Bachitherium

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Bachitherium
Temporal range: late Eocene to late Oligocene, 37.71–24 Ma
Bachitherium cf. insigne skeleton, Pierre Martel Museum of Vachères
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Suborder: Ruminantia
Infraorder: Tragulina
Family: †Bachitheriidae
Janis, 1987
Genus: †Bachitherium
Filhol, 1882
Type species
†Gelocus curtus
(= †Bachitherium curtum)
Filhol, 1877
Other species
  • †Bachitherium insigne Filhol, 1877
  • †Bachitherium vireti Sudre, 1986
  • †Bachitherium lavocati Sudre, 1986
  • †Bachitherium guirounetensis Sudre, 1995
  • †Bachitherium thraciensis Mennecart et al., 2018

Bachitherium is an extinct genus of Paleogene ruminants belonging to the monotypic family Bachitheriidae that lived in eastern Europe and later western Europe from the late Eocene to the late Oligocene. It is known for being the first known ruminant in eastern Europe and later dispersing to western Europe by approximately 31 Ma. It is the namesake genus for the dispersal event after the known as the "Bachitherium Dispersal Event," the second known major dispersal event to occur after the Grande Coupure extinction event in which earlier Asian immigrants had already arrived to western Europe 33.9 to 33.4 Ma.

Taxonomy

Early history

Sketch of a Bachitherium curtum left ramus of a mandible in an 1885 fossil mammals catalogue for the British Museum of Natural History (left) and a photo of a Bachitherium curtum mandible at the same museum (right).

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 must've 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.[1]

In 1885, the English naturalist Richard Lydekker placed Bachitherium provisionally in the family Tragulidae 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).[2][3] 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.[2]

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.[4][3] In 1986, however, Geneviève Bouvrain, Denis Geraads and Jean Sudre revised Cryptomeryx as a synonym of Iberomeryx for which the type and only confirmable species would become Iberomeryx minor.[5][6]

In 1957, the palaeontologist Friedlinde A. Obergfell erected a newer species of Bachitherium named B. serum from the German locality of Wintershof-West. It is said to resemble 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.[7][8]

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 due to the presence of tusk-like P1 (premolar) teeth, and in 1980, S. David Webb and Beryl E. Taylor placed it in the Leptomerycidae based on a large mastoid fissure and enlarged I1 (incisor) teeth.[9][10][11]

Modern revisions of Paleogene ruminants

Genevieve Bouvrain and Denis Geraads reviewed Bachitherium as a genus in 1984, examining the cast of a complete skeleton discovered in the Oligocene limestone of Cereste, France as well as previously collected Quercy materials. The skeleton was referred to as a Bachitherium species of large size (Bachitherium cf. insigne), and until now, it was poorly known and 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 shared derived character from Bachitherium is the presence of a 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.[12][13][11]

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 old 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.[14]

Bachitherium insigne mandible remains, Natural History Museum of Basel

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.[15]

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."[15]

Jean Sudre wrote a research article chapter on the Oligocene artiodactyls of the Quercy collections of France in 1995, including Bachitherium. In the article, he recognized Bachitheriidae as a valid family and that the genus was found in recent decades to be more diverse than previously thought with multiple species known, the genus as a result ranging in the Mammal Paleogene zones from MP23 to MP28. While difficult to discern specimens of B. curtum and B. insigne due to size variations, absences of sufficient skeletons or populations, and historic considerations of the two as dimorphs of one species, he said that recent diagnoses of many Bachitherium remains that supported the criteria on which the species were based (dental dimensions and diastemas of the P1-P2) were not very variable and did not reflect any dimorphism in between species, thereby supporting the distinction between B. curtum and B. insigne. After reviewing the previous species, Sudre ascertained that another species of Bachitherium could be erected from the locality of Le Garouillas in France, Bachitherium guirounetensis, differing from other species by a few astragali whose dimensions are between those of 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. guironetensis. 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. guironetensis, which was found in the same deposit as Bachitherium sp.[16]

Increased role of biostratigraphy

Bachitherium curtum maxillae originally from Tarn-et-Garonne, France dating to the Oligocene, now stored at the Muséum de Toulouse.

