Hylonomus (Greek for "forest mouse"); pronounced high-LON-oh-muss
Forests of North America
Carboniferous (315 million years ago)
Size and Weight:
About one foot long and one pound
Tiny size; sharp teeth
It's always possible that a more ancient candidate will be discovered, but as of now, Hylonomus is the earliest true 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 birds) 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 huge dinosaurs more than 200 million years ago.