# Synaphodus

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***Synaphodus*** is an extinct genus of [artiodactyls](/source/Artiodactyl) belonging to the [basal](/source/Basal_(phylogenetics)) family [Dichobunidae](/source/Dichobunidae). It was endemic to western Europe and lived during the [Oligocene](/source/Oligocene), although its specific temporal range is uncertain. Despite having only one recognized species ***S. brachygnathus***, it has had a complicated taxonomic history due to the genus name, established by the French palaeontologist [Auguste Pomel](/source/Auguste_Pomel) in 1848, being applied early on for a species that was eventually reclassified under the [suoid](/source/Suoidea) genus *[Propalaeochoerus](/source/Propalaeochoerus)*. It was a large-sized member of the [Dichobuninae](/source/Dichobuninae) similar to *[Metriotherium](/source/Metriotherium)* but differed from it by its more [bunodont](/source/Bunodont) (round and low cusps) dentition among other differences. As one of the few dichobunines of the Oligocene, it would have coexisted with immigrant faunas after the [Grande Coupure](/source/Grande_Coupure) extinction event and surviving faunas within western Europe.

## Taxonomy

In 1848, the secretary of French palaeontologist [Jean-Louis Hardouin Michelin de Choisy](/source/Jean-Louis_Hardouin_Michelin_de_Choisy), named Mr. Bayle, wrote about a presentation given by French palaeontologist [Auguste Pomel](/source/Auguste_Pomel), who gave mention to an artiodactyl genus named *Synaphodus*, assigning *[Anthracotherium](/source/Anthracotherium) gergovianum* (originally named by French palaeontologist [Jean-Baptiste Croizet](/source/Jean-Baptiste_Croizet)) to it without further explanation.[1] The same year, Pomel also erected the genus name *Brachygnatus* for the same species.[2] He also wrote about the species *Synaphodus brachygnathus*, explaining that its dentition was similar to that of *[Anoplotherium](/source/Anoplotherium)* and had continuous dentition meaning short [diastemata](/source/Diastema).[3] The etymology of *Synaphodus* derives from the [Ancient Greek](/source/Ancient_Greek) words σύναψη (union) and ὀδούς (tooth) in reference to the continuous dentition on the [holotype](/source/Holotype) [mandible](/source/Mandible).[4]

In 1848–1852, French palaeontologist [Paul Gervais](/source/Paul_Gervais) tentatively listed *A? gergovianum*, explaining that he thought that its [molars](/source/Molar_(tooth)) were more similar to those of *[Dichobune](/source/Dichobune)* than *Anthracotherium*. He noted that the species was listed under three genus names, namely *Cyclognathus*, *Brachygnathus*, and *Synaphodus*.[5] In 1853, Pomel listed the species name *Synaphodus gergovianus* based on a [mandible](/source/Mandible) with dentition, of which *Anthracotherium gergovianus* and *Sinaphodus brachygnathus* (the latter name created by him as well) are synonyms. He also defined *Cyclognathus* as being synonymous with *[Cainotherium](/source/Cainotherium)*.[6] In 1899, Swiss palaeontologist [Hans Georg Stehlin](/source/Hans_Georg_Stehlin) suggested that the synonymous names *Synaphodus brachygnathus* and *Synaphodus gergovianus* are invalid and that their specimens can be classified under *[Palaeochoerus](/source/Palaeochoerus)*. After Stehlin erected *[Propalaeochoerus](/source/Propalaeochoerus)* the same year,[7] *P. gergovianus* as originally named by Croizet has since been assigned to it.[8]

The taxonomic status of *Synaphodus* had often been overlooked by taxonomists of the 20th century, as noted by Jean Sudre.[9] In 1951, French palaeontologist [René Lavocat](/source/Ren%C3%A9_Lavocat) revived *Synaphodus brachygnathus* as a valid genus and species belonging to the [Dichobunidae](/source/Dichobunidae) based on the mandible studied in 1853 by Pomel.[10] The validity of *Synaphodus* was followed by Sudre in 1978, who stated that it was distinct enough from its close relative *[Metriotherium](/source/Metriotherium)*.[9]

