Choeropotamus

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Choeropotamus
Temporal range: Middle Eocene – Early Oligocene 40–33.4  Ma
Choeropotamus parisiensis.jpg
Choeropotamus parisiensis cranium, National Museum of Natural History, France
Scientific classification OOjs UI icon edit-ltr.svg
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Family:† Choeropotamidae
Genus:† Choeropotamus
Cuvier, 1822
Type species
†Choeropotamus parisiensis
Cuvier, 1822
Other species
  • †C. affinis Gervais, 1852
  • †C. lautricensis Noulet, 1870
  • †C. depereti Stehlin, 1908
  • †C. sudreiCasanovas-Cladellas, 1975
Synonyms
Genus synonymy
  • ChaeropotameCuvier, 1822
  • CheropotameCuvier, 1822
  • CheropotamesCuvier, 1822
  • Chaeropotamus Desmarest, 1822
  • Cheiropotamus Ritgen, 1824
  • Cheropotomus Swainson, 1835
  • Cheropotamus von Meyer, 1838
  • Chöropotamus Roger, 1896
Synonyms of C. parisiensis
  • Chaeropotamus GypsorumDesmarest, 1822
  • Choeropotamus Cuvieri Owen, 1841
Synonyms of C. affinis
  • Choeropotamus Matritensisvon Meyer, 1840

Choeropotamus is an extinct genus of Paleogene artiodactyls and the type genus of the family Choeropotamidae. It was endemic to western Europe and lived from the Middle Eocene up to the earliest Oligocene. Choeropotamus was first described and named in 1822 by the French naturalist George Cuvier, who noted its distinct morphology from other contemporary fossil artiodactyls and reused its name from the 17th century Venetian physician Prospero Alpini; Alpini previously used the name towards stuffed hippopotamuses because he incorrectly thought to be distinct species. Since, five species have been described total, with C. parisiensis being the type species.

Contents

Choeropotamus had an elongated but broad and low skull, its mandible being narrow and elongated and its cheek teeth generally having diastemata (or gaps) in between them. Although mostly known from dental fossils and mostly lacking in postcranial evidence, its skull anatomy and evolutionary history have both been well-recorded, with sufficient evidence that the five species form a progressive lineage that gains cusps in the molars and increases in size from the earliest and smallest C. lautricensis to the latest-appearing and largest C. parisiensis. C. parisiensis in particular may have weighed about 58.8 mm (2.31 in), making it sizable compared to many of its contemporary artiodactyl relatives. The bunodont (round-cusped) molars of Choeropotamus point towards either a frugivorous diet or an omnivorous one, in which it could have occasionally consumed meat.

During the times that Choeropotamus existed, western Europe was an archipelago that was isolated from the rest of Eurasia, meaning that it lived in a tropical-subtropical environment with various other animals that also evolved with strong levels of endemism. It went extinct by the Grande Coupure extinction/faunal turnover event, coinciding with shifts towards further glaciation and seasonality plus dispersals of Asian immigrant faunas into western Europe. The specific causes of its extinction are unclear, however.

Taxonomy

Research history

Early history

Choeropotamus parisiensis dentition 1822.jpg
Choeropotamus skull remains 1822.jpg
1822 illustrations of cranial and dental remains of Choeropotamus parisiensis by Georges Cuvier

In 1822, French palaeontologist Georges Cuvier described cranial and dental fossil remains of what he called a distinct "pachyderm" genus. The first fossils that he had were all dental (consisting of molars and canines) and were delivered to him by itinerary author Vaysse de Villiers, who had found them in the gypsum quarries of the French commune of Villejuif. He noted that the canine was small like that of a peccary but would have been larger in overall size than it. He later studied partial dental and skull remains from Montmartre, which include the palatine bone, zygomatic arches, and a lower edge of an orbit on one of the sides. He noted that the molars were roughly square-shaped, were slightly wider than long, and had four main cusps that were blunt along with two smaller ones in between. Cuvier additionally pointed out that the zygomatic arches were different in form from both suids and peccaries. Theorizing that the extinct animal was related to pigs, he had "provisionally" assigned the bones to the new genus Chæropotame, alternatively spelled by Cuvier as "Cheropotame" and "Cheropotames." [1]

A 17th century sketch of a hippopotamus based on stuffed specimens from Egypt. The name "choeropotamus" was used informally on stuffed hippopotamuses by Prospero Alpini and was later reused as a genus name by Cuvier in 1822. Hippopotamus Choeropotamus sketch.png
A 17th century sketch of a hippopotamus based on stuffed specimens from Egypt. The name "choeropotamus" was used informally on stuffed hippopotamuses by Prospero Alpini and was later reused as a genus name by Cuvier in 1822.

