Hadrocodium wui (hadro from Greek ἁδρός/hadros, "fullness"; Latin: codium, from Greek κώδεια/kodeia, "head [of a plant]"; 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  in the Lufeng basin in what is now the Yunnan province in south-western China ( , paleocoordinates ).
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, 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.
Hadrocodium may have been the first animal to have a nearly fully mammalian middle ear. It is the earliest known example of several features possessed only by mammals, including the middle-ear structure characteristic of modern mammals and a relatively large brain cavity. These features had been considered limited to the crown group mammals, who emerged in the Middle Jurassic; the discovery of Hadrocodium suggests that these attributes appeared earlier (45 million years earlier) than previously thought.
- "Hadrocodium wui". Carnegie Museum of Natural History. 2001. Retrieved April 2013.
- "Tiny creature may be ancestor of all mammals". CNN. May 24, 2001. Retrieved April 2013.
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- Luo, Zhe-Xi; Crompton, Alfred W.; Sun, Ai-Lin (2001). "A New Mammaliaform from the Early Jurassic and Evolution of Mammalian Characteristics". Science 292 (5521): 1535–1540. Bibcode:2001Sci...292.1535L. doi:10.1126/science.1058476. PMID 11375489.
- Parsell, D.L. (May 24, 2001). "Tiny Fossil From Early Jurassic Fills New Niche in Mammal Evolution". National Geographic News. Retrieved April 2013.
- Rowe, Timothy; Macrini, Thomas E; Luo, Zhe-Xi (May 2011). "Fossil Evidence on Origin of the Mammalian Brain". Science 332 (955): 955–7. Bibcode:2011Sci...332..955R. doi:10.1126/science.1203117. PMID 21596988. (Supporting online material)
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