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About Hylonomus:
It's always possible that a more ancient candidate will be discovered, but as of now, Hylonomus is the oldest reptile known to paleontologists: this tiny critter scuttled around the forests of the Carboniferous period over 300 million years ago. Based on reconstructions, Hylonomus certainly looked distinctly reptilian, with its quadrupedal, splay-footed posture, long tail, and sharp teeth.
Hylonomus is also a good object lesson in how evolution works. You might be surprised to learn that the oldest ancestor of the mighty dinosaurs (not to mention modern crocodiles and monitor lizards) was about the size of a small gecko, but new life forms have a way of "radiating" from very small, simple progenitors. For example, all mammals alive today--including humans and sperm whales--are ultimately descended from a mouse-sized ancestor that scurried beneath the feet of dinosaurs 100 million years ago.