### Classification

*Synaphodus* belongs to the subfamily [Dichobuninae](/source/Dichobuninae) within the [Dichobunidae](/source/Dichobunidae), an extinct early artiodactyl family within the superfamily [Dichobunoidea](/source/Dichobunoidea).[11] The Dichobunoidea is a paraphyletic group of basal artiodactyls appearing in the Early [Eocene](/source/Eocene) that gave way to various other artiodactyl clades, extant and extinct.[12][13] The Dichobunoidea is considered by researchers to consist of seven families: Cebochoeridae, [Diacodexeidae](/source/Diacodexeidae), Dichobunidae, [Helohyidae](/source/Helohyidae), [Homacodontidae](/source/Homacodontidae), [Leptochoeridae](/source/Leptochoeridae), and [Raoellidae](/source/Raoellidae) (although not all researchers agree that the Raeoellidae is a dichobunoid family). Despite the consensus that the Dichobunoidea is a paraphyletic group, researchers are still investigating the extent to which certain members are stem taxa to other major artiodactyl clades.[14][11][15] At least some dichobunoid families are thought to be [monophyletic](/source/Monophyletic) while others are paraphyletic, some of whom may even be [polyphyletic](/source/Polyphyletic); the latter grouping means that some clades need to be reassessed.[15]

Some of the earliest artiodactyls to have appeared in the fossil record by the Early Eocene are dichobunoids that have simultaneously appeared in North America, Europe, and Asia. In both North America and Europe, species classified to *[Diacodexis](/source/Diacodexis)* are the earliest records of artiodactyls in both continents that extend back to the [Wasatchian](/source/Wasatchian) of the [North American land mammal age](/source/North_American_land_mammal_age) and MP7 of the [Mammal Palaeogene zones](/source/Mammal_Paleogene_zones) of Europe, respectively (*Diacodexis* and the Diacodexeidae are thought to both be polyphyletic). In Asia, some of the earliest artiodactyl genera, who correlate to equivalent ages, are the possible [suiform](/source/Suina) *[Wutuhyus](/source/Wutuhyus)* and dichobunoid *[Tsaganohyus](/source/Tsaganohyus)*. In the Early to Middle Eocene within the three continents, the artiodactyls were common mammals of small to medium sizes that generally had [bunodont](/source/Bunodont) to bunoselenodont (bunodont plus [selenodont](/source/Selenodont)) dentitions, thus making them important for [biostratigraphy](/source/Biostratigraphy).[16][15]

The Dichobunidae is a family of artiodactyls known from both Europe and Asia that contains multiple subfamilies: Dichobuninae, [Hyperdichobuninae](/source/Hyperdichobuninae), [Eurodexinae](/source/Eurodexinae), and [Lantianiinae](/source/Lantianiinae). Members of both Europe and Asia appeared as early as the Early Eocene, evident by the early appearance of *[Eolantianus](/source/Eolantianus)* in Asia and those of other dichobunid genera like *[Protodichobune](/source/Protodichobune)* and *[Aumelasia](/source/Aumelasia)* in Europe by MP10. Both of the early dichobunids *Protodichobune* and *Aumelasia*, along with *Dichobune*, are genera belonging to the Dichobuninae. While most species of the subfamily are recorded exclusively from western Europe, one other species pending assessment as "*Dichobune* sp." is recorded from the Lushi Formation in China, although its status within the Dichobunidae is unclear. The Dichobuninae, and the wider Dichobunidae by extent, lasted up to the Late Oligocene, evident by the range of the dichobunine *Metriotherium* extending up to MP27.[11]

