Euroxenomys
Euroxenomys is an extinct genus of small-size beavers that lived around 23 to 5 million years ago during the Miocene. Although most specimens were identified in Central Europe, the genus was known to be widely distributed across other continents like North America and Eastern Asia. At the time, it comprised several species among which Euroxenomys minutus sp., E. inconnexus sp., E. wilsoni sp., and E. nanus sp. can be named.
Discovery
Due to the lack of fossil evidence, the taxonomy of Euroxenomys remains uncertain and has long been subject to speculations. Von Meyer was the first to describe the species minutus as part of the Chalicomys genus in 1838. This identification was based on the dentary structure relative to the morphology of both the lower premolar and the lower molar.A century later, Schreuder assigned the species to the Trogontherium genus based on minutus enlarged M3 and premolars.
It is only in 1973 that Euroxenomys will get its definitive appellation from Samson and Rädulesco, while being downgraded as a subgenus of Trogontherium. However, recent studies are now leaning towards a new hypothesis placing Euroxenomys as a proper genus morphologically independent from Trogontherium.
Phylogeny
Euroxenomys is part of the Castorids which appeared during the Late Oligocene as Asteneofiber. This stem genus further led to another one, Steneofiber, which later became at the origin of the overall diversification of the family. This radiation essentially took place during the Early Miocene and accounted for up to 30 different genera with different lifestyles and behaviours.Nowadays, only one genus remains, Castor. It is divided into two subspecies: Castor fiber and C. canadensis. Extinct castorids are usually poorly represented in the fossil record as very little material has been found so far. There is therefore relatively little evidence of their taxonomy and ecology.
Phylogenetic trees based on Rybczynski (2007)
Source:Stratigraphic range & diversification
Distribution area
Despite very poor evidence of Euroxenomys in the fossil record of Miocene faunas. the genus has been found in various localities across Central Europe and North America.Some of the newest studies have also identified these small-sized beavers in Eastern Asia, indicating a wider presence than previously expected.
Temporal range
The very first evidence of Euroxenomys originated back to the Early Miocene, whereas the last appearance was dated in the Late Miocene. This relates to different biozones ranging from MN3 to MN13. Although information is lacking to define the precise temporal range as well as the potential demographic trends of the genus, some evidence indicates that Euroxenomys was relatively rare in the Early and early Middle Miocene but quickly became abundant during the Middle and the Late Miocene.Speciation
In terms of diversity, trogontheriine beavers comprised several species which can be divided based on their spatial distribution:- † Euroxenomys minutus
- † Euroxenomys inconnexus
- † Euroxenomys wilsoni
- † ''Euroxenomys nanus''
Description
Morphological features
Euroxenomys was a relatively small-sized castorid probably comparable to common muskrats. Unlike present-time beavers, the genus lacked a wide and flattened tail while showing different long-bone proportions. The dentary structure was also specific to the genus and was characterised by an enlarged upper third molar, an elongated lower fourth premolar, and an upper fourth premolar systematically larger than the upper molars. Just like Trogontherium, Euroxenomys showed a tendency to hypsodonty, with no crown cement, combined with the absence of striated enamel in the upper incisors. Some other features like the hypostriid of the lower fourth premolar are proper to the genus, as they tend to extend down to the crown base.Note: It is worth mentioning that the study of Lechner brought a subsequent contribution to the understanding of Euroxenomys, gathering more than 1,000 new fossil data in only one locality.