Dacrytherium (Ancient Greek: (tear, teardrop) + (beast or wild animal) meaning "tear beast") is an extinct genus of Palaeogene artiodactyls belonging to the family Anoplotheriidae. It occurred from the Middle to Late Eocene of Western Europe and is the type genus of the subfamily Dacrytheriinae, the older of the two anoplotheriid subfamilies. Dacrytherium was first erected in 1876 by the French palaeontologist Henri Filhol, who recognised in his studies that it had dentition similar to the anoplotheriids Anoplotherium and Diplobune but differed from them by a deep preorbital fossa and a lacrimal fossa, the latter of which is where the genus name derives from. D. ovinum, originally classified in Dichobune, is the type species of Dacrytherium. Henri Filhol named D. elegans in 1884, and Hans Georg Stehlin named the species D. priscum and D. saturnini in 1910.
Dacrytherium was a medium-sized artiodactyl that is defined by specific dental traits separating it from the Anoplotheriinae. Typically, its species have deep preorbital fossae that anoplotheriines lack entirely, although the depression of D. elegans is different from other species. Its dental and cranial anatomies, however, were otherwise typical of the Anoplotheriidae, which led to historic confusions regarding whether Dacrytherium belonged to the Anoplotheriidae or its own family. It is also recognised as having two evolutionary paths in the forms of D. elegans-D. saturnini and D. priscum-D. ovinum given the anatomical changes and size increases of dentitions over time, although the researcher who proposed the lineages later expressed doubt that the validity of the latter.
Dacrytherium lived in western Europe during a period when the region was an archipelago isolated from the rest of Eurasia. Contemporary species from this region were not widely dispersed due to high levels of endemism. It co-existed with a wide variety of artiodactyls and perissodactyls in subtropical-tropical environments that supported frugivorous-folivorous mammals. Dacrytherium itself was likely folivorous, but its lifestyle is unknown given the general scarcity of post-cranial evidence and the unusual variations of hypothesized behaviours in the derived anoplotheriines Anoplotherium and Diplobune.
Taxonomy Taxonomic history
In 1876, French palaeontologist Henri Filhol described fossils from recent excavations at the phosphorite deposits of Quercy in France, including bones that he identified as belonging to new genera or species. One identified "pachyderm" has a dental formula, for the incisors, canines, and molars, of , which he considered similar to other fossil mammal genera like Anthracotherium. The specimen was a complete skull with a "peculiar" upper jaw and a lower jaw and all of its teeth. From the skull, he erected the genus name Dacrytherium and established the species D. anthracoides. The same year, French palaeontologist Paul Gervais compared Dacrytherium to hipparionines and merycoidodonts based on the upper maxilla of the complete skull having a similarly deep lateral (or outer) hollowing. The etymology of the genus name derives in Greek from "dacry(o)" (tear, teardrop in reference to lacrimation) and "thḗr" (beast or wild animal) meaning "tear beast", referencing the lacrimal fossa and its function for hosting the lacrimal sac that drains teardrops from the eye's surface.
In 1877, Filhol retained the genus name but replaced the species name D. anthracoides with D. cayluxi, creating figures for the skull of the species. He emphasized that the upper complete skull of Dacrytherium that he found from the Lamandine-Haute deposits in 1876 had a lower jaw and some leg bones that potentially belonged to it. Filhol determined that its dental formula was . The dentition, he said, was more similar to Anoplotherium and Diplobune than Xiphodon.
Filhol described another small species of "pachyderm" from the phosphorites of Lamandine-Haute in 1884 based on a fragment of a skull with premolars and molars. He stated that it was very similar to Anoplotherium, that the first and second premolars were nearly identical to those of Dacrytherium. Based on very specific molar differences, however, he proposed the genus and species name Plesidacrytherium elegans. Like Dacrytherium, the palaeontologist said, P. elegans presented a depression at the area of the infraorbital foramen up the upper maxilla. Filhol already created the name "Plesydacrytherium elegans" as early as 1880, but it lacked any actual definition to make it valid. The genus name derived in Ancient Greek from "plēsíon" (near), "dacry(o)" (tear), and "thḗr" (beast or wild animal) meaning "near tear beast".
British naturalist Richard Lydekker reviewed known species of anoplotheriids in 1885. Previously in 1857, Richard Owen described a purported anoplotheroid the size of Xiphodon gracilis from the Isle of Wight. Owen then determined that because of its dentition corresponding more to Dichobune than to Xiphodon and Dichodon, the specimens belonged to the newly erected Dichobune ovina. Lydekker determined that Dichobune ovinus actually belonged to Dacrytherium. He then said that Dacrytherium ovinum was a larger form of D. cayluxi. In addition, he considered that "Xiphodon platyceps" may be the same as D. ovinum.
