Hylonomus

Hylonomus (; from Greek , meaning , and , meaning ) is an extinct genus of early (possibly stem group) amniote that lived during the Bashkirian stage of the Late Carboniferous. The genus contains a single species, Hylonomus lyelli.

Hylonomus was originally interpreted as a 'microsaur' or ancestral to crown group amniotes. Although a number of early 21st century cladistic analyses have suggested that Hylonomus is the earliest known reptile, a 2025 analysis placed Hylonomus outside of crown group Amniota, which would make it less closely related to modern reptiles than mammals are.