The same year, French zoologist Anselme Gaëtan Desmarest acknowledged the genus as valid but emended the name to Chæropotamus and erected the species name Chæropotamus Gypsorum. [2] In 1824, Cuvier emended the genus name to Chœropotamus and listed the new species name Chœropotamus parisiensis. [3] The genus name Choeropotamus means "river pig," which is a compound of the Greek prefix χοίρος ('choíros') meaning "pig" and the suffix ποτάμι ('potámi') meaning "river." [4] Prior to his publication on the genus Choeropotamus, Cuvier had noted that the name Chœropotame had previously been used in the written scientific account Historiae AEgypti naturalis by Venetian physician Prospero Alpini (his work being published in 1735, over a century after his death). More specifically, Alpini applied the names "Cheropotamo," "Chœropotame", and "Cheropotamus" to two stuffed hippopotamuses from the Nile River in Egypt that he mistakenly thought were distinct animals because of their dentition. In addition, Alpini said that the name was used by ancient Greeks, but Cuvier was unable to confirm such. [5] [6] Cuvier in 1822 directly confirmed in a footnote that he borrowed the genus name from Alpini's work. [1]

In 1829, German palaeontologist Christian Erich Hermann von Meyer said that German physician Samuel Thomas von Sömmerring gave him fossil remains from the German municipality of Georgensgmünd, among them molars and canines were similar to those of the babirusa. He designated the species name Chaeropotamus Sömmeringii in honor of von Sömmerring. The palaeontologist then erected the second species C. Meissneri based on dental fossils from the Swiss locality of Rappehfluh (near Aarberg). [7] Later in 1834, von Meyer described a fragmentary lower jaw that he found back in 1805 at Rappehfluh, noting again that the dentition was similar to those of the babirusa in size, shape, and wear patterns. He cited a monography of Swiss geologist Bernhard Studer regarding why he placed C. Meissneri in Choeropotamus. He then wrote about teeth of C. Sömmeringii from Georgensgmünd, noting that the species placement could be problematic. He explained that unlike C. Sömmeringii, C. Parisiensis seemingly lacked a lower molar in its lower jaw and did not closely match up in dental arrangement. He also erected the suid genus Hyotherium and reassigned C. Sömmeringii to it as H. Sömmeringii. [8] In 1840, He referenced another erected species C. Matritensis based on molars from the Spanish locality of Cerro de San Isidro without further specifying about them. [9] Von Meyer later reassigned C. Meissneri to Hyotherium in 1841. [10]

Illustrations of the fossil jaws of C. parisiensis from Montmartre and the Isle of Wight Choeropotamus parisienis skull mandible illustrations.jpg
Illustrations of the fossil jaws of C. parisiensis from Montmartre and the Isle of Wight

In 1841, British palaeontologist Richard Owen described a right ramus of a mandible of Choeropotamus from the fossil collection belonging to fellow naturalist William Darwin Fox that was uncovered from the Isle of Wight. He renamed the species C. Cuvieri, having sought to synonymize the names C. Parisiensis and C. Gypsorum. He suggested that some of the traits of Choeropotamus were similar to suines but that some others were more convergent with carnivorans. [11] In an 1839–1864 osteography, the French naturalist Henri Marie Ducrotay de Blainville prioritized the name C. parisiensis for the first recognized species. [12]

