This living creature currently exists as two distinct species. It has a tooth structure in which the teeth are fused directly to the jawbone. The female lays from 5 to 18 eggs once every 4 years and the eggs incubate for 12 to 15 months. Preferred body temperature is between 60 and 70 degrees. It has a spiny crest running from head to tail and looks much like a lizard, but isn’t. It may live for 100 years. If it loses its tail it will grow another. What is it?
[Please ensure your answer meets ALL criteria - that's not to much to ask is it?]