DALLAS (AP) — Scientists once thought of dinosaurs as sluggish, cold-blooded creatures. Then research suggested that some could control their body temperature, but when and how that shift came about remained a mystery. Now, a new study estimates that the first warm-blooded dinosaurs may have roamed the Earth about 180 million years ago, about halfway through the creatures’ time on the planet. Warm-blooded creatures — including birds, who are descended from dinosaurs, and humans — keep their body temperature constant whether the world around them runs cold or hot. Cold-blooded animals, including reptiles like snakes and lizards, depend on outside sources to control their temperature: For example, basking in the sun to warm up. Knowing when dinosaurs evolved their stable internal thermometer could help scientists answer other questions about how they lived, including how active and social they were.

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