Extinct genus of mammals
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 ?.?.3.33.1.3.3, 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, "Bachitherium"'s 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."
Jean Sudre wrote a research article chapter on the Oligocene artiodactyls of the Quercy collections of France in 1995, including the Bachitheriidae. He said that recent diagnoses of many "Bachitherium" remains supporting the criteria on which the species were based (dental dimensions and diastemas of the P1-P2) were not variable and did not reflect any dimorphism in between species, thereby supporting the distinction between the smaller "B. curtum" and the larger "B. insigne". After reviewing the previous species, Sudre ascertained that another species of "Bachitherium" could be erected from the Quercy Phosphorites Formation from the locality of Le Garouillas in France, "Bachitherium guirounetensis", differing from other species by a few astragali whose dimensions are between those of "B. curtum" and "B. guirounetensis". He also mentioned a "Bachitherium" sp., which was also known from Le Garouillas based on six astragali and probably a mandible of the P4-M3 series. The mandible fragment was referred to the species with some reservation based on its slightly smaller size compared to those attributed to "B. guirounetensis". Its P4 stood out to Sudre, the paraconid being offset lingually and close to the protoconid, but the palaeontologist speculated that the slight difference is not significant enough for a certain allocation to a different species since it could be interpreted as a variation in the more common "B. guirounetensis", found in the same deposit as "Bachitherium sp."
In 2008, Jan van der Made reviewed the Oschiri fauna from the Mediterranean island of Sardinia, which dates back to MN3 (the early Miocene, or within the Burdigalian), or some 20 Ma. It is the oldest-recognized instance of island endemism within the region (fossils dating to the Eocene are known although rare, but the faunas do not display endemism since the landmass had not separated from the European plate yet). In his previous 1999 list of the large mammals of the early Miocene of Sardinia, he listed "Bachitherium?", in which he would eventually erect the species "Bachitherium sardus", basing the species etymology on the island. He based