| Named By: | Z.inX. Luo, A. W. Crompton & A.inL. Sun in 2001 |
| Time Period: | Early Jurassic, 195 Ma |
| Location: | China, Yunnan Province - Lower Lufeng Formation |
| Size: | Estimated about 3.2 centimetres long |
| Diet: | Insectivore |
| Fossil(s): | Skull |
| Classification: | | Chordata | Mamaliaformes | |
Hadrocodium wui (hadro from Greek adros/hadros, "large, heavy, fullness"; Latin: codium, from Greek kodeia/kodeia, "head [of a plant]" (alluding to its enlarged cranial cavity); and wui, the Latinized version of discoverer Xiao-Chun Wu's name) is an extinct mammaliaform that lived during the Sinemurian stage of the Early Jurassic approximately 195 million years ago in the Lufeng basin in what is now the Yunnan province in south-western China (25.2degN 102.1degE / 25.2; 102.1, paleocoordinates 34.3degN 104.9degE / 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. 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 150 million years ago. Hadrocodium is known only from a skull, but the body is estimated to have been a mere 3.2 cm (1.3 in) in length and about 2 g (0.071 oz) in mass, making it one of the smallest mammals ever.