Duerotherium is an extinct genus of artiodactyl that lived during the Middle Eocene and is only known from the Iberian Peninsula. The genus is a member of the family Anoplotheriidae and the subfamily Anoplotheriinae, and contains one species, D. sudrei. Like other anoplotheriids, it was endemic to Western Europe. The genus was described based on a left fragment of a maxilla (upper jaw) from the Mazaterón Formation of the Duero Basin, from which its name derives, in 2009. Its dentition is mostly typical of the Anoplotheriinae but differs from related genera in the elongated and triangular third upper premolar and traits of the molars. It is thought to have been part of an endemic fauna that evolved in the Iberian Peninsula during the Middle Eocene, when climates were subtropical.
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 Greek, Ancient (to 1453);: θήρ/ 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.[1]
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 origin of anoplotheriids remain uncertain, but they were exclusively distributed in Europe 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 artiodactyl groups are not well-resolved. Some authors assigned them to tylopods (which include camelids and merycoidodonts) or considered them to be close relatives of that group, while some others believed that they were closer to the Ruminantia (which include, amongst others, tragulids).[2] [3]
The Anoplotheriidae consists of two subfamilies, the Dacrytheriinae and Anoplotheriinae, and Duerotherium belongs to the latter. The Dacrytheriinae is the older subfamily and first appeared in the middle Eocene (in the Mammal Palaeogene zones unit MP13, possibly up to MP10), although some authors consider them to be a separate family, as Dacrytheriidae.[1] [4] [5] Anoplotheriines made their first appearances by the late Eocene (MP15-MP16; ~41–40 Ma) with Duerotherium and Robiatherium. After a significant gap of anoplotheriines in MP17a-MP17b, the derived anoplotheriids Anoplotherium and Diplobune made their first appearances by MP18, although their exact origins remain unknown.[1]
The phylogenetic relations within the Anoplotheriidae remain uncertain due to the general scarcity of fossil specimens of most genera.[1] The phylogenetic relations of the Anoplotheriidae itself, as well as the Xiphodontidae, Mixtotheriidae, and Cainotheriidae, have also been elusive due to the selenodont morphologies of the molars, which evolved independently in tylopods or ruminants.[6] Some researchers considered the selenodont families Anoplotheriidae, Xiphodontidae, and Cainotheriidae to be within Tylopoda due to postcranial features that were similar to North American tylopods.[7] Other researchers instead found them to be close relatives of ruminants based on dental morphology. Different phylogenetic analyses have produced different results for the "derived" selenodont Eocene artiodactyl families from Europe, making it uncertain whether they were closer to the Tylopoda or Ruminantia.[8] [9]
In an 2019 article, 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 suggest that Cainotherioidea 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 of the Ruminantia while Tylopoda, along with the Amphimerycidae and Xiphodontidae, split earlier in the tree.[9] The phylogenetic tree published in the article and another work about the cainotherioids is outlined below:[10]
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.[6] [11]
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.[12] [13] 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.[3]
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.[1]
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.[1]
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.[1]
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.[14] [15]
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).[2] 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.[15] The European mammals of the late Eocene (MP17 - MP20) were mostly descendants of endemic middle Eocene groups as a result.[16]
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.[1] 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).[17]