As part of his PhD thesis for the University of Fribourg, Bastien Mennecart reviewed the post-Grande Coupure artiodactyls of Europe in 2012, including all known species of Bachitherium from previous decades. He documented the geographical and biostratigraphical distribution of western European ruminants during the early-middle Oligocene by Mammal Paleogene zones. In the distribution chart, he listed B. curtum as ranging from MP22-MP27, B. insigne from MP23-27, B. vireti in MP23, B. guirounetensis from MP25, B. lavocati from MP28, and Bachitherium sp. from MP25-26. According to Mennecart, Bachitherium sp. was first reported in 1986 by Jean Sudre at Le Garouillas in France and has since been found in multiple sites in France and Italy. He also defined the etymology of the genus, the prefix "Bach" referring to the old collections of the Phosphorites du Quercy and the suffix "therium" meaning "wild beast or animal." He also defined the etymologies of the species names, "curtum" and "insigne" translating in Latin to "shortened" and "distinguished" respectively, "vireti" being in honor of Jean Viret, "guirounetensis" being unknown, and "lavocati" being a tribute to Lavocati who was the first to study in detail the lower dentition of Bachitherium.[9]

In June 2018, Bastien Mennecart et. al described the earliest known European ruminant from the Thrace Basin, which is situated almost entirely within European Turkey but extends westwards of the Tundzha and Maritsa rivers in Bulgaria and Greece. The area is known for having three Paleogene formations that were found during extensive geographical mapping by drilling. The basal breccia-conglomerate-sandstone formation's exact age, although known to be of Eocene age, has an unknown exact age that cannot be possible to deduce because of factors such as a lack of biostratigraphically useful fossils. Since the terrestrial formation is overlain by two marine formations of Priabonian age, however, the basal formation could be estimated to date from the latest Bartonian or early Priabonian (late Eocene). Within the locality, Mennecart et. al reported the oldest Paleogene ruminant from Europe as well as the earliest one known from outside Asia and North America, dating well before the Grande Coupure faunal event in western Europe which dated back to the Eocene-Oligocene transition. The researchers decided that the fossil material belongs to a new species for which they named Bachitherium thraciensis. The holotype and only specimen is a right mandible with P2-M3, which was found south of the Bulgarian village of Sladun near the border of Bulgaria and Turkey. The species was described as small and differing from other species of Bachitherium by the structure of its P3 being similar to that of P2.[17]

The researchers also reviewed reports of the supposed previously oldest Bachitheriidae from the German localities of Herrlingen1, Möhren13, Ronheim1, and Weinheim (Palaeontological Museum, Munich) that date back to 32.5 Ma (MP22). The German ruminant fossils were reclassified to the gelocid species Gelocus villebramarensis and Gelocus communis because they have bifurcated and shortened postentocristids that are close to the diagnosis of Gelocus and not Bachitherium. The fossils also have upper molars that possess a strong cingulum that surrounds the protocone, whereas in Bachitherium, the cingulum is only more emergent. Therefore, the actual oldest record of the Bachitheriidae in western Europe is ca. 31 Ma (MP23).[17]

On November of the same year, Mennecart et. al conducted a study on a fragmented right branch of a ruminant mandible from the area of the Rusce village from the municipality of Vranje in southeastern Serbia. The fossil specimen was originally described in 1939 by Vladimir Laskarev, who attributed it to the moschid species Micromeryx flourensianus based on the small size of the specimen. However, the mandible was reported from the Pčinja basin, which is well-known its Eocene and possibly Oligocene sedimentary fillings as opposed to Neogene sediments. Since the genus Micromeryx is known first from the early Miocene, dating back to the Orleanian or MN4, the mandible, small with deciduous teeth, is unlikely to belong to any moschid. Unfortunately, the whereabouts of the original specimen is unknown, although an old picture of it allows for analysis nonetheless. Mennecart and colleagues recognized the similarities between the Rusce specimen and the Moschidae (Micromeryx and Hispanomeryx), such as the small size of the mandible and the absence of P1. However, moschid characteristics are plesiomorphic, but typical moschid characteristics are missing from the specimen. Bovoidea (Bovidae and Moschidae) is characterized by distally closed molars, while the Rusce specimen has a space between the postentocristid and the posthypocristid on the M1. The Rusce specimens differs from moschids also by the lack of more laterally compressed molars with relatively high crowns, the straight outline of the mandible, and the reduced flatness of M3 lingual wall.[18]

