JOE TUCCIARONE / SCIENCE PHOTO LIBRARY JOE TUCCIARONE / SCIENCE PHOTO LIBRARY
Dimetrodon, computer artwork. Like humans, the predatory synapsid Dimetrodon (meaning 'two-measure tooth') had two kinds of teeth that enabled it to both pierce and chew. It was not a dinosaur, but a synapsid, meaning it was more closely related to the mammals than the reptiles. It lived in the early Permian period, around 270 million years ago. Many scientists believe its dorsal sail was used to regulate its body temperature.
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