Sinoconodon

Sinoconodon is an extinct genus of mammaliamorphs that appears in the fossil record of the Lufeng Formation of China in the Sinemurian stage of the Early Jurassic period, about 193 million years ago. While sharing many plesiomorphic traits with other non-mammaliaform cynodonts, it possessed a special, secondarily evolved jaw joint between the dentary and the squamosal bones, which in more derived taxa would replace the primitive tetrapod one between the articular and quadrate bones. The presence of a dentary-squamosal joint is a trait historically used to define mammals.

==Description== left|thumb|216x216px|Fossil skull, Baoding Natural History Museum This animal had a skull of , which suggests a presacral body length of and a weight of about due to the similar parameters to the European hedgehog. Sinoconodon closely resembled early mammaliaforms like Morganucodon, but it is regarded as more basal, differing substantially from Morganucodon in its dentition and growth habits. Like most other non-mammalian tetrapods, such as reptiles and amphibians, it was polyphyodont, replacing many of its teeth throughout its lifetime, and it seems to have grown slowly but continuously until its death. It was thus somewhat less mammal-like than mammaliaforms such as morganucodonts and docodonts. The combination of basal tetrapod and mammalian features makes it a unique transitional fossil.