400
Reading passage: Natural Sciences
A recent discovery in South Dakota has paleontologists rethinking their theories about dinosaur metabolism. The finding, a fossilized dinosaur heart, more resembles the heart of a bird--an animal with an active metabolism--than it does the crocodile or lizard--animals with less active metabolisms. This might mean that, contrary to popular opinion, dinosaurs were warm-blooded, not cold-blooded.
Consider the structure of the heart of a bird. It has four chambers that separate oxygenated and oxygen depleted blood. Two of these make up the right ventricle, the other two the left. A ventricle is a chamber of the heart which receives blood from a corresponding atrium and from which blood is forced into the arteries. In the bird, the right ventricle pumps oxygen-depleted blood from the body to the lungs, where it becomes oxygenated. In turn, the left ventricle pumps oxygenated blood from the lungs into the body through a single aorta. (The aorta is the channel that carries blood from the heart to the arteries throughout the body.) All this activity evidences the bird's active metabolism. Blood is pumped through the bird's body constantly, whether or not the bird is in motion.
Crocodiles, by contrast, have less active metabolisms. When they dive, their four-chambered hearts shunt oxygen-depleted blood around the lungs through a single valve. Blood from the left ventricle is carried away by a pair of parallel aortae. While this system of oxygenation is more evolved than that afforded by the three-chambered hearts of lizards, it nonetheless limits the crocodile.
As the closest surviving relatives of the dinosaur, both birds and crocodiles have been thought to promise clues about the internal workings of their ancient predecessor. Of late, however, paleontologists have looked to the two species to resolve a long-running debate about dinosaur metabolism.
Due entirely to their perceived similarity to modern reptiles, paleontologists have traditionally considered dinosaurs to be relatively inactive and cold-blooded creatures. The absence of preserved soft-tissues has made the debate largely speculative: arguments for and against the activity of the dinosaur have been contingent upon circumstantial evidence derived from microscopic analysis of bone structure. Now, however, the discovery of what appears to be a fossilized dinosaur heart in the chest cavity of a 650-pound plant-eating animal estimated to have died 66 million years ago may alter previously held conceptions. The structure of this heart, which resembles that of modern birds and mammals, suggests that dinosaurs generated their own body heat, and so may well have led active lives.
The newly-unearthed fossil, a little-known dinosaur called Thescelosaurus, had come to rest in a dry river bed in South Dakota. Because sand had filled the ribcage cavity, the animal's bones were not flattened. A computerized axial tomography (CAT) scan revealed the make-up of the hard, iron-rich clump inside the ribcage: it comprises two ventricular-like cavities and a cardiac structure of the correct volume for an animal of that dimension. An aorta originates and arches back from the left ventricle, which some paleontologists argue points to an active, bird-like metabolism for the dinosaurs. Dissenters, however, caution that, like the pulmonary arch that is also missing, a second aorta may not have been preserved.
Which of the following facts about the Thescelosaurus heart is NOT true?
A. It has two ventricular-like cavities.
B. It has an aorta arching back from the left ventricle.
C. It has a second aorta, like the heart of a crocodile.
D. It oxygenated the dinosaur's blood.
What is It has a second aorta, like the heart of a crocodile.