalist Max Schlosser in 1886 based on a molar tooth set from Quercy Phosphorites deposits. Three additional species were erected and classified to the xiphodontid genus while one other species, first recognized in 1822, was tentatively classified to it and remains unresolved in affinity.
Little is known about Haplomeryx due to its poor cranial and postcranial fossil records. Its dentition is thought to have been typical of xiphodonts such as Xiphodon and Dichodon because of most of its premolars being elongated, its dental differences being more specific compared to the other two genera. Although it may have displayed an evolutionary size increase by species, all of them remained very small in size especially in comparison to the two other xiphodont genera. It lived in 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 including the aforementioned xiphodont genera.
It and other xiphodont genera went extinct by 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 causes of its extinction are attributed to negative interactions with immigrant faunas (resource competition, predation), environmental turnover from climate change, or some combination of the two.
Taxonomy
In 1886, the German naturalist Max Schlosser erected the genus Haplomeryx, known solely from selenodont molars from the Quercy Phosphorites. He proposed that its dentition is most similar to that of Agriochoerus and established the species Haplomeryx zitteli based on an upper jaw fragment consisting of three molars that in total measure long. He noted its small size and theorized that it was the only European member of mammals that were of close affinities with the North American Agriochoerus. The etymology of the genus name derives in Ancient Greek from (simple) and (ruminant) meaning "simple ruminant".
In 1910, the Swiss palaeontologist Hans Georg Stehlin made a review of Haplomeryx amongst other European artiodactyls, stating that he did not notice its fossils having been previously described under any synonymous name and that its overall anatomy is not known. The first species he erected was H. Picteti based on fossil previously described from Mormont in Switzerland, noting that the teeth are smaller than those of H. zitteli and that it has slightly different molar morphologies. The second and other species that Stehlin named was H. egerkingensis based on dentition from the Swiss municipality of Egerkingen. He also tentatively reclassified the species Dichobune obliquum, previously described by the French naturalist Georges Cuvier in 1822 as Anoplotherium (Dichobune) obliquum, to Haplomeryx as H? obliquus. H? obliquus is known only by a single specimen from Montmartre in France, thus making its affinities problematic to resolve.
The French palaeontologist Charles Depéret established the species H. Euzetensis based on dental fossils from the French commune of Euzet in 1917. He said that the species was intermediate in size between H. zitteli and H. picteti.
Classification Haplomeryx belongs to the Xiphodontidae, a Palaeogene artiodactyl family endemic to western Europe that lived from the Middle Eocene to the Early Oligocene (~44 Ma to 33 Ma). Like the other contemporary endemic artiodactyl families of western Europe, the evolutionary origins of the Xiphodontidae are poorly known. The Xiphodontidae is generally thought to have first appeared by MP14 of the Mammal Palaeogene zones, making them the first selenodont dentition artiodactyl representatives to have appeared in the landmass along with the Amphimerycidae. More specifically, the first xiphodont representatives to appear were the genera Dichodon and Haplomeryx by MP14. Dichodon and Haplomeryx continued to persist into the Late Eocene while Xiphodon made its first appearance by MP16. Another xiphodont Paraxiphodon is known to have occurred only in MP17a localities. The former three genera lived up to the Early Oligocene where they have been recorded to have all gone extinct as a result of the Grande Coupure faunal turnover event.
The phylogenetic relations of the Xiphodontidae as well as the Anoplotheriidae, Mixtotheriidae and Cainotheriidae have been elusive due to the selenodont morphologies (or having crescent-shaped ridges) of the molars, which were convergent with tylopods or ruminants. Some researchers considered the selenodont families Anoplotheriidae, Xiphodontidae, and Cainotheriidae to be within Tylopoda due to postcranial features that were similar to the tylopods from North America in the Palaeogene. Other researchers tie them as being more closely related to ruminants than tylopods based on dental morphology. Different phylogenetic analyses have produced different results for the "derived" (or of new evolutionary traits) selenodont Eocene European artiodactyl families, making it uncertain whether they were closer to the Tylopoda or Ruminantia. Possibly, the Xiphodontidae may have arisen from an unknown dichobunoid group, thus making its resemblance to tylopods an instance of convergent evolution.
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 published in the article and another work about the cainotherioids is outlined below:
In 2022, Weppe created a phylogenetic analysis in his academic thesis regarding Palaeogene artiodactyl lineages, focusing most specifically on the endemic European families. He stated that his phylogeny was the first formal one to propose affinities of the Xiphodontidae and Anoplotheriidae. He found that the Anoplotheriidae, Mixtotheriidae, and Cainotherioidea form a clade based on synapomorphic dental traits (traits thought to have originated from their most recent common ancestor). The result, Weppe mentioned, matches up with previous phylogenetic analyses on the Cainotherioidea with other endemic European Palaeogene artiodactyls that support the families as a clade. As a result, he argued that the proposed superfamily Anoplotherioidea, composing of the Anoplotheriidae and Xiphodontidae as proposed by Alan W. Gentry and Hooker in 1988, is invalid due to the polyphyly of the lineages in the phylogenetic analysis. However, the Xiphodontidae was still found to compose part of a wider clade with the three other groups. Within the Xiphodontidae, Weppe's phylogeny tree classified Haplomeryx as a sister taxon to the clade consisting of Xiphodon plus Dichodon.
Description
Unlike both Xiphodon and Dichodon with known evidence of complete sets of 44 teeth, the number of teeth present in Haplomeryx is unclear since it is only known by its sets of 3 molars, 4 premolars, and, in the case of H. zitteli, possibly a canine. Xiphodonts are characterized by indistinct canines in comparison to other teeth and elongated premolars. Xiphodontids additionally have molariform P4 teeth, upper molars with 4 to 5 crescent-shaped cusps, and selenodont lower molars with 4 ridges, compressed lingual cuspids, and crescent-shaped labial cuspids.
Haplomeryx is not as well-described compared to Xiphodon or Dichodon. The upper molars are brachyodont (low-crowned) and have four crescent-shaped cusps, although H. egerkingensis has an additional small paraconule cusp. They also have W-shaped ectolophs (crests or ridges of upper molar teeth), curved mesostyle cusps, hollowed labial walls of the paracone plus mesocone cusps, and conical protocone cusps that are connected to the parastyle cusps. The P4 tooth appears short and triangular, and its lingual cusp is crescent-shaped. The lower premolars, with the exception of P4, are elongated in form. Its dentition appears similar to that of Dichodon, the main difference being that the parastyle cusp of Haplomeryx appears to be more prominent. The astragalus originally assigned to H. euzetensis is narrow and elongated, appearing slightly slanted at its bottom end similar to those of Dacrytherium and Xiphodon. According to Hooker and Weidmann, however, it actually belonged to Pseudamphimeryx; they instead reassigned an astragalus from Leptotheridium to Haplomeryx. The astragalus that was reassigned to Haplomeryx was described by Depéret as being very narrow and elongated with a narrow tibial groove and a straight bone axis.
Haplomeryx is also diagnosed as being very small in size. While Haplomeryx may have displayed evolutionary size increases, it remained small-sized unlike Xiphodon and Dichodon, which both were both capable to growing to medium sizes. According to Jean Sudre, the upper molars belonging to H. picteti, H. euzetensis, and H. zitteli progressed in evolutionary chronology by increased sizes.
Palaeoecology
Middle Eocene
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 (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 million years ago) 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.