What it is
A small, slim animal shaped like a lizard, about eight inches long with its tail. It lived about 315 million years ago, in a swamp forest in what is now Nova Scotia. Its name means forest dweller.
A tree that became a trap
When a river burst its banks, sand buried the feet of the trees and killed them. Their insides rotted, and the stumps stood hollow. Snails, millipedes and small four-legged animals found their way in. The next flood filled the stump with mud and sealed them there. Hylonomus was found in one of those stone stumps at Joggins, on the Bay of Fundy, by the geologist William Dawson in 1859.
Why it matters
Reptiles lay their eggs on land, so they never have to go back to the water. In Dawson's day the word reptile covered amphibians too, and it took until 1964 for a scientist named Robert Carroll to show that Hylonomus was really built like one. For a hundred and sixty years, it was the oldest reptile anyone had found. Then, in 2025, tracks from Australia pushed reptile-like animals back about 35 million years, and a new study argued Hylonomus sits just outside the reptile family. The title is being argued again.
Read the sources
- The Joggins Fossil Cliffs UNESCO World Heritage site: a review · Atlantic Geology (2011)
- Coal Age Galapagos: Joggins and nineteenth-century geology · Atlantic Geology (2006)
- The earliest reptiles, by Robert Carroll · Journal of the Linnean Society (1964)
- Earliest amniote tracks recalibrate the timeline · Nature (2025)