Hadrocodium

Hadrocodium wui (hadro from Greek ἁδρός/hadros, "fullness";[1] Latin: codium, from Greek κώδεια/kodeia, "head [of a plant]";[2] and wui, the Latinized version of discoverer Xiao-Chun Wu's name[3]) is an extinct mammaliaform that lived during the Sinemurian stage of the Early Jurassic approximately 195 million years ago[4] in the Lufeng basin in what is now the Yunnan province in south-western China (

25.2°N 102.1°E, paleocoordinates

34.3°N 104.9°E).[5]

The fossil of this mouse-like, paper-clip sized animal was discovered in 1985 but was then interpreted as a juvenile morganucodontid.[1] Hadrocodium remained undescribed until 2001; since then its large brain and advanced ear structure[6] 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.[7] 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,[8] including the middle-ear structure characteristic of modern mammals and a relatively large brain cavity.[6] 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.

Whether Hadrocodium was warm-blooded or cold-blooded has not been settled, although its apparent nocturnal features would seem to place it in the endotherm group.