When: Mid-Carboniferous/Early Pennsylvanian (315-312 million years ago)
Where: Nova Scotia, Canada
What: Hylonomus is the oldest well preserved fossil reptile. This small (20 cm/8 inch) long fossil would have looked in life much like a modern lizard. One key feature that separates this early reptile from basal tetrapods is a proportionally much smaller skull that had a relatively much stronger bite. A reorganization of the skull bones allowed for more growth of the muscles involved in biting, giving Hylonomus and its relatives the ability to bite though the hard shells of insects. It differs from most later reptiles in lacking any temporal fenestrae; openings behind the orbit that not only lighten the skull but provide additional surfaces for jaw muscle attachment.
Most Hylonomus fossils have been found within stumps of the ancient club moss Sigillaria. When nearby lakes flooded and these trees died, the portion of the trunk below the newly deposited lake sediments remained after the rest rotted away. Over time the insides of these buried stumps also rotted away, creating natural pit traps. Dozens of these stumps have been found in Nova Scotia, containing a wide variety of invertebrate and vertebrate fossils, including many specimens of Hylonomus.