In an 1848–1852 source, French palaeontologist Paul Gervais erected C. affinis based on a partial lower jaw with three molars that was found in the French locality of La Débruge. [13] French naturalist Jean-Baptiste Noulet erected C. lautricensis in 1870, having named the species after the French commune of Lautrec. He specified that the lower jaw specimens were found from a rich deposit accidentally discovered from a railway project from Castres to Albi and that C. lautricensis was half the size of C. parisiensis. In addition, he did a brief review of some of the previously named species, considering C. parisiensis and C. affinis to be valid and accepting that the two species, C. Sömmeringii and C. Meissneri were reassigned to Hyotherium. [14] The same year, German palaeontologist Oscar Fraas recognized another species named C. steinheimensis, its fossils having been found in the German municipality of Steinheim am Albuch. [15] While British palaeontologist Richard Lydekker in 1885 listed C. cuvieri, C. parisiensis, and C. affinis as synonyms of C. gypsorum and suggested that C. steinheimensis belongs to Hyotherium, [16] Karl Alfred von Zittel in a 1991-1993 handbook suggested that both C. parisiensis and C. affinis were valid species. [17]

Later history

Illustrations of the upper and lower dentitions of C. parisiensis with labels Choeropotamus parisiensis jaw.jpg
Illustrations of the upper and lower dentitions of C. parisiensis with labels

In 1908, Swiss palaeontologist Hans Georg Stehlin confirmed the validities of C. parisiensis, C. affinis, and C. lautricensis in his review of Choeropotamus. He also erected C. Depéreti, which he said was based on a mandibular fragment from a Basel collection that was originally uncovered from the French commune of Saint-Hippolyte-de-Caton. He additionally provisionally recognized C. parisiensis var. minor, named for a small-sized mandible from a gypsum in the French commune of Argenteuil. Stehlin also stated that it was intermediate in size between the small-sized C. lautricensis and the larger C. affinis and C. parisiensis. [18] French palaeontologist Charles Depéret in 1917 listed a species variation named C. lautricensis mut. major without further elaboration. [19] In 1930, American zoologist Glover M. Allen designated C. matritensis as both a nomen nudum (inadequately defined name) and a synonym of C. affinis. [20]

In 1975, Spanish palaeontologist María Lourdes Casanovas Cladellas explained in her thesis that C. lautricensis mut. major was applied to an upper jaw and a left hemimandible, both of which were found in the French locality of Robiac. In both her thesis and research paper, she recognized the new species C. sudrei based on multiple mandible fragments from the Spanish locality of Roc de Santa 1; the species was named after French palaeontologist Jean Sudre. [21] [22] "C." steinheimensis was eventually classified to another suid genus Parachleuastochoerus , first recognized in the 20th century. [23]

Classification

Reconstructed skeleton of Amphirhagatherium, an endemic European artiodactyl genus classified within the Choeropotamidae. The polyphyletic nature of the Choeropotamidae makes the placements of its genera uncertain and inadequately defined. Amphirhagatherium Geiseltal-Skeleton.jpg
Reconstructed skeleton of Amphirhagatherium , an endemic European artiodactyl genus classified within the Choeropotamidae. The polyphyletic nature of the Choeropotamidae makes the placements of its genera uncertain and inadequately defined.

Choeropotamus is the type genus of the Choeropotamidae, a Palaeogene artiodactyl family endemic to western Europe that lived from the Middle Eocene to the Early Oligocene. Choeropotamidae had a particularly complicated taxonomic history, as many genera now classified in the family, other than Choeropotamus, were not originally recognized as such in the 20th century. In particular, many genera like Haplobunodon , Lophiobunodon , and Rhagatherium were classified in a separate family called the "Haplobunodontidae." As a result, Choeropotamidae had, by some sources, been considered a monotypic family, meaning that Choeropotamus was the only genus classified in it. However, the Haplobunodontidae had been synonymized by later taxonomists, its genera being reclassified into the Choeropotamidae. However, some authors have suggested that the Haplobunodontidae should be revived due to questions relating to the polyphyly of the Choeropotamidae. [24] [25] [26]

The Choeropotamidae as currently defined is polyphyletic. This is because cladistic analyses of "choeropotamids" based on dental anatomy reveal that not all genera share close synapomorphies (or traits of the latest common ancestor) to prove close relations between them. However, disagreements remain on whether or not Haplobunodon lies within the Choeropotamidae sensu stricto (in a strict sense) to justify a separate Haplobunodontidae family. [27] [28] [29] Because the definition of the Choeropotamidae is polyphyletic and not agreed upon, the Choeropotamidae requires further analyses. [25]

