Docodonta is an order of extinct mammaliaforms...
that lived during the mid- to late-Mesozoic era. Their most distinguishing physical features were their relatively sophisticated set of molars, from which the order gets its name. In the fossil record, Docodonta is represented primarily by isolated teeth and bits of jawbones.
While most of these specimens have been found across former Laurasia (modern-day North America, Europe, and Asia), some have also been found from Gondwana (modern-day India and Southern Hemisphere).
Docodonts are not quite as closely related to the placentals and marsupials as the monotremes are, so they are not included among the crown-group mammals. Because of the complexity of their molars and the fact that they possess the dentary-squamosal jaw joint, though, they have generally been regarded as mammals. Some authors do limit the term "Mammalia" to the crown group, however, excluding mammaliaforms like the docodonts.
Castorocauda (pictured above) is a genus of small, semi-aquatic mammal relatives living in the Jurassic period, around 164 million years ago, found in lakebed sediments of the Daohugou Beds of Inner Mongolia. It contains the single species Castorocauda lutrasimilis. They were highly specialized, with adaptations evolved convergently with those of modern semi-aquatic mammals such as beavers, otters, and the platypus...
illustration by Nobu Tamura
