Hadrocodium

Hadrocodium; Temporal range: Sinemurian; ~195 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
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;

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 in what is now the Yunnan province in south-western China^[2] (25°12′N 102°06′E / 25.2°N 102.1°E / 25.2; 102.1, paleocoordinates 34°18′N 104°54′E / 34.3°N 104.9°E / 34.3; 104.9).^[3] 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.^[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]