2002. It 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.

Discovery and naming

Hylonomus lyelli was first described by John William Dawson in 1860. The species'

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