The authors then investigated the family that the mandible belonged to. Although the D3 tooth is absent from occlusal view, the lingual view shows that two elongated cristids go towards the distal end of the D3 from the mesolabial conid, which eliminates pecorans from being the possible owner of the mandible and leaves the tragulines. The molars possess anteriorly closed trigonids that are confined by the anterior fusion of the premetacristid and preprotocristid, differentiating from the molars of the Lophiomerycidae and stem Tragulidae, which do not show developed and elongated premetacristids. The Rusce specimen's molars are bunodont and low-crowned, which match the molars of the Bachitheriidae but contradict the molars of the Asian Praetragulidae, the latter of which are laterally compressed and relatively high crowned similar. Finally, the straight ventral outline of the mandible differs from the Tragulidae, Lophiomerycidae, and Gelocidae but are similar to the Bachitheriidae. Therefore, the mandible most likely belonged to Bachitherium based on its traits. Although the lower permanent premolars and limb bone proportions typically used to discern the different species are not known in the sample, the size of the mandible is in agreement with the smaller species B. viretti and B. thraciensis, the oldest ones known in the palaeontological record. The roughness of the enamel and pronounced bunodont character of the cuspids led the authors to think that it probably belongs to B. thraciensis, but they intentionally an open nomenclature, assigning the Rusce specimen to B. cf. thraciensis.[18]

Classification

Portrait of Henri Filhol, who erected the genus Bachitherium.
Picture of Christine Janis, who created the family Bachitheriidae.

Bachitherium is the type and only genus of the family Bachitheriidae within the infraorder Tragulina in the artiodactyl suborder Ruminantia. Tragulina is more basal than Pecora and is characterized by doglike astragalus trochleas, a lack of any posterolingual cristid on P4, a Dorcatherium fold (or similar cuspid structures) on the lower molars, and relatively bunoselenodont molars. The Tragulidae (its members commonly known as "chevrotains" or "mouse deer") is the only extant family of the infraorder and are, as a result, considered living fossils, since they've changed little in the evolutionary record.[9][19] Bachitherium is technically considered a mammal of Asian origin based on its geographical origins being that of eastern Europe as opposed to the Eocene landmass Balkanatolia or western Europe, which became isolated from North America and Greenland by the later early Eocene and thereby had faunas that evolved in the form of distinct endemism compared to the rest of Eurasia (Bachitherium is not found in Asia).[17][20][21] This is especially evident after the abrupt Grande Coupure extinction event within the MP20-MP21 boundary (which includes the Eocene-Oligocene boundary), which marks the base of the Rupelian at 33.9 Ma. The first true ruminants to appear in western Europe, the Gelocidae and Lophiomerycidae, were immigrant taxa that dispersed from Asia while approximately 60% of existing mammal lineages of western Europe, most of which were endemic to the continent, disappeared.[17][22][23]

Several researchers such as Innessa Anatolevna Vislobokova have considered the two superfamilies within the infraorder, the Traguloidea (Tragulidae, Gelocidae, Leptomerycidae, Archaeomerycidae, Lophiomerycidae, Bachitheriidae) and Hypertraguloidea (Hypertragulidae, Praetragulidae), to be valid.[24] The superfamily Traguloidea is sometimes used in systemic palaeontological diagnoses, plus Janis and Jessica M. Theodor defined Hypertraguloidea as a valid superfamily consisting of the Hypertragulidae and Praetragulidae based on the fusion of the magnum and trapezoid bones in the carpus, a trait unseen in other artiodactyls. The two superfamilies are not often mentioned in research papers on members of the Tragulina, however.[25][26][27]

