CORVALLIS, Ore. - Scientists at Oregon State University who just completed a study of what they say is the world's most perfectly preserved fossil of a theropod, or meat-eating dinosaur, say it provides an unprecedented view of the biology of these ancient reptiles and new clues to their lifestyle.
The bottom line is: You wouldn't want to meet one of them in a dark alley.
The research, to be published Friday in the journal Science, offers insights into dinosaur metabolism, the warm-blooded versus cold-blooded debate, the question of whether or not they might have been the ancestors of birds, and even the biology that first helped them dominate the world and eventually may have led to their extinction.
"This fossil is helping confirm that the dinosaurs were indeed, by definition, cold-blooded, and that in all likelihood birds are not the descendants of any known group of dinosaurs," said Nicholas Geist, a paleobiologist at OSU. "But the extraordinary condition of the fossil allows us to hang some meat on the bones of these animals and bring them back to life a little bit. It's almost like a dinosaur dissection."
What that analysis reveals, Geist said, is an animal that had the best of both worlds. Like other cold-blooded animals, these theropod dinosaurs had low metabolic rates while at rest, which is an excellent strategy for conserving energy. But its enhanced lung ventilation capacity gave it the potential for the type of aggressive, extended activity typical of birds and mammals.
"These theropod dinosaurs were fast, dangerous animals," Geist said, "certainly not slow or sluggish. They could conserve energy much of the time and then go like hell whenever they wanted to. That might go a long way towards explaining why they were able to dominate mammals for 150 million years."
Geist and OSU colleague Terry Jones made these observations after study in Salerno, Italy, of a fossil first discovered just a few years ago of a baby Scipionyx, a meat-eater that lived about 110 million years ago and bore some similarity to a velociraptor. They were only the second group of paleontologists in the world to study the fossil.
"Besides an intact skeleton, this fossil shows remnants of liver, large intestine, windpipe and even muscles," Jones said. "The baby dinosaur probably died in a shallow, still, saltwater marsh that preserved its structure incredibly well. It's like a Rosetta stone for paleontology, and shows us more about dinosaur biology than we ever knew before."
Of particular importance, the researchers said, is the clear presence of a complete separation of the body cavity into two parts, one containing the lungs and heart, and the other holding the liver and guts.
This sort of body cavity partitioning is only seen in living animals that use an active diaphragm to help ventilate their lungs, such as mammals and crocodilians. Like crocodiles and alligators, the liver of these carnivorous dinosaurs was pulled back by large muscles that attached to a distinctive bony element of the pelvis. These muscles pulled the liver back, causing it to act like a piston.
This "hepatic piston" mechanism probably enhanced the capacity for high levels of oxygen exchange and the fast-paced activity associated with it - that could have rivaled that of some mammals.
"This type of physiology would provide some metabolic advantages unlike that of any animal still alive today," Jones said. "But for various reasons it only works well in a warm, equitable climate, which most of the world had during the age of dinosaurs. When the climate turned colder or more seasonal variation developed, what had once been the advantage of the dinosaurs became their problem."
The lungs and other structures found in this dinosaur fossil bears little similarity to that of modern birds, the researchers said, and they continue to believe it's unlikely that birds could have evolved from any known dinosaurs. The fossil also showed none of the types of nasal "turbinates" that previous OSU research has linked to nearly all warm-blooded animals.
The only living animals that have a lung structure similar to what is being found in dinosaurs, the researchers said, are the modern crocodilians. But that may be an evolutionary remnant of a physiology which once served a different purpose, they said. Prehistoric crocodilian species which are now extinct were more lightly built, fast animals that could stand upright on land and run.
The view of theropod dinosaurs that is emerging, the OSU researchers said, is reptiles that could save energy while idling but had the potential for enormous, sustained bursts of activity when needed. This was not a sluggard of a lizard.
"A lot of people who only see cold-blooded reptiles moving slowly in temperate zones have no concept of what they can do in warmer climates and how well they can function," Geist said. "Then if you add in the lung capacity that we're finding for meat-eating dinosaurs, what you have is a turbocharged reptile.
"If you could go back in time and see one of them, that's probably the last thing you'd ever see."