In 2020, Vincent Luccisano et al. created a phylogenetic tree of the basal artiodactyls, a majority endemic to western Europe, from the Palaeogene. The results found the Dichobunidae, except for *Aumelasia*, as a paraphyletic stem group in relation to other artiodactyls. Both the Dichobuninae and Hyperdichobuninae are recovered as paraphyletic groups. Luccisano et al. noted the lack of phylogenetic resolution of the dichobunid subfamilies to each other and to other artiodactyl clades, which follows results from earlier studies and means that more research needs to be done for dichobunid phylogenetics. The phylogenetic tree as produced by the authors is shown below:[17]

In 2023, Abhay Rautela and Sunil Bajpai created an analysis on the phylogenetic relationships between basal artiodactyls by compiling a matrix of dental remains of 34 artiodactyl species; most of these artiodactyl species are dichobunoids (Diacodexeidae, Dichobunidae, Homacodontidae, Cebochoeridae, Leptochoeridae, Raoellidae), but some are members of the [Pakicetidae](/source/Pakicetidae) and one other species is a member of the [Helohyidae](/source/Helohyidae) (the basal placental mammal *[Protungulatum](/source/Protungulatum)* is the outgroup taxon in the analysis). Below is a cladogram by Rautela and Bajpai of the artiodactyl taxa based on a 50% majority consensus:[15]

As seen in the above phylogeny, one clade pairs *Dichobune* with *[Homacodon](/source/Homacodon)*, *[Buxobune](/source/Buxobune)*, and *[Gobiohyus](/source/Gobiohyus)* based on specific dental traits. Based on the cladogram, Rautela and Bajpai defined *Diacodexis*, the Diacodexeidae, and Dichobunidae as all polyphyletic taxa. In the case of the dichobunines, this is because they are more closely paired with non-dichobunids than with the lantianiines (*Eolantianus*, *[Elaschitotherium](/source/Elaschitotherium)*) and hyperdichobunines (*[Mouillacitherium](/source/Mouillacitherium)*).[15]

In 2022, Weppe conducted a phylogenetic analysis in his academic thesis regarding Palaeogene artiodactyl lineages, focusing most specifically on the endemic European families but also on European dichobunids. He found that the Dichobuninae was more closely related to the Cebochoeridae and species classified to the polyphyletic [Choeropotamidae](/source/Choeropotamidae), contrasting with the Hyperdichobuninae, which was paraphyletic in relation to the other endemic European artiodactyl groups ([Amphimerycidae](/source/Amphimerycidae), Anoplotheriidae, [Xiphodontidae](/source/Xiphodontidae), [Mixtotheriidae](/source/Mixtotheriidae), and [Cainotherioidea](/source/Cainotherioidea)). Within the dichobunine clade, which includes *Dichobune* and *Metriotherium*, *D. robertiana* is defined as the plesiomorphic species that makes up the first branch and is followed by those of *D. sigei*, *M. mirabile*, and a clade consisting of *D. jehennei* and *D. leporina*. He also stated that the species named *D.* aff. *robertiana* had even more plesiomorphic traits than the other *Dichobune* species and supported the idea from prior literature that *M. mirabile*, *D. jehennei*, and *D. leporina* were more derived species within their subfamily. He defined *Dichobune* as being paraphyletic in relation to *Metriotherium*.[18]