Regardless of taxonomic problems, the Choeropotamidae has been well-defined as one of the endemic European Palaeogene artiodactyl groups that lived primarily during the Eocene. The Choeropotamidae itself from the middle Eocene up to the earliest Oligocene. Masillabune , arguably the oldest choeropotamid, made its first appearance during the unit MP11 of the Mammal Palaeogene zones. It was followed by the likes of Haplobunodon and Amphirhagatherium in MP12 and Hallebune in MP13. Two of the latest-appearing choeropotamids, Choeropotamus and Thaumastognathus , made their first appearances in MP16. Both Choeropotamus and Amphirhagatherium, the latest-surviving choeropotamids, extended up to MP20, coinciding with the Grande Coupure extinction event in western Europe. [25] They are generally regarded as close relatives of the Cebochoeridae, another endemic European artiodactyl family. [27] [28] One phylogenetic tree example of the Choeropotamidae and Haplobunodontidae being defined as separate families according to Vincent Luccisano et al. in 2020 is shown below: [27]

Artiodactyla

In 2022, Weppe conducted a phylogenetic analysis in his academic thesis regarding Palaeogene artiodactyl lineages, focusing most specifically on the endemic European families. One monophyletic set consisted of the Dichobuninae, Cebochoeridae, and two clades of the polyphyletic Choeropotamidae. He suggested that Cuisitherium and Lophiobunodon were not very closely related to the true clade of choeropotamids, consisting of Thaumastognathus, Choeropotamus, Amphirhagatherium, Rhagatherium, and the polyphyletic Haplobunodon; his analysis disagrees with other sources suggesting that some genera like Haplobunodon were not closely related to Choeropotamus and states that the Haplobunodontidae is therefore not valid. More specifically, he defined Thaumastognathus, Choeropotamus, Haplobunodon lydekkeri, and "H." solodurense as forming one specific clade of the Choeropotamidae sensu stricto. [28]

Description

Skull

Choeropotamus affinis.jpg
Choeropotamus depereti.jpg
C. affinis mandible (left) and C. depereti dentition (right), Natural History Museum of Basel

The skull of Choeropotamus is elongated but low and broad, its snout and zygomatic arches being narrow with sideway projections and its crest on the parietal bone being very high. The neurocranium is elongated, and the mastoid part of the temporal bone is amastoid, meaning that it does not have a surface exposure. The latter trait is evolutionarily derived to the mastoid trait of Haplobunodon in which the mastoid part has a surface exposure. The postglenoid process, found on the temporal bone, is separated from a different posttympanic process, the latter of which the paroccipital process projects far behind. The projection of the paroccipital process contrasts strongly to those of the Anthracotheriidae, in which the process is attached to the posttympanic process. [30] [24] [26] [25]

The mandible is both narrow and elongated, the angle of the mandible being prominent. The ascending ramus of the mandible is low but relatively vertical and very wide. The mandibular symphysis is lengthy, extending all the way back to the area of P2 or P3. [24] [25] The anatomy of the raoellid Khirtharia inflata has been recorded as similar to Choeropotamus in part due to the similar mandible shapes, such as the well-developed angular region and non-straight lower edge. [24]

Dentition

Image of the right maxilla (fig. 1-2) and left mandible (fig. 3-7) of C. depereti Choeropotamus depereti cranial remains.jpg
Image of the right maxilla (fig. 1–2) and left mandible (fig. 3–7) of C. depereti

Choeropotamus was originally described as having an incomplete lower dental set in comparison to its upper counterpart, the P1 (first lower premolar (P/p)) likely being the tooth that was evolutionarily lost. [24] Thus, its defined dental formula was 3.1.4.33.1.3.3 for a total of 42 teeth. [17] However, Jerry J. Hooker and Marc Weidmann pointed out that the enlarged tooth that was separated from P2 was misinterpreted by some authors like Jean Viret and Sudre as a canine (C/c). Instead, as Stehlin in 1908 suggest, the enlarged tooth was actually P1, evident by a C. affinis fossil from La Débruge that has four equal-sized dental alveoli for the canine and three incisors (I/i) that were in front of the P1 tooth. Therefore, it has the complete dental formula of 3.1.4.33.1.4.3 for a total of 44 teeth, [31] consistent with the primitive dental formula for early-middle Palaeogene placental mammals. [32] The dental anatomy of primitive Choeropotamus species is best represented by C. depereti, but the derived C. affinis and C. parisiensis remain the best-known species in terms of such. [26]

