Scientists say they have found a well-preserved, fossilized heart inside the skeleton of a dinosaur. Although not all paleontologists are convinced yet, the team believes that images show that this small, 66-million-year-old dinosaur pumped blood in much the same way as birds do.
Michael Hammer, a professional fossil preparator from Jacksonville, Oregon, chanced on the skeleton of a plant-eating dinosaur called Thescelosaurus in South Dakota's Hell's Creek Formation in 1993. Tucked beneath its upper ribcage was rust-colored rocky material, the kind that paleontologists typically chisel away when cleaning a specimen. Hammer thought the hunk might be a fossil-bearing concretion. "You never know what's going to be in them," he says.
To peer inside without damaging the concretion, Hammer teamed up with physician Andrew Kuzmitz and several scientists to make computerized tomography (CT) scans. The images revealed a heart, with two ventricles divided by a septum, and a single, large blood vessel--the systemic aorta--leading from the heart toward the back of the chest. One image of the rib cage and heart is so clear, Kuzmitz says, that it looks like a "carcass that should be hanging from a meat hook." The team suggests that chemical reactions between the blood- and iron-rich heart and minerals in the groundwater preserved it.
The heart's anatomy is more like that of birds and mammals than crocodiles or other reptiles, the team reports in the 21 April issue of Science  and on a special Web site . And a heart that beats like a bird's suggests that this dinosaur's metabolic rate would have been more akin to that of an endotherm (a warm-blooded animal) than an ectotherm (a cold-blooded animal), providing another feature shared by dinosaurs and their putative feathered relatives.
The fossil heart is "a fantastic discovery," says Jack Horner, a dinosaur paleontologist at Montana State University's Museum of the Rockies in Bozeman. "It just shows you what you can find if you keep your eyes and mind open." But others are skeptical, pointing out that sandstone doesn't normally preserve soft tissue. "That, and the absence of any other traces of nonskeletal tissues, raises a major red flag for me," says Paul Sereno, a paleontologist at the University of Chicago. "I'd need to examine this before I'd agree that it's a heart."