Docodonta is an order of extinct mammaliaforms that lived during the Mesozoic, from the Middle Jurassic to Early Cretaceous. They are distinguished from other early mammaliaforms by their relatively complex molar teeth, from which the order gets its name. Until recently, Docodonta were represented primarily by teeth and jaws found across former Laurasia,. However, recent discoveries in China include some exceptionally well preserved, almost completebody fossils.
Description
Docodontans are an early branch of the mammaliaform tree, but unlike other early mammals they developed a more complex tooth shape, allowing them to pierce and crush food. This tooth shape includes a series of tall cusps in two rows, and a basin between them called a pseudotalonid. The 'pseudo-' refers to this structure's resemblance to the later true talonid basins on the tribosphenic teeth of crownMammalia, which evolved convergently with those of the early docodontans. Docodonts and other Mesozoic mammals were traditionally thought to have been primarily ground dwelling and insectivorous, but recent more complete fossils from China have shown this is not the case. Castorocauda from the Middle Jurassic of China, and possibly Haldanodon from the Upper Jurassic of Portugal, were specialised for a semi-aquatic lifestyle. Castorocauda had a flattened tail and recurved molars, which suggests possible fish or aquatic invertebrate diet. It was thought possible that docodonts had tendencies towards semi-aquatic habits, given their presence in wetland environments, although this could also be explained by the ease with which these environments preserve fossils compared with more terrestrial ones. Recent discoveries of other complete docodontans such as the specialised digging species Docofossor, and specialised tree-dweller Agilodocodon suggest Docodonta was more ecologically diverse than previously suspected. Docofossor shows many of the same physical traits as the modern daygolden mole, such as wide, shortened digits in the hands for digging.
Taxonomy
Docodonts are not as closely related to the placentals and marsupials as the monotremes, and are not included in the crown-group mammals. The complexity of their molars and the fact that they possess the dentary-squamosal jaw joint, means that they were previously sometimes regarded as belonging to Mammalia. Modern authors usually limit the term "Mammalia" to the crown group, which excludes earlier mammaliaforms like the docodontans, but they are considered to be closer to Mammalia than many other early mammalian groups such as Morganucodonta, Kuehneotheriidae, Haramiyida and Sinoconodon. One disputed docodont, Gondtherium, has been described from India, which was previously part of the Southern Hemisphere continent of Gondwana. However this identification is not certain, and some authors have chosen to exclude it from their analyses of Docodonta. In recent analyses, Gondtherium falls outside the docodont family tree. There has also been a suggestion of erecting two sub-families within Docodonta, but these groups are not found in any other analyses, and therefore not accepted by all mammal palaeontologists.
