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.

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.

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 14 mm (0.55 in) long.

[2] 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.

[8] 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.

[9] 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.

[4] 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.

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.

[12] The phylogenetic tree published in the article and another work about the cainotherioids is outlined below:[13] Eurodexis russelli Dichobune leporina Amphimeryx murinus Xiphodon castrense Paratoceras coatesi Eotylopus reedi Parvitragulus priscus Lophiomeryx chalaniati Archaeomeryx optatus Mixtotherium cuspidatum Anoplotherium latipes Dacrytherium ovinum Robiacina lavergnesis Robiacina minuta Robiacina quercyi Palembertina deplasi Paroxacron bergeri Paroxacron valdense Oxacron courtoisii Cainotherium laticurvatum Caenomeryx filholi Caenomeryx procommunis Plesiomeryx cadurcensis Plesiomeryx huerzeleri In 2022, Weppe created a phylogenetic analysis in his academic thesis regarding Palaeogene artiodactyl lineages, focusing most specifically on the endemic European families.

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.

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.

[21] 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.

[24][6][25] The stratigraphic ranges of the early species of Haplomeryx also overlapped with metatherians (Herpetotheriidae), cimolestans (Pantolestidae, Paroxyclaenidae), rodents (Ischyromyidae, Theridomyoidea, Gliridae), eulipotyphlans, bats, apatotherians, carnivoraformes (Miacidae), and hyaenodonts (Hyainailourinae, Proviverrinae).

[27] Based on the Egerkingen α + β locality, H. egerkingensis coexisted with the herpetotheriid Amphiperatherium, ischyromyids Ailuravus and Plesiarctomys, pseudosciurid Treposciurus, omomyid Necrolemur, adapid Leptadapis, proviverrine Proviverra, palaeotheres (Propalaeotherium, Anchilophus, Lophiotherium, Plagiolophus, Palaeotherium), hyrachyid Chasmotherium, lophiodont Lophiodon, dichobunids Hyperdichobune and Mouillacitherium, choeropotamid Rhagatherium, anoplotheriid Catodontherium, amphimerycid Pseudamphimeryx, cebochoerid Cebochoerus, tapirulid Tapirulus, mixtotheriid Mixtotherium, and the xiphodont Dichodon.

[28][29] In Lavergne, fossils of the two Haplomeryx species were found with those of the herpetotheriids Amphiperatherium and Peratherium, pseudorhyncocyonid Leptictidium, nyctitheres Euronyctia and Saturninia, omomyids Necrolemur and Pseudoloris, theridomyids (Burgia, Elfomys, Glamys, Idicia), bats (Carcinipteryx, Hipposideros, Vaylatsia), proviverrine Allopterodon, carnivoraformes Quercygale and Paramiacis, cebochoerids Acotherulum and Cebochoerus, anoplotheriids Catodontherium and Dacrytherium, mixtothere Mixtotherium, dichobunids Dichobune and Mouillacitherium, amphimerycid Pseudamphimeryx, and the xiphodont Dichodon.

[6][26][30][31] 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.

[36] In the MP19 locality of Escamps, H. zitteli is recorded to have cooccurred with the likes of the herpetotheriids Amphiperatherium and Peratherium, pseudorhyncocyonid Pseudorhyncocyon, nyctitheres Saturninia and Amphidozotherium, bats (Hipposideros, Vaylatsia, Stehlinia), theridomyids (Paradelomys, Elfomys, Blainvillimys, Theridomys), adapid Palaeolemur, hyainailourine Pterodon, amphicyonid Cynodictis, palaeotheres Palaeotherium and Plagiolophus, dichobunid Dichobune, choeropotamid Choeropotamus, anoplotheriids Anoplotherium and Diplobune, cainotheres Oxacron and Paroxacron, amphimerycid Amphimeryx, and the other xiphodonts Xiphodon and Dichodon.

The Turgai Strait, which once separated much of Europe from Asia, is often proposed as the main European seaway barrier prior to the Grande Coupure, but some researchers challenged this perception recently, arguing that it completely receded already 37 Ma, long before the Eocene-Oligocene transition.

[8][38][4] The extinctions of Haplomeryx and many other mammals have been attributed to negative interactions with immigrant faunas (competition, predations), environmental changes from cooling climates, or some combination of the two.