Hadrocodium

Hadrocodium wui is an extinct mammaliaform that lived during the Sinemurian stage of the Early Jurassic approximately 195 million years ago[1] in the Lufeng Formation of the Lufeng Basin in what is now the Yunnan province in south-western China[2] (25.2°N 102.1°E / 25.2; 102.1, paleocoordinates 34.3°N 104.9°E / 34.3; 104.9).[3] It is considered as the closest relative of the class Mammalia.

Hadrocodium Temporal range: Sinemurian ~
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Synapsida
Clade:	Therapsida
Clade:	Cynodontia
Clade:	Mammaliaformes
Genus:	†Hadrocodium Luo, Crompton, & Sun, 2001
Species
H. wui Luo, Crompton, & Sun, 2001

The fossil of this mouse-like, paper-clip sized animal was discovered in 1985 but was then interpreted as a juvenile morganucodontid.[4] Hadrocodium remained undescribed until 2001; since then its large brain and advanced ear structure[5] have greatly influenced the interpretation of the earliest stages of mammalian evolution, as these mammalian characters could previously be traced only to some 150 million years ago.[6] Hadrocodium is known only from a skull 1.2 cm (0.47 in) long, and its body would have measured 3.2 cm (1.3 in) long in total and weighed up to 2 g (0.071 oz), making it one of the smallest Mesozoic mammaliaforms.[1][7][8] The specimen is thought to have been that of a mature adult.[9]

The name Hadrocodium alludes to its large cranial cavity,[10] deriving from the Greek word hadrós (ἁδρός 'large, heavy, fullness')[4] and the Latin word codium, from Greek kṓdeia (κώδεια 'head [of a plant]').[11] The species name, wui, is the Latinized version of discoverer Xiao-Chun Wu's name.[2]

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.[9]