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).

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.

[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]').

[12] Dvinia Procynosuchidae Thrinaxodon Cynognathus Tritylodontidae Traversodontidae Tritheledontidae Chiniquodontidae Prozostrodon Morganucodontidae Docodonta Hadrocodium Kuehneotheriidae crown group Mammals Adelobasileus Sinoconodon Morganucodon Megazostrodon Haramiyida Haldanodon Castorocauda Hadrocodium Mammalia