Bachitherium is an extinct genus of Paleogene ruminants that lived in Europe from the late Eocene to the late Oligocene. The genus was erected in 1882 by Henri Filhol based on fossil remains found in the Quercy Phosphorites Formation. Bachitherium curtum was defined the type species, and another species called B. insigne; five more species have since been named although one, B. sardus, is currently pending reassessment. The genus name derives from "Bach", the French locality where its first fossils were found, and the Greek / meaning "beast". Bachitherium has historically been assigned to various families within the ruminant infrorder Tragulina, but was reclassified to its own monotypic family Bachitheriidae by Christine Janis in 1987.
For much of its taxonomic history, Bachitherium was only known from incomplete remains, making assessments difficult. However, a nearly complete skeleton of B. cf. insigne was uncovered in the commune of Céreste in France in 1981, which helped palaeontologists understand its morphology compared to other ruminants. The complete fossil revealed that Bachitherium had cursorial limb builds unlike typical tragulines and like smaller-sized members of the infraorder Pecora. In addition, its dentition differs from other traguline families such as the Tragulidae, Hypertragulidae, and Leptomerycidae by a combination of a tusklike but reduced bottom first incisor, strong upper canine, caniniform first bottom premolar, and other evolutionarily derived traits unique to the genus.
Based on its dental and limb morphologies, B. curtum, weighing to , was well-adapted to closed forest environments and ate a mix of leaves, fruit, and herbages. In comparison, the larger B. insigne, weighing up to , was more cursorial and therefore built for more open forested environments and a diet consisting mainly of leaves. The morphological differences between the species, combined with their frequent appearances in the same fossil deposits, imply niche partitioning based on their different ecological niches. The last species B. lavocati probably evolved from B. curtum and was likely better adapted to open forested environments than other species of its genus, coinciding with major climatic and faunal restructurings.
Bachitherium is the earliest-known ruminant to have appeared in the European fossil record, originating in eastern Europe during the late Eocene and migrating to western Europe by the early Oligocene after major seaway barriers fell. The genus remained endemic to the continent and survived several waves of climatic changes of the Oligocene without evolving into a new genus. The Microbunodon Event in the late Oligocene, however, led to its extinction as it faced a combination of a warming climate plus subsequent habitat turnover and competition from a new wave of migrating species.
Taxonomy Early history
The French palaeontologist Henri Filhol described two fossil artiodactyl genera from the lime phosphate deposits of the French province of Quercy that he thought indicated the Eocene epoch in 1882. He thought that the first genus belonged to the Moschidae family and must be closely affiliated with Gelocus. According to Filhol, the lower dental formula of the genus was 3:1:3:3, and the genus was characterized by its compressed lower premolars, with the posterior edge extending as a deep furrow. It was therefore differentiated from Gelocus in that the newer genus had all compressed lower premolars whereas only the last premolar of Gelocus, which has four premolars total, was compressed. The incisors, he observed, were quite small, while the canine immediately following them was strong. Based on these traits, Filhol erected the genus name Bachitherium, basing its etymology on "Bach," a location of phosphorite deposits. The first species, B. insigne, was determined to be a large animal while B. medium was much smaller than B. insigne but larger than the smallest species B. minus.
In 1885, the English naturalist Richard Lydekker placed Bachitherium provisionally in the family Tragulidae (chevrotains/mouse-deer/tragulids) and listed the species Bachitherium curtum, for which he made "Gelocus curtum" and "Bachitherium medium" synonyms of because the specimens appeared to have belonged to the same species (notably, Filhol described the species "Gelocus curtus" and "Gelocus insignis" in 1877 prior to the erection of Bachitherium). Lydekker stated that Bachitherium was known only by some portions of the mandible and maxilla, that it was said to be closely allied to Gelocus but differed by its first lower premolar. He stated that the dental formula of Bachitherium was , that the upper cheek of the genus closely resemble that of Prodremotherium. Additionally, he also confirmed B. insigne as a valid species.
In 1886, the German palaeontologist Max Schlosser erected the genus Cryptomeryx for which he made Lophiomeryx gaudryi, as described by Filhol in 1877, and Bachitherium minus synonyms of the species Cryptomeryx gaudryi. In 1986, however, Geneviève Bouvrain, Denis Geraads and Jean Sudre revised Cryptomeryx as a synonym of Lophiomeryx.
In 1957, the palaeontologist Friedlinde A. Obergfell erected a newer species of Bachitherium named B. serum from the German locality of Wintershof-West. He said that it resembled B. insigne from the phosphorite sites of Quercy based on the entoconid being connected to the metaconid on both the P3 and P4 teeth, leaving a valley on the tip from the protoconid to the entostylid. In 1971, Léonard Ginsburg created a genus named Andegameryx, of which A. andegaviensis is the type species. He acknowledged the similarities between B. insigne and "B. serum" but noticed that unlike Bachitherium, the tooth valley of "B. serum" is not, at least on the P4, closed on the rear; it instead turns inward and opens at the posterior lingual angle of the tooth. Because of this and other dental traits of the species, the taxon was reclassified to Andegameryx as the species A. serum.