In 1891–1893, German palaeontologist Karl Alfred von Zittel synonymized Mixtotherium and Plesidacrytherium with Diplobune, and Adrotherium (species A. depressum) with Dacrytherium. He supported the validity of D. cayluxense and D. ovinum as well as the synonymy of "X. platyceps" with the latter species. However, in 1892, Lydekker made reviews of the species of Dacrytherium. He contextualized that his 1885 reassignment of Dichobune ovina to Dacrytherium was based on the dental series of a cranium. At that time, he thought that D. cayluxense was one of two distinct species of Dacrytherium because he thought that the former differed from the latter based on dental differences including the canines. However, before Lydekker wrote his 1892 article, Arthur Smith Woodward presented him with more fossil evidence from the French phosphorites including a well-preserved mandible of D. cayluxi, leading him to synonymize it with D. ovinum. In addition, Lydekker considered his previous synonymy of X. platyceps with Dacrytherium "unfounded" since the former clearly lacked any lacrimal fossa that the latter has.
In 1908, Hans Georg Stehlin synonymized Adrotherium with Mixtotherium instead of Dacrytherium. In 1910, Stehlin reaffirmed the validity of Mixtotherium but synonymized Plesidacrytherium with Dacrytherium, reclassifying its species to D. elegans. He synonymized "X. platyceps" with Dichodon cuspidatum because of their close dentitions combined with the former lacking the preorbital fossa that Dacrytherium has. In his revisions of the Palaeogene artiodactyls, he erected two more species of Dacrytherium: D. priscum and D. saturnini. The name D. priscum refers to a larger species of the genus. According to Stehlin, Ludwig Rütimeyer knew of specimens of Dacrytherium in 1891 but doubted that it actually belonged to the genus based on Lydekker's specimen illustrations, a conclusion that Stehlin considered a mistake. The second species D. saturnini was also based on specific cranial plus dental differences and was named after the locality of Saint-Saturnin-lès-Apt in the department of Vaucluse, France where the fossils came from.
Classification
Dacrytherium is the type genus of the subfamily Dacrytheriinae, which belongs to the Palaeogene artiodactyl family Anoplotheriidae. The family was endemic to western Europe and lived from the middle Eocene to the early Oligocene (~44 to 30 Ma, possible earliest record at ~48 Ma). The exact evolutionary origins and dispersals of the anoplotheriids are uncertain, but they exclusively resided within the continent when it was an archipelago that was isolated by seaway barriers from other regions such as Balkanatolia and the rest of eastern Eurasia. The Anoplotheriidae's relations with other members of the Artiodactyla are not well-resolved, with some determining it to be either a tylopod (which includes camelids and merycoidodonts of the Palaeogene) or a close relative to the infraorder and some others believing that it may have been closer to the Ruminantia (which includes tragulids and other close Palaeogene relatives).
The history of dacrytheriines has been contentious as a result of disagreements as to whether they constitute a subfamily of the Anoplotheriidae or a distinct family named "Dacrytheriidae". The family name was first proposed by Charles Depéret in 1917; some palaeontologists like Jean Sudre in 1978 opted to follow the family rank while some others like Jerry J. Hooker in 1986 considered the clade to be an anoplotheriid subfamily. A 2007 source supports dacrytheriines as a subfamily based on recent phylogenetic analyses determining that Dacrytherium falls within the Anoplotheriidae. It is one of two subfamilies of the Anoplotheriidae, the other being the Anoplotheriinae.
The Dacrytheriinae is the older anoplotheriid subfamily, but the actual first appearance by Mammal Palaeogene zone range is uncertain. The first undisputed appearance of anoplotheriids is by MP13, but their range may have extended, in the case of Catodontherium, into MP11 or even MP10. Dacrytherium itself made its first undisputed appearance by MP13 as an artiodactyl leaning towards bunoselenodont dentition. The younger subfamily Anoplotheriinae made their first appearances by the late Eocene (MP15-MP16), or ~41-40 Ma, within western Europe with Duerotherium and Robiatherium. After a significant gap of anoplotheriines in MP17a-MP17b, the derived anoplotheriids Anoplotherium and Diplobune made their first appearances in western Europe by MP18, although their exact origins are unknown. The Dacrytheriinae has recently been suggested to have been a paraphyletic subfamily based on dental morphology from which the Anoplotheriinae stemmed, but further research is required to confirm if this is true.
Conducting studies focused on the phylogenetic relations within the Anoplotheriidae has proven difficult due to the general scarcity of fossil specimens of most genera. The phylogenetic relations of the Anoplotheriidae as well as the Xiphodontidae, Mixtotheriidae, and Cainotheriidae have also been elusive due to the selenodont morphologies 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" 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. The phylogenetic tree published in the article and another work about the cainotherioids is outlined below: