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 1.2 cm (0.47 in) long, and its body would have measured 3.2 cm (1.3 in) long in total and weighed up to 2 g (0.071 oz), making it one of the smallest Mesozoic mammaliaforms.[1][7][8] The specimen is thought to have been that of a mature adult.[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]').[11] The species name, wui, is the Latinized version of discoverer Xiao-Chun Wu's name.[2]
While initially suggested to have possessed a fully mammalian ear akin to those of modern mammals, a 2022 restudy suggested that it actually had a primitive mandibular middle ear similar to those of other primitive mammaliaforms.[9]
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