'', an early amynodont known from the Early to Middle Eocene of
Inner Mongolia in China Amynodonts probably originated in Asia during the
Early Eocene. The oldest known Asian amynodont is
Andarakodon, from the Early Eocene of
Kyrgyzstan. The Early–Middle Eocene deposits of the
Irdin Manha Formation in China have yielded fossils of the oldest well-defined member of the group,
Rostriamynodon.
Caenolophus, contemporary with
Rostriamynodon, has been suggested to be the most primitive amynodont but its placement in Amynodontidae is disputed;
Caenolophus has also been classified as a hyracodont, and appears to be anatomically intermediate between the two families.
Teilhardia, known from the same area, has also been suggested to be either a hyracodont or a primitive amynodont but it is fragmentarily known and possibly synonymous with
Caenolophus. Amynodonts spread to North America shortly after their initial appearance and achieved a
Holarctic distribution. They became one of the dominant herbivore groups of their time. In North America, the temporal range of the amynodonts extends from the Middle Eocene to the
Early Oligocene. The earliest known North American example of the group is
Amynodon, from the late
Bridgerian and
Uintan land mammal ages. The amynodonts were at their greatest taxonomic diversity in Asia, especially in
Central Asia. Amynodonts increased in body size over the course of their evolutionary history, the smallest known amynodont is the relatively basal "
Amynodon"
sinensis, estimated at 127 ± 15 kg (280 ± 33 lbs), whereas the largest are the derived
Zaisanamynodon borisovi, at 2442 ± 257 kg (5384 ± 567 lbs), and
Procadurcodon orientalis, at 2720 kg (5997 lbs).) of
Cadurcotherium nouleti, known from the Oligocene of France and SwitzerlandMigration across
Beringia was highly important in amynodont evolution. The ancestors of the North American
Amynodon and
Amynodontopsis bodei were probably amynodonts that migrated from Asia separately;
Amynodontopsis bodei is more derived than
Amynodon but likely descended from the Asian species
Amynodontopsis jiyuanensis. Phylogenetic analyses suggest that there may have been as many as four or five dispersal events where new amynodonts spread from Asia into North America. There were also several dispersal events into Europe.
Cadurcotherium is the only genus recorded from Western Europe but
Amynodontopsis,
Cadurcodon, and
Sellamynodon are all known from Eastern Europe. Amynodonts do not appear to have reached Northern Europe. The migration of amynodonts into Europe in the Oligocene may have been facilitated by the preceding
Grande Coupure extinction event. The Middle to Late Eocene was a time of great environmental change, which may have facilitated the
evolutionary radiation of the amynodonts. In North America, the environment changed during this time from lush
semitropical forests to being dominated by open forests and grasslands. The timeframe corresponding to the late Uintan and the
Duchesnean land mammal ages in North America saw the emergence of the cadurcodontines and metamynodontines, illustrating diversification and pursuits of different lifestyles. Similar to North American examples, Asian amynodonts also appear to diversify around the boundary between the Eocene and Oligocene. The metamynodontines experienced evolutionary radiation in North America, represented by the genera
Megalamynodon and
Metamynodon. The record of carcudontines in North America is sparse and depends on the phylogenetic position of
Amynodontopsis and the assessments of the species
Zaisanamynodon protheroi and
Amynodon reedi. Amynodonts were more successful in Asia than in North America, perhaps due to the increasing aridity in North America going into the Oligocene. Metamynodontines survived in North America into the Oligocene;
Metamynodon lived in river system across most of the continent, but was a rare element of the Oligocene fauna and was extinct by the end of the
Whitneyan land mammal age (late Early Oligocene). Amynodonts in Europe disappeared in the latest Oligocene. Amynodonts survived the longest in Asia, where
Cadurcotherium persisted in modern-day Pakistan until the latest Oligocene. The last Asian amynodonts were previously believed to have reached into the early
Miocene, but this dating has proven incorrect. ==References==