Several complete skeletons hint at sudden, mass extinctionResearchers from the Whiteside Museum of Natural History in Seymour are unraveling an ancient mystery. "This is life," said Coleton Caldwell, assistant director of the museum. "The first time life is living on land, solely on land, and it's still trying to figure things out, you know? What works, what doesn't work? So it's just really, really, really unique." Working southwest of Wichita Falls, near the shore of Lake Kemp in Baylor County, the researchers have uncovered the skeletal remains of seven dimetrodons. The mammal-like finback reptiles roamed parts of North Texas 60...