Produced by the Population Genetics and Evolution class, Furman University

The Jurassic: Hadrocodium
Hadrocodium is the earliest known mammal, differentiated from non-mammalian vertebrates by its large brain and advanced ear structure. It is also one of the smallest known mammals, with an estimated adult body weight of only two grams. This incredibly small size would make sense for a large rate of morphological innovations due to a high rate of reproduction (White 2005). Several suites of mammalian apomorphies are observed. Inside the jaw, the lack of a concavity and a postdentary trough suggests that the machinery of the inner ear had already begun to move towards the cranium. Also, jaw articulaton is moved anteriorly, spreading jaw musculature forward, allowing room for a larger brain - likely due to the separation of hearing from eating (Luo 2001). Hadrocodium’s small size and large brain would most likely have given it a very high metabolism. It would have had to eat almost constantly, likely a diet of small insects and worms. This diet is also backed up by its “triconodont-like” teeth, for insectivory (Luo 2001). It is more closely related to the mammals of today then cynodonts (mammal-like reptiles; White 2008).

Page by Megan Aprill
Hadrocodium. Picture From:darwiniana.org

Luo Z. 2001. A new mammaliaform from the early Jurassic and evolution of mammalian characteristics Science 292: 1535-1540

White AT. 2005. Hadrocodium. www.palaeos.com. Accessed March 28, 2010.

White R. 2008. Evolution- Charles Darwin & A.R. Wallace. Accessed March 28, 2010.