In 2015, Bastien Mennecart and Grégoire Métais created a hypothesis on the phylogenetic relationships of Oligocene ruminants by compiling a matrix of 40 dental, mandibular, and postcranial features known from them from both direct observation and literature of the taxa. The taxa included in the analysis are 20 ruminant species representative of the European and Asian "Gelocidae" (a known wastebasket family), Bachitheriidae, late Oligocene or early Miocene Pecora of ambiguous affinities, Lophiomerycidae, Tragulidae, Leptomerycidae, Archaeomerycidae, and Hypertragulidae. Below are two phylogenetic trees of the defined Oligocene ruminants, the first based on a 50% majority consensus and the second based on a strict consensus:[28]




Mennecart and Métais stated that the systematic position of the monogeneric family Bachitheriidae is somewhat problematic since the phylogenetic relationship of Bachitherium with other Oligocene ruminants remains ambiguous or at least not consensual in palaeontological literature. They suggested that the "moving" position of the Bachitheriidae is probably linked with convergent evolution of postcranial and dental features. Bachitherium is considered primitive in retaining a caniniform P1 and a traguloid type of the astragalus, but the selenodont molars and fusion of the carpal bones were probably a result of convergent evolution/parallel evolution. The convergent traits, the researchers suggested, caused misinterpretations of the phylogenetic relations of Bachitherium in various phylogenetic trees of Paleogene ruminants, most of which were done by hand. Despite having a complete skeleton, the phylogenetic position of the Bachitheriidae with respect to the "crown" Ruminantia (ie. below or above the Tragulidae) remained unstable. They suggested that basicranial morphological analyses will provide key features to better understanding the evolutionary history of bachitheriids.[28]

In 2021, Bastien Mennecart et. al reused the morphological matrix of some 40 characters from 2015 by Mennecart and Métais (as explained above) to produce a hypothesis regarding the phylogenetic affinities of the Oligocene tragulid Nalameryx using additional remains from the genus. Similar to the research article from 2015, Amphirhagatherium weigelti and Merycoidodon culbertsoni are classified as outgroups within the Artiodactyla due to not falling within the Ruminantia suborder. However, the 2021 phylogenetic tree also adds the enigmatic artiodactyl Stenomeryx, which is typically considered to be either next to the outgroup Amphirhagatherium or as sister taxa of the Bachitheriidae, favouring the former hypothesis over the latter. The Bachitheriidae are considered the sister family to the Tragulidae and are thus phylogenetically within the crown Ruminantia, seemingly supported in part thanks to the Eocene-aged remains recently described from the Balkans. Below is an updated tree based on the 2015 phylogenetic trees by Mennecart and Métais resulting from the cladistic analysis of 41 dental, cranial, and postcranial features:[29]


Ruminantia

Description

Although Bachitherium and most other Paleogene ruminants are typically known mostly or only from jaw remains, Bachitherium is also known by a complete skeleton from the limestone deposits of Cereste, France that is attributed to Bachitherium cf. insigne, allowing for more thorough diagnoses for the Bachitheriidae.[12] Unfortunately, the skeleton is strongly flattened transversely, limiting observations of it to only its left side. The sutures of the skull, as a result, are little or cannot be observed.[30] Nonetheless, the complete skeleton and various other remains of Bachitherium allow for distinguishing it as a distinct family compared to other members of the Tragulina infraorder such as the Gelocidae, Lophiomerycidae, Hypertragulidae, Leptomerycidae, and Tragulidae.[15]

Since the two species are commonly confused with each other, the major differences between B. curtum and B. insigne are the former species' smaller teeth, diastema, and postcranial bones compared to the latter.[9]

Skull

Skeleton of Tragulus javanicus at the Smithsonian National Museum of Natural History in a close-up view of the skull.

