A computerized tomography (CT) scan of the chest cavity of a new dinosaur fossil reveals a heart more closely resembling a bird or mammal organ, rather than a modern reptile's. This discovery, which suggests that the dinosaur was warm-blooded, with a relatively high metabolism, is reported in the 21 April issue of Science. When Michael Hammer, professional fossil collector and co-author of the Science paper, discovered the remains of a dinosaur in South Dakota, he guessed from the unusually well-preserved ribs that its chest cavity might hold some internal organs.
Although researchers don't fully understand how this happens, animals' soft tissues can sometimes become fossilized in oxygen-poor environments, such as the sediment beneath a streambed.
Instead of paring away the bones from their surroundings, Hammer carefully cleaned the surface of the skeleton. Before sending the specimen to its new owners at the North Carolina State Museum of Natural Sciences, physician Andrew Kuzmitz had its chest region CT scanned. A team from the Museum and from North Carolina State University produced three-dimensional images from the CT data and analyzed the results.
The images revealed two neighboring cavities and a single tubular structure positioned like an aorta. The cavities are probably the heart's lower chambers, the ventricles. The walls of the upper chambers, the atria, are usually very thin and probably collapsed when the dinosaur died. Birds and mammals also have four-chambered hearts with a single aorta.
Dinosaurs have traditionally been considered to be more closely related to modern reptiles, which have two aortas and a simpler heart structure that allows some mixing between the oxygen-rich blood from the lungs and the oxygen-poor blood from the body. This mixing reduces the overall oxygen content of the blood supplied to the reptiles' tissues.
"It's truly amazing that this animal seems to have had such a highly-evolved heart. The implications completely floored me," said Science co-author Dale Russell, of the North Carolina State Museum of Natural Sciences and North Carolina State University. Russell's research team also includes Paul Fisher, Michael Stoskopf, and Reese Barrick, of North Carolina State University, Kuzmitz, a physician in private practice, and Hammer, whose company is called Hammer and Hammer Paleotek.
Four-chambered hearts deliver completely oxygenated blood to the body, which fuels the relatively fast metabolisms of birds and mammals. In contrast, living reptiles tend to be more sluggish and can get by with less oxygen.
The team therefore suspects that their dinosaur may have also had a metabolic rate higher than that of a typical reptile. Since most modern animals with high metabolic rates are also warm-blooded, this prospect leads to the tantalizing possibility that the dinosaur was also warm-blooded.
The dinosaur has yet to be formally classified, but currently goes by the name "Willo." It's a two-legged, 13 foot-long thescelosaur, a member of the ornithiscian, or "bird-hipped," group of dinosaurs.
Although their name indicates otherwise, the saurischian, or "lizard-hipped," dinosaurs are actually the ones that many researchers suspect eventually gave rise to birds. Therefore it might be concluded that the ancestors to both ornithiscian and saurischian dinosaurs also had advanced hearts and high metabolisms. The age difference between Willo and the ancestral dinosaurs is so great, however, that four-chambered hearts may also have arisen independently in the dinosaurean to bird lineage and in ornithiscians, the researchers say.
What the heart structure can reveal about dinosaurs' activity levels and the cold- v. warm-blooded debate is a question that "begs for more work," Russell said.
To that end, the authors urge in their paper that fossil hunters make a point of looking for traces of soft tissue in their discoveries. In the past, much of this evidence was probably overlooked by researchers seeking only bones.
"In order to clean a specimen up to display as a skeleton, you'd throw everything away that wasn't bone. It makes you wonder how much stuff we've missed," said co-author Paul Fisher.