## Description

Dichobunoids are known for having the complete dental formula of for a total of 44 teeth, consistent with the primitive dental formula for early-middle Palaeogene [placental](/source/Placental) mammals.[19][20] This is the case for the Dichobunidae, whose teeth are not much separated by [diastemata](/source/Diastema) and are [bunodont](/source/Bunodont) (low and rounded cusps). Except for some of the oldest genera, dichobunids are also described as having molars (M/m) that generally have five to six [tubercles](/source/Tubercle) (or cusps) each.[9][21] The Dichobuninae is described as having unspecialized and rounded dentition, although it is more bunodont than in the earlier Diacodexeidae. In the upper [premolars](/source/Premolar) (P/p), the [metaconule](/source/Metaconule) cusp is larger than the [paraconule](/source/Paraconule) cusp. P3 has a [protocone](/source/Protocone) cusp while P4 has a [metaconid](/source/Metaconid) cusp. P1 is premolariform in shape. The upper molars in dichobunines usually have three wide distal cusps along with a [hypocone](/source/Hypocone) cusp.[11] Within the six-cusped molars, the [paracone](/source/Paracone), [metacone](/source/Metacone), protocone, and metaconule cusps are the major types present while the paraconule and hypocone cusps are the secondary ones.[22] In terms of cranial features, the Dichobuninae is diagnosed as having slightly elongated snouts.[11]

*Synaphodus* is diagnosed as being a large-sized dichobunine similar to *Metriotherium*. Only the lower dentition of *Synaphodus* is known due to the upper dentition never having been found. Unlike with the selenodont (crescent-shaped ridges) *Metriotherium*, the dentition of *Synaphodus* is more bunodont (round and low cusps). Its canine is large-sized. The premolars have a [paraconid](/source/Paraconid) cusp on them individually, and there is a noticeable lack of diastema between P2 and P3. P4 in *Synaphodus* is longer than that of *Metriotherium* and has a well-developed [metaconid](/source/Metaconid) at the lingual side's centre area.[11]

## Palaeoecology

Although the Eocene-Oligocene transition marked long-term drastic cooling global climates, western Eurasia was still dominated by humid climates, albeit with dry winter seasons in the Oligocene. Europe during the Oligocene had environments largely adapted to winter-dry seasons and humid seasons that were composed of three separate vegetational belts by latitude, with temperate [needleleaf](/source/Conifer)-[broadleaved](/source/Broadleaved) or purely broadleaved deciduous forests aligning with the northernmost belt between 40°N and 50°N, the middle belt of warmth-adapted mixed [mesophytic](/source/Mesophytic) and [evergreen](/source/Evergreen) broadleaved forests aligning between 40°N and 30°N, and the last belt containing tropical vegetation aligning below 30°N.[23][24]

The exact stratigraphic range of *Ephelcomenus*, while suggested to be exclusive to the Oligocene, remains uncertain.[9][11] The Eocene environmental trends and faunal assemblages of western Europe differed dramatically from those of the continent in the Oligocene due to warmer and subtropical climates plus strong levels of endemism given the isolation of western Europe as an [archipelago](/source/Archipelago) from other landmasses by the early Eocene.[25][26] The Dichobuninae, which *Synaphadus* belongs to, was one of the European artiodactyl groups endemic to Europe.[27]

Given that it was most likely exclusive to the Oligocene, it would have coexisted with post-Grande Coupure survivors as well as non-endemic immigrant faunas originating from eastern Eurasia. Examples of immigrant faunas include later anthracotheres, ruminants ([Gelocidae](/source/Gelocidae), [Lophiomerycidae](/source/Lophiomerycidae), and [Bachitheriidae](/source/Bachitheriidae)), [rhinocerotoids](/source/Rhinocerotoid) ([Rhinocerotidae](/source/Rhinocerotidae), [Amynodontidae](/source/Amynodontidae), and [Eggysodontidae](/source/Eggysodontidae)), carnivorans ([Nimravidae](/source/Nimravidae), [Ursidae](/source/Ursidae) and later Amphicyonidae), eastern Eurasian rodents ([Eomyidae](/source/Eomyidae), [Cricetidae](/source/Cricetidae), and [Castoridae](/source/Castoridae)), and [eulipotyphlans](/source/Eulipotyphla) ([Erinaceidae](/source/Erinaceidae)).[28][29][30][31]

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Adapted from the Wikipedia article [Synaphodus](https://en.wikipedia.org/wiki/Synaphodus) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Synaphodus?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.