Its teeth have thick and wrinkled enamel, the premolars and molars (M/m) having strong cingula. [25] The incisors (I/i) are weak in form and are shaped upright, [17] the first incisors being caniniform (canine-shaped). The upper canine (C/c) is vertical, compressed, and is sharp at its rear, forming a dagger-like shape. The lower canine in comparison is incisiform (incisor-shaped). P1 is separated from P2 by a diastema. P2 is slightly more elongated than P3, which is triangular, has a simple form consisting of a single tubercle (or cusp), and lacks a protocone cusp. P4 has a metacone cusp on the short postprotocrista ridge. P1 is caniniform and is separated from P2 by a long diastema. P2 and P3 are both one-cusped and compressed in shape, whereas P4 can be one-cusped or two-cusped. The upper molars, shaped similarly to those of anthracotheres in their lowness and bunodont (low and rounded) cusps, increase in size from the first to third upper molars. More derived species have an accessory cusp that appears in the tooth's centre. The lower molars are elongated and have bunodont internal cusps along with external cusps that have V-shaped crests. M3 has two main cusps on the hypoconulid lobe. [24] [31] [24] Choeropotamus has been compared to the cebochoerid Cebochoerus due to their shared caniniform first incisors, dagger-shapped upper canines, and incisiform lower canines. All four Choeropotamus species are part of a singular lineage of anagenesis, in part proven by the addition of cuspules (or small cusps) in later species. [31]

Both Choeropotamus and its relative Thaumastognathus have long diastemata and caniniform P1 teeth. [25] The dentition of Choeropotamus had been compared to other artiodactyls such as "Haplobunodon" solodurense and the Anthracotheriidae due to similarities in molar morphology, seemingly the result of parallel evolution. Both Khirtharia inflata and Choeropotamus have been compared in dental anatomy due to the presence of diastemata in between their premolars, but the former has more bunodont dentition than C. lautricensis and C. depereti and has similarly bunodonty to that of C. parisiensis. [24] However, the evolutionary relation of Choeropotamus to Palaeogene artiodactyls endemic to Asia remain uncertain, the similarity of the former to other artiodactyls like Khirtharia probably being a case of parallel evolution. [25]

Postcranial skeleton

In terms of postcranial remains, Choeropotamus is only known from leg bone remains like the astragalus. [31] Such remains were first by Depéret in 1917, who attributed an astragalus and calcaneum to C. depereti. [19] However, in 1947, Jean Viret and J. Prudant stated that the astragalus originally attributed to Choeropotamus actually belonging to Dacrytherium while the astragalus assigned initially to the latter actually belonging to the former. [33] The calcaneum attributed to Choeropotamus has been described as being similar to those of the pig Sus and the anthracotheres Anthracotherium and Brachyodus , differing by the more compressed transverse surface and the articular surface for the fibula being narrower and more elongated; this suggests that at least the lower end of the fibula was proportionally large. [19] The astragalus has a trochleated (pulley-like) and slightly bent head along with a distally (situated away from the center) extensive sustentacular facet. [31] Depéret described it as being narrow plus slender in shape similar to those of the anthracothere Elomeryx and the wild boar (Sus scrofa). [19]

