Hadrocodium

Hadrocodium wui is an extinct mammaliaform that lived during the Sinemurian stage of the Early Jurassic approximately in the Lufeng Formation in what is now the Yunnan province in south-western China (25.2°N, 102.1°W, paleocoordinates 34.3°N, 104.9°W). It is considered as the closest relative of the class Mammalia.

The fossil of this mouse-like, paper-clip sized animal was discovered in 1985 but was then interpreted as a juvenile morganucodontid. Hadrocodium remained undescribed until 2001; since then its large brain and advanced ear structure have greatly influenced the interpretation of the earliest stages of mammalian evolution, as these mammalian characters could previously be traced only to some. Hadrocodium is known only from a skull 1.2 cm long, and its body would have measured 3.2 cm long in total and weighed up to 2 g, making it one of the smallest Mesozoic mammaliaforms. The specimen is thought to have been that of a mature adult.

The name Hadrocodium alludes to its large cranial cavity, deriving from the Greek word hadrós ( 'large, heavy, fullness') and the Latin word codium, from Greek kṓdeia ( 'head [of a plant]'). The species name, wui, is the Latinized version of discoverer Xiao-Chun Wu's name.

While initially suggested to have possessed a fully mammalian ear akin to those of modern mammals, a 2022 restudy suggested that it actually had a primitive mandibular middle ear similar to those of other primitive mammaliaforms.

Phylogeny

 * Phylogeny