Next to the mystery of what exactly killed the dinosaurs, the biggest head scratcher has been whether they were warm- or cold-blooded. Many studies have hinted that dinosaurs had an active lifestyle , a sign of warm blood, but their bones tell a different story: They often contain tree-ring-like features, called lines of arrested growth (LAGs), which have been linked to lengthy episodes of slow metabolism in a variety of modern-day reptiles and other cold-blooded creatures. In some studies of well-preserved fossils, researchers have used LAGs to help determine the ages of Tyrannosaurus rex and other dinosaurs when they died. But in a first-of-its-kind analysis, published online today in Nature, researchers report that the bones of many mammals contain LAGs, too . The scientists looked at bone samples from more than 100 wild individuals representing 41 species of ruminants—plant-eating mammals that have a four-chambered stomach—from a variety of ecosystems ranging from Norway's high-arctic islands of Svalbard to the southern tip of Africa. They found that all of the species, including the red deer Cervus elaphus (right), had LAGs (denoted by arrows in bone sample from the species, left). Although the causes of the features in these mammals aren't clear, the new finding shows that LAGs do not a cold-blooded creature make. So consider this one more dino mystery that has yet to be solved.
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