Size

The species of Choeropotamus are described as medium to large in size, the genus being one of the larger representatives of its family. [24] [25] Choeropotamus could have been about the size of a domestic pig. [17] The linear evolutionary stage, beginning from C. lautricensis and ending up to C. parisiensis, shows a progressive increase in size. For C. lautricensis, the length of M1-M3 is 30 mm (1.2 in) while that of M1-M3 ranges from 28 mm (1.1 in) to 33 mm (1.3 in). The second earliest species C. sudrei has M1-M3 and M1-M3 lengths that measure 31 mm (1.2 in) and 33.5 mm (1.32 in), respectively. The M1-M3 of C. depereti ranges in length from 33 mm (1.3 in) to 36 mm (1.4 in) while its M1-M3 ranges from 38 mm (1.5 in) to 45 mm (1.8 in). The M1-M3 length of the second-latest species C. affinis ranged from 42 mm (1.7 in) to 46 mm (1.8 in), and its M1-M3 length is 54 mm (2.1 in). As for the latest species C. parisiensis, its M1-M3 and M1-M3 dental rows measured 52 mm (2.0 in) and 62 mm (2.4 in) long, respectively. [24] An M2 of C. parisiensis individually measures 14 mm (0.55 in) long. [25]

The estimated body mass of C. parisiensis has been calculated by Helder Gomes Rodrigues et al. in 2019 based on an astragalus from the National Museum of Natural History, France that measured 40 mm (1.6 in) long and 19 mm (0.75 in) wide, yielding a weight of 58.8 kg (130 lb). The body mass formula based on astragali was previously established by Jean-Noël Martinez and Sudre in 1995 for Palaeogene artiodactyls, although Choeropotamus was not included in the initial study. [34] [35]

Palaeobiology

Choeropotamids are among the early artiodactyls that have brachyodont (low-crowned) and bunodont cheek teeth, suggesting that they had either frugivorous or omnivorous diets. [36] Choeropotamus specifically is consider a large-sized bunodont artiodactyl of western Europe that had elongated skulls, long diastemata, caniniform P1 teeth, and molars with additional cusps. Given that the molars were used to grind food, Choeropotamus could have possibly been an "unspecialized" omnivore. They also suggested the possibility that they probably had robust limbs. [25] The sharp anterior teeth on choeropotamids like Choeropotamus infers occasional carnivorous dietary habits and/or sparring in intraspecific competition. The possibility of Choeropotamus feeding on hard fruits is supported by the thick enamel on its teeth like in similar modern mammals. The feeding behaviours would have differed from other purported choeropotamids like Amphirhagatherium, which would have been a ground feeder of fruits and leaves, and Masillabune, which based on gut contents consumed plants of the family Lauraceae and other unidentified plants. [26] Weppe in his thesis brought up a hypothesis in which the cranial and dental morphologies of Choeropotamus to members of the Hippopotamoidea (i.e. their diastemata and incisors) could possibly point to more semi-aquatic behaviours of the former just like with behaviours of some members of the latter. [28]

Palaeoecology

Middle Eocene

Palaeogeography of Europe and Asia during the Middle Eocene with possible artiodactyl and perissodactyl dispersal routes. Middle Eocene Paleogeography Tethys Dispersals.jpg
Palaeogeography of Europe and Asia during the Middle Eocene with possible artiodactyl and perissodactyl dispersal routes.

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 mya), most of the ungulate form dentitions shifted from bunodont (or rounded) cusps to cutting ridges (i.e. lophs) for folivorous diets. [37] [38]

Land connections between western Europe and North America were interrupted around 53 Ma. From the Early Eocene up until the Grande Coupure extinction event (56–33.9 mya), western Eurasia was separated into three landmasses: western Europe (an archipelago), Balkanatolia (in-between the Paratethys Sea of the north and the Neotethys Ocean of the south), and eastern Eurasia. [39] The Holarctic mammalian faunas of western Europe were therefore mostly isolated from other landmasses including Greenland, Africa, and eastern Eurasia, allowing for endemism to develop. [38] Therefore, the European mammals of the Late Eocene (MP17–MP20 of the Mammal Palaeogene zones) were mostly descendants of endemic Middle Eocene groups. [40]

The five known species of Choeropotamus form a single successive evolutionary line beginning with C. lautricensis, which is exclusive to MP16 based on its appearances in the contemporary localities of Le Castrais and Robiac. [25] By then, it would have coexisted with perissodactyls (Palaeotheriidae, Lophiodontidae, and Hyrachyidae), non-endemic artiodactyls (Dichobunidae and Tapirulidae), endemic European artiodactyls (Choeropotamidae (possibly polyphyletic, however), Cebochoeridae, Mixtotheriidae, Anoplotheriidae, Amphimerycidae, and other members of Xiphodontidae), and primates (Adapidae, Omomyidae). [36] [41] [42] It also cooccurred with metatherians (Herpetotheriidae), rodents (Ischyromyidae, Theridomyoidea, Gliridae), eulipotyphlans, bats, apatotherians, carnivoraformes (Miacidae), and hyaenodonts (Hyainailourinae, Proviverrinae). [43]

