Docodonta

Docodontans Temporal range: Middle Jurassic-Early Cretaceous Bathonian–Aptian PreꞒ Ꞓ O S D C P T J K Pg N
Skeletal diagrams of Borealestes serendipitus (green) and B. cuillinensis (blue) Scale bars = 10 mm
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Synapsida
Clade:	Therapsida
Clade:	Cynodontia
Clade:	Mammaliaformes
Order:	†Docodonta Kretzoi, 1946
Genera
See text.

Docodonta is an order of extinct Mesozoic mammaliaforms (advanced cynodonts closely related to true crown-group mammals). They were among the most common mammaliaforms of their time, persisting from the Middle Jurassic to the Early Cretaceous across the continent of Laurasia (modern-day North America, Europe, and Asia). They are distinguished from other early mammaliaforms by their relatively complex molar teeth. Docodontan teeth have been described as "pseudotribosphenic": a cusp on the inner half of the upper molar grinds into a basin on the front half of the lower molar, like a mortar-and-pestle. This is a case of convergent evolution with the tribosphenic teeth of therian mammals. There is much uncertainty for how docodontan teeth developed from their simpler ancestors. Their closest relatives may have been certain Triassic "symmetrodonts", namely Woutersia, and Delsatia.^[1] The shuotheriids, another group of Jurassic mammaliaforms, also shared some dental characteristics with docodontans. One study has suggested that shuotheriids are closely related to docodontans,^[2] though others consider shuotheriids to be true mammals, perhaps related to monotremes.^[3]

For much of their history of study, docodontan fossils were represented by isolated teeth and jaws. The first docodontan known from decent remains was Haldanodon, from the Late Jurassic Guimarota site of Portugal. Recently, exceptionally preserved skeletons have been discovered in the Jurassic Tiaojishan Formation of China. Chinese docodontans include otter-like,^[4] mole-like,^[5] and squirrel-like species,^[6][7] hinting at impressive ecological diversity within the group. Many docodontans have muscular limbs and broad tail vertebrae, adaptations for burrowing or swimming. Like true mammals, docodontans have hair,^[4] a saddle-shaped hyoid apparatus,^[7] and reduced postdentary jaw bones which are beginning to develop into middle ear ossicles. On the other hand, the postdentary bones are still attached to the jaw and skull, the nares (bony nostril rims) have yet to fuse, and in most species the spine's thoracic-lumbar transition is rather subdued.^[5][6]