Metriotherium is an extinct genus of artiodactyls belonging to the basal family Dichobunidae. It was endemic to western Europe and lived from the Early to Late Oligocene. Metriotherium and its type species M. mirabile were first named by the French naturalist Henri Filhol in 1882, although it was originally named "Mesotherium" in 1880, a name that had been preoccupied decades prior by a separate mammal genus in South America. In total, there are four valid species assigned to the genus. In comparison to its likely ancestor Dichobune, Metriotherium consists mainly of larger-sized species relative to other members of the Dichobuninae and had selenodont (crescent-like ridges) dentition. It is defined as a distinct dichobunine genus on the basis of some very specific dental morphologies, with some species being known only from isolated dental remains. Whereas the earliest-appearing species M. minutum and the latest M. sarelense were recorded to be smaller-sized species, M. mirabile, with a temporal range occurring roughly in between the two species, was larger in size. It lived in humid but seasonal climates and only briefly overlapped in range with Dichobune in the Early Oligocene before being the last dichobunid known to have existed in Europe up to the Late Oligocene.
Taxonomy In 1880, the French palaeontologist Henri Filhol said that he studied a lower maxilla with continuous dentition from the deposits of Caylux, recording that its teeth was similar to that of Anoplotherium but still had specific differences from it. He opted to assign to it the name Mesotherium mirabile. In 1882, Filhol recognized that the name Mesotherium was already used prior to his 1880 naming and hence was preoccupied. Thus, he gave it the new genus name Metriotherium, giving it a more thorough diagnosis based on its lower dentition but noting that the upper dentition was unknown. He confirmed the dental similarities to the anoplotheriids Anoplotherium and Diplobune but also stated its crest arrangements somewhat mirrored those of both Lophiodon and Pachynolophus. Filhol also named two additional genera based on dental fossils similar to that of Dichobune, each with a newly named species: Deilotherium simplex and Spaniotherium speciosum. As explained by American zoologist Theodore Sherman Palmer in 1904, the name Mesotherium as given by Filhol in 1880 was preoccupied due to it already having been used towards a genus of mammal belonging to the suborder Typotheria by the French naturalist Étienne Serres beginning in 1857. The etymology of the name Metriotherium is derived from the Ancient Greek words μέτρῐος (moderate) and θήρ (beast or wild animal) meaning "moderate beast". The other two names Deilotherium and Spaniotherium, in Ancient Greek, mean "cowardly beast" and "rare beast", respectively. In 1885, British naturalist Richard Lydekker suggested that Metriotherium might be a synonym of Lophiomeryx. On the other hand, multiple other palaeontologists of the 19th century treated Metriotherium as a valid genus, namely the Swiss palaeontologist Ludwig Rütimeyer in 1891 and Karl Alfred von Zittel in 1891–1893. The latter palaeontologist appeared to have questioned the validities of Deilotherium and Spaniotherium, evident by the question marks placed before their names. In 1906, the Swiss palaeontologist Hans Georg Stehlin discussed the taxonomic status of Metriotherium. Due to the similarities of the dental fossils assigned to the genera, Stehlin synonymized Deilotherium and Spaniotherium with Metriotherium as well as the former two genera's species (D. simplex and S. speciosum) with M. mirabile. He then erected the species M. paulum based on upper molars from the French commune of Bach. In 1980, Michel Brunet and Jean Sudre erected M. minutum based on a right-side mandible with dentition from the French locality of Villebramar. The species name is simply based on the species' small size relative to others in the same genus. In 2003, Jean Guy Astruc et al. listed the new species M. sarelense, a name credited to Sudre, one of the paper's coauthors. M. sarelense is based on dental remains, and its name is derived from its type locality of Sarèle, found in the French department of Gard. According to Jessica M. Theodor in 2007, Metriotherium is "in need of revision" due to some species only being defined by isolated teeth.
Classification
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