Extinct genus of endemic Palaeogene European artiodactyls
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 3.1.73.1.7, 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 3.1.4.33.1.4.3. 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, 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 1855 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, Mixtotheriidae, and Cainotherioidea 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:
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 created a phylogenetic analysis in his academic thesis regarding Palaeogene artiodactyl lineages, focusing most specifically on the endemic European families. The phylogenetic tree, according to Weppe, is the first to conduct phylogenetic affinities of all anoplotheriid genera, although not all individual species were included. 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. He also proposed that "Leptotheridium", previously relocated from the "Dacrytheriidae" to the Xiphodontidae, composes part of a paraphyletic anoplotheriid clade with the dacrytheriines "Catodontherium" and "Dacrytherium".
Description.
Skull.
"Dacrytherium" has fairly complete skull material since 1876 and is best known for its large-sized lacrimal fossa in front of its eye, or "tear pit", hence the derivation of the genus name. Such a depression as seen in "Dacrytherium" occurs in extant and extinct ruminants. "Dacrytherium" is also known by the presence of a preorbital fossa, a feature where the preorbital gland is located in living animals; it is also present in some extant artiodactyls including bovids and deer. The preorbital fossae, which occupy the