A unique trait of Bachitherium is the great development of its face, as the anterior edge of the orbit, although just slightly above the M2, is as far from the anterior edge of the premaxilla as it is from the occipital crest. The premaxillary, being edentulous (lacking teeth), upright, and short, comes into contact with long nasals, broadened behind and ending towards the front. There is a small gap in the ethmoid bone probably bordered by the nasal, maxillary, frontal, and lacrimal bones. The lacrimal fossa appears to be absent from the skull, the lacrimal hole instead opening inside the orbit which is small and closed posteriorly by a complete and robust postorbital bar. The back of the skull, which is slightly inclined with respect to the face, is surmounted by a strong sagittal crest protruding above the occipital face. Forward of the nuchal lines is the auditory hole, separated from a strong paramastoid (or near the mastoid) apophysis (normal developmental outgrowth of a bone) by a mastoid of unknown size. The squamosal suture, visible for a short distance, is oblique forwards and backwards and is pierced by several small foramina.[30][31]

The horizontal region of the mandible increases in height towards the back, the lower edge having two concavities. Of the two concavities, one is in front of P2 while the other is behind M3. The upward angular region of the mandible is wide, its posterior edge connecting to the neck of the condyle in a condyloid process similar to camelids.[30]

Cranial lengths (mm & in) of the Bachitherium cf. insigne skeleton from Cereste, France[30]
Front of premaxilla to occipital condyle Front of premaxilla to top of occipital crest Front of nasals to top of occipital crest
183 mm (7.2 in) 208 mm (8.2 in) 197 mm (7.8 in)

Dentition

The various specimens of Bachitherium cf. lavocati from Mailhat, France (Specimen 1) and B. lavocati from Pech Desse, France (2-3) and Gaimersheim, Germany (4-9).

Bachitherium is distinguished by its dentition that is closer to the Tragulina than other artiodactyls but are distinct enough to justify it belonging to its own family. The dental formula of the genus is 0.1.3.33.1.4.3, where the P1 is evolutionarily lost, thus explaining why it has one less premolar in its upper jaw than its lower jaw. There are also no upper incisors recorded from the skeleton or other related specimens, typical of ruminant traits. The upper canine is curved in a hook-like manner and strong but relatively short since it does not exceed the height of the upper edge of the mandible. The lower canine is also short similar to the upper canine and the lower incisors but is incisiform in shape. In terms of occlusion, the C1 is lodged between the caniniform P1 of similar size for wearing and the frontal teeth. The diastemas of C1-P2 and P1-P2 are recorded to be lengthy. The complex of front teeth being very distant from the cheekbones, probably in relation to the strengthening of the adductor musculature, constitutes the main autapomorphy of Bachitherium.[30][31]

The Bachitheriidae is defined as a traguline family that possesses a narrow and "pinched" hypoconulid on M3, a strong presence of a distally isolated posterolabial cristid on P3 and P4, and a strong "Dorcatherium" fold on the molars.[30][15][17] It is also different from other traguline genera by the outline of its mandible in relation to its dentition. The lower part of the mandible is straight, and the elongated diastema is highly constricted just after P2. The typical traguline-shaped lower premolar, defined by no metaconid and two elongated parallel cristids forming the edges of the enlarged back valley, is only known in Europe from the Tragulidae (including Iberomeryx of the Paleogene and Dorcatherium of the Neogene) and the Bachitheriidae. The lower molars are derived, having relatively selenodont cusps, typical of most ruminants, as well as possessing no cingulum. The upper molars of the Bachitheriidae do not possess any aligned paracone and metacone.[9] Some more recently described dental autapomorphies of the Bachitheriidae are the diastema between P1-P2 being longer than that of P2-P4 and a shallow external postprotocristid that is not linked to the prehypocristid.[17]

