Anoplotheriidae is an extinct family of artiodactyls exclusive to the Palaeogene that was endemic to western Europe and lived from the Middle Eocene to the Early Oligocene. The Anoplotheriidae is divided into two subfamilies: the Anoplotheriinae and the Dacrytheriinae. The type genus of the Anoplotheriinae is Anoplotherium and includes several other genera like Diplobune and Ephelcomenus. In contrast, Dacrytheriinae was limited to Dacrytherium and Catodontherium. The taxonomic history of anoplotheriids began as early as 1804 with the naming of Anoplotherium. The Anoplotheriidae was erected by the British zoologist John Edward Gray in 1821. Many other anoplotheriid genera were named from between the late 19th and early 21st centuries, although the dacrytheriines were initially assigned to their own family before eventually being reclassified as an anoplotheriid subfamily. The superfamily "Anoplotherioidea" had sometimes been utilized by taxonomists, although its taxonomic definition is uncertain. Anoplotheriids are thought to have been closely related to other extinct and contemporaneous artiodactyl families in western Europe, but the broader-level classifications of them are yet to be resolved.
Anoplotheriids were relatively medium to large in size, ranging in estimated weights from about in Dacrytherium to about in Anoplotherium. Anoplotheriid taxa vary from one another, but usually display a degree of selenodonty (crescent-shaped ridges), which point towards leaf-eating diets, and a complete dental row of 44 teeth with limited amounts of diastemata (gaps between teeth). Dacrytheriines differ from anoplotheriines in that they have a preorbital (in front of orbit) fossa (depression in bone) and several minute dental distinctions. Their skulls are low and their muzzles are elongated. Anoplotheriids with adequate postcranial fossil evidence are two-toed (Anoplotherium commune) to three-toed (other taxa like Diplobune and Anoplotherium latipes), the latter trait being unknown among other artiodactyls. Because of the lack of analogues for the limb morphologies and mobilities of anoplotheriids, unusual palaeobiological behaviours have been theorized by researchers, namely bipedalism in Anoplotherium, arboreal locomotion (tree-climbing) from Diplobune, and fossorial (digging) capabilities in Ephelcomenus.
Anoplotheriidae is endemic to western Europe, which was fragmented by oceans into a subtropical to tropical archipelago during the Eocene. This region sustained a diverse array of mammal groups. Dacrytheriines appeared earlier than anoplotheriines and lived from the Middle Eocene to the Late Eocene, whereas Anoplotheriines lived from the Middle Eocene to the Early Oligocene. Following the extinctions of early anoplotheriines, Anoplotherium and Diplobune filled their niches in the Late Eocene. After the Grande Coupure extinction and faunal turnover event, a combination of cooling climates and dispersal of various immigrant faunas to western Europe, the diversity of the Anoplotheriidae was drastically reduced. Only one anoplotheriid species, Diplobune minor, survived into the late Early Oligocene.
Taxonomy Research history
The taxonomic history of the Anoplotheriidae began as early as 1804 when the French palaeontologist Georges Cuvier erected the genus Anoplotherium based on fossils found at the gypsum quarries at the outskirts of Paris (known as the Paris Basin). The etymology of Anoplotherium derives in Ancient Greek from (, 'not'), (, 'armor, large shield'), and (, 'beast, wild animal') meaning "unarmed beast". He later named several species within Anoplotherium the same year. In 1812, he published a skeletal drawing of A. commune based on the incomplete skeletons he previously studied and speculated on the behaviours of the multiple species he assigned to Anoplotherium including A. commune. In 1821, British zoologist John Edward Gray established the family name "Anoplotheriadæ", providing the taxonomic diagnosis and assigning Anoplotherium as the sole member. He defined the Anoplotheriidae as having two long toes along with six total incisors, four canines, and twenty-eight cheek teeth, none of which are spaced out by diastemata (dental row gaps). In 1822, Cuvier assigned several species of Anoplotherium to its then-recognized subgenera Xiphodon and Dichobune. Because of its palaeontological significance, Anoplotherium became a key figure of 19th-century texts on palaeontology, although its iconicity declined in the 20th century. Several sculptures of A. commune were created by English sculptor Benjamin Waterhouse Hawkins in the Crystal Palace Dinosaurs attraction in the Crystal Palace Park in the United Kingdom, open to the public since 1854. The three sculptures were based on Cuvier's descriptions of the species along with his unpublished reconstructions showing that it was muscular, maintaining a degree of accuracy to the modern day.
