Respiratory physiologist John Ruben has cast a whole new light on the controversy over whether dinosaurs were warm-blooded. Using ultraviolet (UV) light from an ordinary UV lamp, Ruben was able to identify the outlines of the intestines, liver, trachea, and muscles of an incredibly well-preserved 100-million-year-old Scipionyx, a raptor or small meat eater. Writing in today's issue of Science, Ruben and his colleagues at Oregon State University in Corvallis argue that the arrangement of those internal organs suggests that dinosaur lungs were structurally simple. That in turn suggests these animals were basically cold-blooded, with spurts of high metabolic activity.
Ruben's adventure began when he hauled an 80-watt UV lamp from his lab to an archaeology office in Salerno, Italy. The UV light, which can coax out patterns invisible in ordinary light, revealed Scipionyx's liver and a muscle next to the pubic bone that looked remarkably like that of modern crocodiles. In today's crocs, the muscle runs from the pubis to the liver and helps move the liver back and forth like a piston, causing the lungs to expand and contract.
Finding the same arrangement in theropod dinosaurs rules out the possibility that they breathed with the type of sophisticated lung that supports birds' high metabolism, says Ruben. But because raptors like Scipionyx were among the most dynamic dinosaurs, Ruben began to question the assumption that a bird or mammal lung is needed for high metabolism. These simple lungs were also able to power periods of high metabolism and intense activity, he now believes, a theory that provides a hybrid answer to the old question of whether dinosaurs were cold- or warm-blooded creatures.
"If they're right, this could represent landmark work suggesting a whole new way to view dinosaur physiology," says anatomist Lawrence Witmer at the Ohio University College of Osteopathic Medicine in Athens. Coupling an economical resting metabolism with a capacity for bursts of activity may have been the best of all possible metabolic worlds, says paleontologist James Farlow at the Indiana University-Purdue University in Fort Wayne. If dinosaurs possessed that combination of features, Farlow reasons, "it's not surprising that they ruled Earth for over 100 million years."