Leptotheridium
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| Leptotheridium Temporal range: Middle Eocene to Late Eocene | |
|---|---|
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Genus: | †Leptotheridium Stehlin, 1910 |
| Type species | |
| †Leptotheridium lugeoni Stehlin, 1910 | |
| Other species | |
| |
Leptotheridium is an extinct genus of Palaeogene artiodactyl endemic to western Europe that lived from the Middle to Late Eocene. It was erected by the Swiss palaeontologist Hans Georg Stehlin in 1910 and contains the species L. lugeoni and L. traguloides. Its phylogenetic position is unclear, with researchers determining that it belonged to either the Anoplotheriidae (specifically the subfamily Dacrytheriinae) or the Xiphodontidae due to its dental and postcranial anatomy. The small-sized artiodactyl genus is unique from its close relatives in that it seemingly lacks the first lower premolars, weak selenodonty (crescent-like ridges) in its dentition, and has three-lobed canines. It is one of the earlier artiodactyl species to have appeared in the fossil record of Europe.
Classification
In 1910, the Swiss palaeontologist Hans Georg Stehlin erected the genus Leptotheridium, which he stated had a dental form similar to that of Catodontherium. The first species that he created was Leptotheridium Lugeoni, designating it as the type species. The species was established based on a partial maxilla with dentition from the Swiss locality of Mormont and was first designated to Rhagatherium valdense by François Jules Pictet de la Rive and Aloïs Humbert in 1869. The second species that he named was L. traguloïdes based on a maxilla fragment from a locality in Egerkingen that was initially classified to Mixtotherium gresslyi by Ludwig Ruetimeyer in 1891.[a][1][2][3]
The taxonomic position of Leptotheridium had long been in dispute as palaeontologists had classified it either to the Anoplotheriidae or Xiphodontidae, two artiodactyl families that were endemic to western Europe during the Palaeogene. In 1910, Stehlin suggested that Leptotheridium was close in affinity to Dacrytherium and Catodontherium, members of the anoplotheriid subfamily Dacrytheriinae.[4][1] In 1917, the French palaeontologist Charles Depéret placed Leptotheridium in the Dacrytheriidae (now an anoplotheriid subfamily).[5] The systematic placement of Leptotheridium within the Dacrytheriinae (or Dacrytheriidae) had been followed by other palaeontologists like Jean Viret in 1961 and Jean Sudre in 1978.[6][7]
In 2000, the palaeontologists Jerry J. Hooker and Marc Weidmann reclassified Leptotheridium to the Xiphodontidae, rejecting its previous classification to the Dacrytheriinae due to differences in dental and postcranial anatomy as well as the lack of any preorbital fossa. He argued that its dentition was very to that of Xiphodon, thus further supporting the reclassification.[8] In 2006, Miguel Angel Cuesta et al. chose to follow Hooker and Weidmann in the classification of Leptotheridium to the Xiphodontidae instead of the Anoplotheriidae.[9]
In 2022, Weppe created a phylogenetic analysis in his academic thesis regarding Palaeogene artiodactyl lineages, focusing most specifically on the endemic European families. The phylogenetic tree, according to Weppe, is the first to conduct phylogenetic affinities of all anoplotheriid genera, although not all individual species were included. His research placed Leptotheridium into a clade with the dacrytheriines Catodontherium and Dacrytherium, thus positioning it as a member of the Dacrytheriinae rather than the Xiphodontidae.[10]
Description
Leptotheridium is described as being a small-sized artiodactyl that lacks preorbital fossae and is diagnosed as having poorly developed selenodont (crescent-like ridges) crests in its dentition, especially evident by the forms of the postprotocrista and postparaconule ridges on the upper molars (M/m).[4][7] It was smaller than Catodontherium and Dacrytherium in size and has overall bunoselenodont (bunodont (rounded) and selenodont dentition).[11] Unlike with other members of the Anoplotheriidae and Xiphodontidae known by dental sets,[12][13][14] that of Leptotheridium is incomplete (meaning that it has fewer than 44 total teeth) because of the lack of a first lower premolar (P/p), or P1.[4]
The canines (C/c) are incisiform (incisor-shaped (I/i)) but are divided into three deep lobes (trilobed). The premolars are not shaped like molars, with P4 being elongated and having an external conical point. P4 is slightly elongated with a slightly wide back area. P2 is very similar to P3 in shape but differs by its narrower shape. The upper molars are roughly quadrangular in shape and contain prominent paraconule cusps, which along with the mesostyle cusps are prominent like in Catodontherium but less spherical than in Dacrytherium. There are diastemata in between both the lower canine and P2 and the P2 and P3 teeth; the canine still remains in contact with the incisors.[4][7]
Leptotheridium was additionally previously by an astragalus previously assigned to it that was described as being very narrow and elongated with a narrow tibial groove and a straight bone axis. However, it was later reassigned to Haplomeryx while some postcranial fossils that were assigned previously to Xiphodon were reclassified to Leptotheridium. In terms of the reassigned astragalus, it is narrow plus elongated in form, its tibial groove appearing narrow but deep. The back calcaneal facet, occupying a significant portion of the astragalus' back face, is wide. The calcaneum appears similar to that of Dacrytherium but differs by a more elongated back tuberosity.[5][8]
