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 of the in what is now the Yunnan province in south-western China[2]
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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, 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 mammaliaform ever (in contrast, the related ancestral mammal would have been bigger, about 10 cm or 4 inches).
The name Hadrocodium alludes to its large cranial cavity,[7] deriving from the Greek word hadrós (ἁδρός 'large, heavy, fullness')[4] and the Latin word codium, from Greek kṓdeia (κώδεια 'head [of a plant]').[8] The species name, wui, is the Latinized version of discoverer Xiao-Chun Wu's name.[2]
Hadrocodium might 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,[9] including the middle-ear structure characteristic of modern mammals and a relatively large brain cavity.[5] These features had been considered limited to the crown group mammals, which emerged in the Middle Jurassic; the discovery of Hadrocodium suggests that these attributes appeared 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 former group.
Hadrocodium are capable of laying eggs like today's platypus and echidna.
Rowe, Timothy; Macrini, Thomas E; Luo, Zhe-Xi (2006). "Hadrocodium wui (On-line)". Digital Morphology at the University of Texas. Retrieved 1 April 2013. — 3D models from CT scans of the original fossil
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