Duerotherium

Duerotherium is an extinct genus of Palaeogene artiodactyls known only from the Iberian Peninsula during the Middle Eocene, which contains one species D. sudrei. It, like other members of the Anoplotheriidae, was endemic to western Europe. The anoplotheriine was described from a left fragment of a maxilla from the Mazaterón Formation of the Duero Basin (where its name derives from) in 2009. Its dentition is mostly typical of the Anoplotheriinae but differs by an elongated plus triangular 3rd upper premolar and very specific traits of the molars. It is thought to have been part of an endemic faunal assemblage that evolved within the Iberian Peninsula by the Middle Eocene, where climates were subtropical.

Taxonomy
In 2009, Spanish palaeontologists Miguel-Ángel Cuesta and Ainara Badiola described a newly erected anoplotheriine genus from the Mazaterón Formation near the village of Mazaterón, which are located within the Duero Basin. The genus and type species Duerotherium sudrei was created based on a fragment of a left fragment of a maxilla with a dental series of P3-M3 (specimen STUS 11562), which was deposited in the "Sala de las Tortugas" of the University of Salamanca. The genus etymology derives from the Duero Basin for where the fossil was described plus the Greek θήρ/therium meaning "beast" or "wild animal." The etymology of the species name was dedicated in honour of Jean Sudre for his studies on endemic European Palaeogene artiodactyls.

Classification
Duerotherium belongs to the Anoplotheriidae, a Palaeogene artiodactyl family endemic to western Europe that 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 Anoplotheriidae consists of two subfamilies, the Dacrytheriinae and Anoplotheriinae, the latter of which is the subfamily that Duerotherium belongs to. The Dacrytheriinae is the older subfamily of the two that first appeared in the middle Eocene (since the Mammal Palaeogene zones unit MP13, possibly up to MP10), although some authors consider them to be a separate family in the form of the Dacrytheriidae. Anoplotheriines 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.

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 used for the journal and another published 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. 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 said that Ephelcomenus, Duerotherium, and Robiatherium compose a clade of the Anoplotheriidae.

Description
The dental formula of the Anoplotheriidae is for a total of 44 teeth, consistent with the primitive dental formula for early-middle Palaeogene placental mammals. Anoplotheriids have selenodont or bunoselenodont premolars and molars made for folivorous/browsing diets, consistent with environment trends in the late Eocene of Europe. The canines of the Anoplotheriidae are premolariform in shape, meaning that the canines are overall undifferentiated from other teeth like incisors. The lower premolars of the family are piercing and elongated. The upper molars are bunoselenodont in form while the lower molars have selenodont labial cuspids and bunodont lingual cuspids. The subfamily Anoplotheriinae differs from the Dacrytheriinae by the lower molars lacking a third cusp between the metaconid and entoconid as well as molariform premolars with crescent-shaped paraconules.

Duerotherium is diagnosed based on its small size and, most exclusively, its dental traits based on the maxilla fragment. The P3 tooth is in a mesiodistal position, is elongated plus triangular in shape, and has a distolingually positioned protocone cusp plus a noticeable posterolingual talon. The morphology of the tooth of Duerotherium is similar to the P3 tooth of Dacrytherium based on the positions of the cusps, although the latter differs by it being mesiodistally elongated compared to the former. The morphology of P4 is typical of the Anoplotheriinae and has only has externally-positioned cusp.

The upper molars of Duerotherium also have similar morphologies to those of other anoplotheriines. They are bunoselenodont and have large and conical protocone cusps in the near-front of the paracone in the front areas of the teeth. The metaconule is slightly asymmetric, and the postmetaconule ridge is moderate in form. The parastyle and metastyle cusps are divergent, revealing a moderate W-shaped ectoloph ridge. The molars are heterodont in dentition form and increase in size sequencing from M1 to M3. In a top (or occlusal) outline view, the M1 is quadrate in shape while M2-M3 appears more trapezoidal. It differs from each anoplotheriine genus based on various specific morphologies of the molars.

Duerotherium is described as being slightly larger than Robiatherium but having a smaller size than Ephelcomenus. D. sudrei is especially smaller than Anoplotherium and most species of Diplobune. It might be similar in size to Diplobune minor.

Palaeoecology
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 Ma) along with the archaic "condylarths". By the late Eocene (approx. 37 - 33 Ma), most of the ungulate form dentitions shifted from bunodont cusps to cutting ridges (i.e. lophs) for folivorous diets.

Land-based connections to the north of the developing Atlantic Ocean were interrupted around 53 Ma, meaning that North America and Greenland were no longer well-connected to western Europe. From the early Eocene up until the Grande Coupure extinction event (56 Ma - 33.9 Ma), the western Eurasian continent was separated into three landmasses, the former two of which were isolated by seaways: western Europe (an archipelago), Balkanatolia, and eastern Eurasia (Balkanatolia was in between the Paratethys Sea of the north and the Neotethys Ocean of the south). The Holarctic mammalian faunas of western Europe were therefore mostly isolated from other continents including Greenland, Africa, and eastern Eurasia, allowing for endemism to occur within western Europe. The European mammals of the late Eocene (MP17 - MP20) were mostly descendants of endemic middle Eocene groups as a result.

The Mazaterón Formation of the Duero Basin dates to the middle Late Eocene (Robiacian of Europe) and ranges in faunal level from MP15-MP16. Fossils of testudines, crocodilians, rodents, primates, hyaenodonts, artiodactyls, and perissodactyls were collected from the site. The mammal taxa collected from the Eocene of the Iberian region differ from contemporary faunas in other areas of Europe, supporting the hypothesis of a division of the Iberian Peninsula as a semi-separate bioregion. The taxa known from the Mazaterón fossil site with Duerotherium include the testudines Hadrianus and Neochelys, alligatoroid Diplocynodon, baurusuchid Iberosuchus, adapoid Mazateronodon, omomyid Pseudoloris, pseudosciurid rodent Sciuroides, theridomyid rodents Pseudoltinomys and Remys, hyaenodont Proviverra, palaeotheres (Paranchilophus, Plagiolophus, Leptolophus, Palaeotherium, Cantabrotherium, Franzenium, and Iberolophus), dacrytheriines (cf. Dacrytherium), and xiphodonts (cf. ''Dichodon).