Hylonomus (/haɪˈlɒnəməs/; hylo- "forest" + nomos "dweller") is an extinct genus of reptile that lived 312 million years ago during the Late Carboniferous period. It is the earliest unquestionable reptile (Westlothiana is older, but in fact it may have been an amphibian, and Casineria is rather fragmentary). The only species is the type species Hylonomous lyelli.
Hylonomus was 20 centimetres (8 in) long (including the tail) and probably would have looked rather similar to modern lizards. It had small sharp teeth and likely ate small invertebrates such as millipedes or early insects.
Fossils of Hylonomus have been found in the remains of fossilized club moss stumps in Joggins, Nova Scotia, Canada. It is supposed that, after harsh weather, the club mosses would crash down, with the stumps eventually rotting and hollowing out. Small animals such as Hylonomus, seeking shelter, would enter and become trapped, starving to death. An alternative hypothesis is that the animals made their nests in the hollow tree stumps. Fossils of the basal pelycosaur Archaeothyris and the basal diapsid Petrolacosaurus are also found in the same region of Nova Scotia, although from a higher stratum, dated approximately 6 million years later.
Fossilized footprints found in New Brunswick have been attributed to Hylonomus, at an estimated age of 315 million years.
This animal was discovered by John William Dawson in the mid-19th century. The species' name was given it by Dawson's teacher, the geologist Sir Charles Lyell. While it has traditionally been included in the group Protothyrididae, later studies have shown that it is probably more closely related to diapsids.
Hylonomus lyelli was named the Provincial Fossil of Nova Scotia in 2002.