|Subfamilies and Genera|
Phenacodontidae is an extinct family of large herbivorous mammals traditionally placed in the “wastebasket taxon” Condylarthra, which may instead represent early-stage perissodactyls. They lived between the Paleocene and Eocene epochs (about 60–50 million years ago) and their fossil remains have been found in North America and Europe.
These animals had a variety of body sizes, and could be as small as domestic cats (Tetraclaenodon and Ectocion) and as large as sheep (Phenacodus). The skull of phenacodontids is long and narrow, and equipped with a small braincase.
The skeleton of phenacodontids show several primitive characteristics (the long and heavy tail for example) but also a number of advanced, Perissodactyla-like adaptations: Their long legs, for example, had five fingers, but the first finger showed a clear reduction, and in some forms (like Phenacodus) the fifth finger was reduced as well.
Some species had tapir-like adaptations suggestive of the presence of a short proboscis or a strong prehensile lip.
The teeth of phenacodontids, particularly in the latter forms, were quite specialized: The molars and premolars were equipped with low cusps that sometimes joined in ridges, similar to the condition found in some perissodactyls. Some forms, like Meniscotherium, had enlarged ridges. This adaptation is unusual for mammals as old as phenacodontids.
The phenacodontids evolved in the middle Paleocene in North America. Early forms were usually small; Tetraclaenodon, for example, was the size of a fox. Later forms were much larger and invaded Europe, although they never became as plentiful as in North America.
Towards the beginning of the Eocene these animals slowly disappeared from the fossil record. Only a few forms survived into the middle Eocene: the Phenacodus in Europe and North America, Almogaver in Europe and Ectocion in North America.
An exception to the scarcity of Eocene phenacodontids is the dog-sized genus Meniscotherium, whose fossils are very abundant.
According to more recent views, instead of a monophyletic clade, the Condylarths are better understood as an evolutionary grade that lead to the true ungulates. Indeed, recent phylogenetic studies confirm that phenacodonts were most closely related to modern odd-toed ungulates.
The specialized teeth found in at least some phenacodontids seem to indicate a primary herbivorous lifestyle. The shape of the legs indicated that some phenacodontids (like Phenacodus) were swift runners.
- Radinskya, a basal perissodactyl from the Paleocene of China
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