Dental lengths (mm & in) of the Bachitherium cf. insigne skeleton from Cereste, France[30]
C1-P2 diastema P2-P4 M1-M3 P2-M3 C1-P1 diastema P1-P2 diastema P2-P4 M1-M3 P2-M3
41 mm (1.6 in) 25.7 mm (1.01 in) 30.2 mm (1.19 in) 56.5 mm (2.22 in) 7.8 mm (0.31 in) 27.2 mm (1.07 in) 27.7 mm (1.09 in) 33.4 mm (1.31 in) 62.5 mm (2.46 in)
Measurements (mm & in) of the teeth and diastemas of Bachitherium species holotypes[9][17]
Species Holotype Type Locality Age Diastema P1-P2 P2-M3 P2-P4 M1-M3
B. thraciensis NMNHS FM3320 Sladun, Bulgaria Late Eocene >14 mm (0.55 in) 18 mm (0.71 in)
B. vireti UCBL FSL9672 Nassiet (Landes, France) Early Oligocene (MP23) 10 mm (0.39 in) - 15 mm (0.59 in) 40 mm (1.6 in) - 42 mm (1.7 in) 18 mm (0.71 in) - 18.2 mm (0.72 in) 23 mm (0.91 in) - 25.2 mm (0.99 in)
B. insigne MNHN QU4258 Old collections of Quercy, France Oligocene (MP23-MP27) 38 mm (1.5 in) 63 mm (2.5 in) 25 mm (0.98 in) 37 mm (1.5 in)
B. curtum MNHN Qu3917 Old collections of Quercy, France Oligocene (MP23-MP27) 19.5 mm (0.77 in) - 22 mm (0.87 in) 50.5 mm (1.99 in) - 51 mm (2.0 in) 20 mm (0.79 in) - 22 mm (0.87 in) 26.5 mm (1.04 in) - 28 mm (1.1 in)
B. guirounetensis UM GAR441 Le Garouillas, France Middle Oligocene (MP25) 27.5 mm (1.08 in) 57.6 mm (2.27 in) 26 mm (1.0 in) - 28 mm (1.1 in) 30.5 mm (1.20 in) - 35.7 mm (1.41 in)
B. lavocati UM PDS1330 Pech Desse (Lot, France) Late Oligocene (MP28) 32 mm (1.3 in) - 34 mm (1.3 in) 51 mm (2.0 in) - 52 mm (2.0 in) 20.4|mm} - 22.5 mm (0.89 in) 26 mm (1.0 in) - 32.4 mm (1.28 in)

Vertebrae

The cervical vertebrae of the Bachitherium skeleton is short and robust, its total length measuring less than that of the head. On the scapula (or shoulder blade), the spine ends in a strong, hooked acromion, reaching the level of the glenoid fossa. The atlas, also known in the anatomy of the neck vertebrae as C1, possesses wide, rounded anatomical processes (or "wings") that terminate posteriorly at a point exceeding the level of articulation with the axis of the cervical vertebrae (C2). The joints of the atlas reach dorsally almost to the top of the neural canal, of which the anterior member is much shorter than the posterior one. The axis's body is short, but the axis itself is noted to be remarkable for the vertical and especially caudal extension of its high spinous process which reaches the level of C4. The unusual traits of the cervical vertebrae could be connected with the strong development of the large oblique muscles of the head.[12][30]

The spinal processes are short on C3 and C4 but lengthen from C5 to D1. The lengths of the spinal processes are more or less constant up to D7. The spine is slightly curved backwards from vertebrae C7-D2, then straight on D3-D4, and finally slightly inclined forward until D9. In the spine, there are 13 dorsal vertebrae, 7 lumbar vertebrae (probably sacral), and 17 caudal vertebrae. The sacrum is short because it does not exceed the cavities of the acetabulum. The first ribs of the bachitheriid are wide, short, and straight, contrasting with the later ribs that are thin, long, and curved. The number of sternebrae is not determinable because of the transversely flattened state of the sub-complete skeleton.[30]

Limbs

In bachitheriids, the neck of the humerus is slightly marked, and the tuberosity (prominence where muscles and connective tissues attach) of the tendon of the infraspinatus muscle is strong and oval. There is neither any triceps curve nor deltoid tuberosity present within the humerus. In the distal part of the bone, the diameter of the condyle is lower than that of the trochlea. The ulna, or one type of long bone found in the forearm, is separated from the radius (the other type of long bone found in the forearm) along its entire length. The olecranon of the ulna is wide and located in the axis of the diaphysis. The pelvis does not appear anything particularly peculiar except for the elongation of the acetabular portion of the hip bone compared to the ilium, although it is still shorter than that of the Tragulidae. The trochanter of the femur is moderately elevated compared to the femoral head. The calcaneus is robust but is shorter than that of camelids.[30]

