Choeropotamus is an extinct genus of Palaeogene 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. Cuvier reused the name Choeropotamus from its use by 17th century Venetian physician Prospero Alpini, who previously used it to describe stuffed hippopotamuses that he incorrectly thought to be distinct species. There are currently five described species of Choeropotamus, with C. parisiensis being the type species.
Choeropotamus had an broad and low, yet elongated skull, with a similarly elongated but narrow mandible. Its cheek teeth generally possess 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 of the five species forming a progressive lineage that gained cusps in the molars and increased in size; from the earliest and smallest C. lautricensis to the latest-appearing and largest C. parisiensis, which may have weighed about , sizable compared to many contemporary artiodactyls. The bunodont (round-cusped) molars of Choeropotamus point towards either a frugivorous or omnivorous diet, in which it could have occasionally consumed meat.
During the time that Choeropotamus was extant, western Europe was an archipelago with a tropical-subtropical climate that was isolated from the rest of Eurasia, living alongside various other animals which were also highly endemic. 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. However, the specific causes of Choeropotamus extinction are unclear.
Taxonomy Research history Early history
In 1822, French palaeontologist Georges Cuvier described cranial and dental fossil remains of what he called a distinct "pachyderm" genus. The first fossils in his possession consisted 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 canines were small like that of a peccary but that this new pachyderm species would have been larger-bodied than a peccary. 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 provisionally assigned the bones to the new genus Chæropotame, alternatively spelled by Cuvier as "Cheropotame" and "Cheropotames."
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. In 1824, Cuvier emended the genus name to Chœropotamus and listed the first species name Chœropotamus parisiensis. The genus name Choeropotamus means "river pig," which is a compound of the Greek prefix () meaning "pig" and the suffix () meaning "river." 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). Alpini applied the names "Cheropotamo," "Chœropotame", and "Cheropotamus" to two stuffed hippopotamuses from the Egyptian part of the Nile River that he mistakenly thought were distinct animals because of the two individuals' dentitions being supposedly unique from other hippopotamuses. In addition, Alpini said that the name was used by ancient Greeks, but Cuvier was unable to confirm such. Cuvier in 1822 directly confirmed in a footnote that he borrowed the genus name from Alpini's work.
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 that 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). 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 that of the babirusa in size, shape, and wear patterns. He cited a monography of fossil from the Molasse basin written by 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. 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. Von Meyer later reassigned C. Meissneri to Hyotherium in 1841.
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. In an 1839–1864 osteography, the French naturalist Henri Marie Ducrotay de Blainville prioritized the name C. parisiensis for the first recognized species.
In an 1848–1852 work, 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. French naturalist Jean-Baptiste Noulet erected C. lautricensis in 1870, naming the species after the French commune of Lautrec. He specified that the lower jaw specimens were found from a rich fossil deposit in Castres accidentally discovered at 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. 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. 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 belonged to Hyotherium, Karl Alfred von Zittel in a 1991-1993 handbook suggested that both C. parisiensis and C. affinis were valid species.
Later history
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. French palaeontologist Charles Depéret in 1917 listed a species variation named C. lautricensis mut. major without further elaboration. In 1930, American zoologist Glover M. Allen designated C. matritensis as both a nomen nudum (inadequately defined name) and a synonym of C. affinis.
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. "C." steinheimensis was eventually reclassified and moved to the genus Parachleuastochoerus, first recognized in the 20th century.
Classification
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 was synonymized with Choerpotamidae by later taxonomists, though other authors have suggested that the Haplobunodontidae should be revived due to questions relating to the polyphyly (lacking in relations from closest ancestors) of the Choeropotamidae.
The Choeropotamidae as currently defined is polyphyletic; cladistic analyses of "choeropotamids" based on dental anatomy reveals 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. Because the current definition of the Choeropotamidae is polyphyletic and not agreed upon, the Choeropotamidae requires further analyses.
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 but extended up to the earliest Oligocene. Masillabune, arguably the oldest choeropotamid, first appeared during the unit MP11 of the Mammal Paleogene 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. They are generally regarded as close relatives of the Cebochoeridae, another endemic European artiodactyl family. 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: