Hylonomus and the egg that freed vertebrates from the water

What happened

Near Joggins, Nova Scotia, fossil hunters found the remains of Hylonomus lyelli sealed inside the hollow, rotted-out stumps of fossilized club-moss trees, a small, 20-to-25-centimetre lizard-like animal that had likely crawled in seeking shelter and become trapped. Hylonomus is considered the earliest known reptile, and, more specifically, the earliest known amniote, the group whose eggs carry their own protective membrane and no longer need to be laid in water. That single innovation, a self-contained egg an animal could lay on dry land, is what let its descendants push inland, away from the rivers, swamps, and coasts every vertebrate before them had stayed tied to.

How we know

Hylonomus is known from multiple articulated skeletons preserved inside the fossilized tree stumps of the Joggins Formation, now a UNESCO World Heritage Site specifically recognized for this fossil record. Its classification as the earliest amniote rests on skeletal features, including skull and limb characteristics, that distinguish amniotes from the amphibians that came before them, though the egg itself does not fossilize and its structure is inferred from its descendants.

Sources

• Smithsonian Magazine. Tiny Animals Trapped in Fossil Trees Help Reveal How Fauna Moved Onto Land · Reputable sourcesmithsonianmag.com · The domain "smithsonianmag.com" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
• CBC News. Oldest reptile fossil returns to N.S. (2011) · Reputable sourcecbc.ca · The domain "cbc.ca" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)