Hylonomus ("forest mouse") was an early reptile. It lived 315 million years ago during the Carboniferous period. As of 2006 it is the earliest confirmed reptile (Westlothiana is older but may be an amphibian). It was 20 cm long (including the tail) and probably would have looked rather similar to modern lizards. It had small sharp teeth and probably ate millipedes and early insects. It is a precursor of later reptiles. Its predators were the giant dragonflies, Mesothelae spiders, large arthropods, and other tetrapods such as Eryops, and Archaeothyris.
Fossils of Hylonomus have been found in the remains of fossilized tree stumps in Joggins, Nova Scotia. It is thought that after harsh weather, the tree tops would crash down and the stumps became hollowed out. Hylonomus individuals seeking shelter would enter but get trapped, starving to death.
This animal was discovered by Sir William Dawson in the 1800s. The name comes from the Greek word for wood, the Latin for mouse (forest mouse), and the name of the geologist, and Dawson's teacher, Sir Charles Lyell.