The forelimbs of Bachitherium are shorter than the hind limbs, but the difference is smaller than in tragulids. The proportions of the limbs are similar to that of Moschus or small cervids (Muntiacus, Hyelaphus), although the distal limbs in Bachitherium are a little less elongated. The metatarsal bones II and IV (or long bones of the feet) are partially fused (fused except at their distal extremities without a groove (or furrow) on the anterior face) while the metacarpal bones III and IV (or long bones of the hands coming after wrist bones) are unfused. The lateral metacarpals appear to be absent and all of the front phalanges are missing, the former trait being suggested as a autapomorphy of the monotypic traguline family since the skeleton specimen is excellently preserved that the bones being missing couldn't have been an accident. The first phalanges of the back feet are short compared to the second, while the third are very long with an articular surface being almost perpendicular to the body of the bone. [12][30][17] The fusions of the bones seem to point towards a general trend in ruminant evolution since it is present within pecoran evolution while the fusions aren't always represented in the Tragulidae. Because of this, the Bachitheriidae are considered more evolved than the Lophiomerycidae but more primitive than the Protopecora.[9]

Body mass

In September 1995, prior to Sudre's publication on B. guirounetensis, he and Jean-Noël Martinez used the dimensions of the astragali of Paleogene artiodactyls to estimate their body masses, since the astragalus is the most frequently postcranial bone in fossil assemblages from its reduction of vulnerability to fragmentation due to its stocky shape and compact structure. Four known Bachitherium species, B. vireti, B. curtum, B. insigne, and B. nov. sp. (the last of which would later be known as B. guirounetensis) were analyzed using two different methods of medial weight estimations: the astragalus size and the M1 area, both results shown in the below table:[32][16]

Comparisons (g and oz) of medial weight estimates from the astragalus size and the M1 area[32]
Species M1 Area Weight Estimate Astragalus Size Weight Estimate Weight Limit Values
Bachitherium vireti 4,736 g (167.1 oz) 4,553 g (160.6 oz) 4,089 g (144.2 oz) - 5,229 g (184.4 oz)
Bachitherium curtum 6,905 g (243.6 oz) 6,114 g (215.7 oz) 4,772 g (168.3 oz) - 6,959 g (245.5 oz)
Bachitherium lavocati 6,905 g (243.6 oz) 7,722 g (272.4 oz) 4,906 g (173.1 oz) - 12,202 g (430.4 oz)
Bachitherium nov. sp. 12,913 g (455.5 oz) 18,647 g (657.8 oz) 14,104 g (497.5 oz) - 23,808 g (839.8 oz)


In 2014, Takehisa Tsubamoto reexamined the relationship between astragalus size and estimated body mass based on extensive studies of extant terrestrial mammals, reapplying the methods to Paleogene artiodactyls previously tested by Sudre and Martinez. The researcher used linear measurements and their products with adjusted correction factors. The recalculations resulted in somewhat lower estimates compared to the 1995 results (with the exception of the anoplotheriid Diplobune minor, which as a shorter astragalus proportion than most other artiodactyls, displayed in the below graph:[33]


Estimated body masses (kg) of Paleogene artiodactyls based on recalculated trochlear widths (Li1) in comparison to estimates from Martinez and Sudre (1995)


B. lavocati, according to its dental traits, was barely larger as a species than B. curtum, considered to be the potential ancestor of the former species. There is no evidence that B. lavocati, a common species at the Pech Desse locality in France, had any dimorphism, indicating that it was homogenous in nature. Both B. lavocati and B. curtum have been estimated to weigh 7 kg (15 lb) - 8 kg (18 lb), consistent with the idea of similar weights. B. insigne, however, was unusually large in comparison to not other other bachitheriid species but also other Paleogene tragulines, being estimated to weigh up to 36 kg (79 lb). B. insigne overlapping in the MP zones with other species such as B. curtum and B. guirounetensis offers hints of different ecological niches being filled by them within the European continent, as the larger size of B. insigne likely means that it lived in a different habitat and had slightly different eating habits compared to them.[34][35][9]

References

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