Restoration of Acotherulum, a contemporary artiodactyl that would have coexisted with Choeropotamus in the Middle and Late Eocene Acotherulum quercyi.png
Restoration of Acotherulum , a contemporary artiodactyl that would have coexisted with Choeropotamus in the Middle and Late Eocene

Other fossil mammals found from the deposits of Robiac include the herpetotheriids Amphiperatherium and Peratherium , apatemyid Heterohyus , nyctithere Saturninia , omomyids ( Necrolemur , Pseudoloris , and Microchoerus ), adapid Adapis , ischyromyid Ailuravus, glirid Glamys , pseudosciurid Sciuroides , theridomyids Elfomys and Pseudoltinomys , hyaenodonts ( Paracynohyaenodon , Paroxyaena , and Cynohyaenodon ), carnivoraformes ( Simamphicyon , Quercygale, and Paramiacis ), cebochoerids Cebochoerus and Acotherulum , choeropotamid Haplobunodon, tapirulid Tapirulus , anoplotheriids Dacrytherium , Catodontherium , and Robiatherium , dichobunid Mouillacitherium , robiacinid Robiacina , xiphodonts ( Xiphodon , Dichodon , Haplomeryx ), amphimerycid Pseudamphimeryx , lophiodont Lophiodon , hyrachyid Chasmotherium , and palaeotheres ( Palaeotherium , Plagiolophus Leptolophus , Anchilophus , Metanchilophus , Lophiotherium , Pachynolophus , Eurohippus ). [44]

By MP16, a faunal turnover occurred, marking the disappearances of the lophiodonts and European hyrachyids as well as the extinctions of all European crocodylomorphs except for the alligatoroid Diplocynodon . [41] [45] [46] [47] The causes of the faunal turnover have been attributed to a shift from humid and highly tropical environments to drier and more temperate forests with open areas and more abrasive vegetation. The surviving herbivorous faunas shifted their dentitions and dietary strategies accordingly to adapt to abrasive and seasonal vegetation. [48] [49] The environments were still subhumid and full of subtropical evergreen forests, however. The Palaeotheriidae was the sole remaining European perissodactyl group, and frugivorous-folivorous or purely folivorous artiodactyls became the dominant group in western Europe. [50] [36]

Late Eocene

Restoration of Pterodon, which appeared in the Late Eocene and coexisted with later Choeropotamus species Pterodon Life Reconstruction.png
Restoration of Pterodon , which appeared in the Late Eocene and coexisted with later Choeropotamus species

Two species of Choeropotamus made their appearances in the Late Eocene: C. sudrei and C. depereti, the latter arguably descending from the former. Whereas C. sudrei is exclusively recorded from MP17a localities, C. depereti is recorded from both MP17a and MP17b (the two do not overlap in localities). [25] [28] [51] Choeropotamus in the Late Eocene largely coexisted with the same artiodactyl families as well as the Palaeotheriidae within western Europe, [43] although the Cainotheriidae and the derived anoplotheriids Anoplotherium and Diplobune all made their first fossil record appearances by MP18. [25] [52] In addition, several migrant mammal groups had reached western Europe by MP17a-MP18, namely the Anthracotheriidae, Hyaenodontinae, and Amphicyonidae. [43] In addition to snakes, frogs, and salamandrids, rich assemblage of lizards are known in western Europe as well from MP16-MP20, representing the Iguanidae, Lacertidae, Gekkonidae, Agamidae, Scincidae, Helodermatidae, and Varanoidea, most of which were able to thrive in the warm temperatures of western Europe. [53]

