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.
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, 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, which emerged in the Middle Jurassic; the discovery of Hadrocodium suggests that these attributes appeared 45 million years earlier than previously thought.
^Close, Roger A.; Friedman, Matt; Lloyd, Graeme T.; Benson, Roger BJ (2015). "Evidence for a mid-Jurassic adaptive radiation in mammals". Current Biology. 25 (16): 2137–2142. doi:10.1016/j.cub.2015.06.047. PMID26190074.
"Hadrocodium wui". Carnegie Museum of Natural History. 2001. Archived from the original on 2012-06-17. Retrieved April 2013. Cite uses deprecated parameter |deadurl= (help); Check date values in: |accessdate= (help)
Rowe, Timothy; Macrini, Thomas E; Luo, Zhe-Xi (2006). "Hadrocodium wui (On-line)". Digital Morphology at the University of Texas. Retrieved April 2013. Check date values in: |accessdate= (help) — 3D models from CT scans of the original fossil