In 1848, French palaeontologist Auguste Pomel reclassified one Anoplotherium species to Amphimeryx. As written in his two 1848–1852 works, French palaeontologist Paul Gervais treated both Xiphodon and Dichobune as distinct genera from Anoplotherium, thus classifying their respective species outside of Anoplotherium. French naturalist Charles Lucian Bonaparte wrote the family name "Anoplotheriidae" in his taxonomy list dating to 1850. Swiss palaeontologist François Jules Pictet de la Rive erected the clade Anoplothérioides in 1853, defining it as a broad-level clade (more specifically a superfamily). Besides Anoplotherium and "Eurytherium" (since synonymized with the former), he also classified multiple other non-anoplotheriid genera into the clade like Chalicotherium, Dichobune, Adapis, Dichodon, and Merycopotamus. In 1862, Swiss palaeontologist Ludwig Rütimeyer erected a subgenus for Dichobune named Diplobune, suggesting that A. secundarium was a transitional species from Anoplotherium to Dichobune. In response, German palaeontologist Oscar Fraas in 1870 confirmed that Diplobune was a distinct genus with close relations to Anoplotherium but not Dichobune. Following up on higher-level taxa involving Anoplotherium, American mammalogist Theodore Gill in 1872 listed the superfamily name Anoplotherioidea, of which the two family members were the Anoplotheriidae and Dichobunidae; the two genera listed under the former were Anoplotherium and Eurytherium. In 1876, French palaeontologist Henri Filhol erected Dacrytherium while also naming a new species under it. British palaeontologist Richard Lydekker defined the Anoplotheriidae as a family consisting of Xiphodon, Dacrytherium, and Anoplotherium in his 1885 work on artiodactyl taxonomy. In a book dating to 1891–1893, German palaeontologist Karl Alfred von Zittel suggested that there were four total subfamilies within the Anoplotheriidae: the "Anoplotherinae" (the three genera of which he said consisted of Anoplotherium, Diplobune, and Dacrytherium), "Dichobuninae", "Caenotherinae", and "Xiphodontinae".
In 1906, French palaeontologist Charles Depéret erected another genus Catodus, later changing the genus name to Catodontherium in 1908. In 1910, Swiss palaeontologist Hans Georg Stehlin discussed the clade "Anoplotheridae", noting that previous authors used to group under it artiodactyls from the Eocene that had diastemata-free dental rows and premolariform canines. He continued that many of the artiodactyls previously classified under the family cannot continue to be under it because of various other distinct dental characteristics, meaning that the cladistic definition of the family needed to be revised. Thus, Stehlin explained, the family only consisted of Anoplotherium and Diplobune. According to Depéret (1917), the newly established "Dacrythéridés" (or Dacrytheriidae) was closely aligned with Anoplotheriidae and Anthracotheriidae but restricted to only a few genera like Dacrytherium, Catodontherium, and Leptotheridium. In 1938, Swiss palaeontologist Johannes Hürzeler erected another anoplotheriid genus Ephelcomenus. In 1945, American palaeontologist George Gaylord Simpson recognized only the family name Anoplotheriidae, its genera being Anoplotherium, Diplobune, "Hyracodontherium" (since synonymized with Diplobune), Dacrytherium, Catodontherium, Leptotheridium, and Tapirulus.
In a new vein, a 1961 paper by French paleontologist Jean Viret recognized two subfamilies within the Anoplotheriidae: the Anoplotheriinae (Anoplotherium, Diplobune, and Ephelcomenus) and the Dacrytheriinae (Dacrytherium, Catodontherium, Leptotheridium, and Tapirulus). In 1977, French palaeontologist Jean Sudre suggested that Robiacina be classified into its own anoplotheriid subfamily "Robiaciinae", making it one of two subfamilies. He also revived the clade Anoplotherioidea, defining it as a group consisting of the Anoplotheriidae, Dacrytheriidae, and Cainotheriidae. He reaffirmed the validity of the Anoplotherioidea in 1978, considering it to have consisted of the Anoplotheriidae and "Dacrytheryidae"; he also classified Robiacina into the latter subfamily. In 1986, British palaeontologist Jerry J. Hooker agreed with Viret in there being two subfamilies under the Anoplotheriidae. In 1988, Sudre named another anoplotheriine Robiatherium. In two papers involving Hooker dating to 2000 and 2001, he and the papers' respective coauthors (Marc Wedmann and Katherine M. Thomas) further restricted Anoplotheriidae, removing Robiacina and then Tapirulus. In 2009, Spanish palaeontologists Miguel-Ángel Cuesta and Ainara Badiola erected Duerotherium and classified it into the Anoplotheriinae. The higher-level taxonomic placement of Leptotheridium remains in dispute, with authors continuing to classify it into either the Dacrytheriinae or Xiphodontidae.
Classification