Other mammals found in the MP17a locality of Fons 4 with C. sudrei include the herpetotheriids Amphiperatherium and Peratherium, glirid Glamys, theridomyids ( Paradelomys , Theridomys , Estellomys , Elfomys , Remys ), pseudosciurid Treposciurus , omomyid Necrolemur , adapid Leptadapis , hyaenodont Hyaenodon , palaeotheres (Anchilophus, Pachynolophus, Plagiolophus, Palaeotherium), dichobunid Mouillacitherium, amphimerycid Pseudamphimeryx, cebochoerid Cebochoerus, anoplotheriid Dacrytherium, and xiphodonts (Xiphodon, Dichodon, Haplomeryx, Paraxiphodon ). [43]

The last two species of Choeropotamus to have made their appearances were C. affinis at MP18 and its descendent C. parisiensis at MP18, MP19, and MP20, the two species only overlapping at the MP18 locality of La Débruge. [25] At the MP20 locality of Saint-Capraise-d'Eymet, fossils of C. parisiensis have been found along with those of the theridomyid Patriotheridomys , hyainailourid Pterodon , palaeotheres Plagiolophus and Palaeotherium, anoplotheriids Anoplotherium and Diplobune , xiphodont Xiphodon, and the gelocid Paragelocus . [43]

Extinction

A panorama of the Headon Hill Formation in the Isle of Wight. The stratigraphy of it and the Bouldnor Formation led to better understandings of faunal chronologies from the Late Eocene up to the Grande Coupure. Headon2.jpg
A panorama of the Headon Hill Formation in the Isle of Wight. The stratigraphy of it and the Bouldnor Formation led to better understandings of faunal chronologies from the Late Eocene up to the Grande Coupure.

The Grande Coupure event during the latest Eocene to earliest Oligocene (MP20-MP21) is one of the largest and most abrupt faunal turnovers in the Cenozoic of Western Europe and coincident with climate forcing events of cooler and more seasonal climates. [54] The event led to the extinction of 60% of western European mammalian lineages, which were subsequently replaced by Asian immigrants. [55] [56] [57] The Grande Coupure is often dated directly to the Eocene-Oligocene boundary at 33.9 Ma, although some estimate that the event began slightly later, at 33.6–33.4 mya. [58] [59] The event occurred during or after the Eocene-Oligocene transition, an abrupt shift from a hot greenhouse world that characterised much of the Palaeogene to a coolhouse/icehouse world from the Early Oligocene onwards. The massive drop in temperatures results from the first major expansion of the Antarctic ice sheets that caused drastic pCO2 decreases and an estimated drop of ~70 m (230 ft) in sea level. [60]

Many palaeontologists agree that glaciation and the resulting drops in sea level allowed for increased migrations between Balkanatolia and western Europe. The Turgai Strait, which once separated much of Europe from Asia, is often proposed as the main European seaway barrier prior to the Grande Coupure, but some researchers challenged this perception recently, arguing that it completely receded already 37 Ma, long before the Eocene-Oligocene transition. In 2022, Alexis Licht et al. suggested that the Grande Coupure could have possibly been synchronous with the Oi-1 glaciation (33.5 Ma), which records a decline in atmospheric CO2, boosting the Antarctic glaciation that already started by the Eocene-Oligocene transition. [39] [61]

The Grande Coupure event also marked a large faunal turnover marking the arrivals of later anthracotheres, entelodonts, ruminants (Gelocidae, Lophiomerycidae), rhinocerotoids (Rhinocerotidae, Amynodontidae, Eggysodontidae), carnivorans (later Amphicyonidae, Amphicynodontidae, Nimravidae, and Ursidae), eastern Eurasian rodents (Eomyidae, Cricetidae, and Castoridae), and eulipotyphlans (Erinaceidae). [62] [63] [55] [64]

While the presence of Amphirhagatherium in the German locality of Bohnerz and the British Bouldnor Formation may infer possible survival of the Choerpotamidae at MP21, the latest-surviving Choeropotamus species, C. parisiensis, is not recorded from any site that may overlap with the MP21 unit. Many other artiodactyl genera from western Europe disappeared also as a result of the Grande Coupure extinction event. [65] [55] [25] The extinctions of Choeropotamus and many other mammals have been attributed to negative interactions with immigrant faunas (competition, predations), environmental changes from cooling climates, or some combination of the two. [58] [65]

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