Linked from
The 7 pages that link to Hylonomus, each with the reason it gives.
CarboniferousBroader topic: It is among the oldest known animals commonly classified as a reptile.
SynapsidRelated: Its age places early amniotes near the beginning of synapsid history.
Amniotic eggBroader topic: Its fossils are among the oldest evidence of animals with amniote traits.
ReptiliaBroader topic: It is among the oldest known animals often identified as a reptile.