As a result of the perceived closeness of the genus to Gelocus, Bachitherium was normally placed within the Gelocidae, although the palaeontologist René Lavocat placed it in the Hypertragulidae in 1946 based on the bachitheriid's tusk-like P1 (premolar) teeth, and in 1980, S. David Webb and Beryl E. Taylor placed it in the Leptomerycidae by diagnosis of a large mastoid fissure of the temporal bone and enlarged I1 (incisor) teeth.
Modern revisions of Paleogene ruminants Genevieve Bouvrain and Denis Geraads reviewed Bachitherium as a genus in 1984, examining the cast of a sub-complete skeleton from the Oligocene limestone locality of Cereste, France in addition to previously collected Quercy materials. The skeleton was referred to as a Bachitherium species of large size (Bachitherium cf. insigne), and until then, it only had jaws for fossil remains. The researchers discussed Webb and Taylor's 1980 placement of Bachitherium in the Leptomerycidae. They first reviewed the basic diagnosis of the hypertragulids, a primitive ruminant family with numerous characteristics distinct from other ruminants such as sharp cutting premolars and loss of the mesostyle in the molars. In comparison, the only dental trait the hypertragulids shared with Bachitherium is the caniniform P1, validating the previous reassignment from the Hypertragulidae. However, Bouvrain and Garaads also observed that although it shared I1 being developed into a tusk similar to leptomerycids, it was very reduced unlike them, therefore not belonging to the family due to the lack of autapomorphy shared with them. Furthermore, the C1 is reduced and the P1 tooth is small and separated by the diastema of the canine as with the P2 in early leptomerycids like Leptomeryx, the later leptomerycids such as Pseudoparablastomeryx losing their P1. These traits strongly contrast with Bachitherium, which has a strong C1 and a caniniform P1 tooth.
By 1986, Jean Sudre conducted a close study of Bachitherium, which then had just two species in its name and were reported additionally in Italy, Switzerland, and Germany. Bachitherium was considered to have characterized the middle Oligocene (Stampian), although the specimens from southern Germany were thought to characterize the lower Oligocene. In the palaeontological locality of Itardies in Quercy, two species were determined by Sudre to have characterized the 19th-century Quercy collections of the site: the abundant B. curtum and the newer species B. vireti, which was much rarer. At the Pech Desse locality in Quercy (the most recent site at the former province at the time), a second new species, B. lavocati was recognized by the palaeontologist, differing from B. curtum by the size of the diastema between the P1 and P2. He also reviewed the systematics of the genus, confirming that B. curtum (Filhol 1877) was the type species.
In 1987, the British palaeontologist Christine Marie Janis wrote a journal questioning and revising traguline clades such as Bachitherium, in which the recent discovery of the complete skeleton of the genus and her investigation of the European dental material of it could enable a reassessment of the ruminant. Janis verified Bachitherium not belonging to the Hypertragulidae in that despite the tusk-like caniniform P1, the former was more advanced based on dental and postcranial evidence. Bachitherium, the palaeontologist argued, could be excluded from the Tragulidae and Hypertragulidae families because of a combination of features of its limb anatomy. Nonetheless, Bachitheriums caniniform P1 was more similar to hypertragulids than those of the Leptomerycidae which was characterized by their more conical P1 teeth. Janis said that the hypothesis that a caniniform P1 is a primitive ruminant character would entail the loss of the character twice within the evolution of ruminants in the Tragulidae and Pecora, which she felt was less likely than the alternate hypothesis that it is a derived character state, which would involve three independent evolutions of the character in the Hypertragulidae, the Leptomerycidae, and Bachitherium.
Bachitherium was discerned to have a mixture of primitive and derived dental characteristics, the lower molars retaining the "Dorcatherium" fold ("fold on the posterolingual face of the metaconid") and the posterior lower premolars possessing primitive ruminant conditions of elongated, posteriorly directed metaconids. The hypoconulid of the M3 is narrow and somewhat "pinched" in appearance similar to Lophiomeryx. However, Bachitherium was thought to be separate from the leptomerycids based on derived characteristics more similar to "gelocids" and its lack of derived features with the leptomerycids. Unlike the leptomerycids, the I1 is small, the "Dorcatherium fold" is retained in the lower molars, and the internal cingulum is retained in the upper molars. Bachitherium is also excluded from the Pecora by the presence of a caniniform P1 tooth and the possession of a traguloid type of astragalus (also known as the talus bone or ankle bone). As a result, Christine Janis suggested that the it should be placed as a monotypic genus within the Tragulina, for which she named the family "Bachitheriidae."