A small dinosaur with a heart of stone answers an old question

If you happen to be one of the very small number of people that sit around wondering what kind of heart dinosaurs had, last Friday was your lucky day. A team of researchers from North Carolina say they have found one – a fossil dinosaur heart. Because soft tissues rarely form fossils, we know little of the internal organs of dinosaurs. No fossil dinosaur heart has ever been found before this one.

That has not stopped paleontologists from speculating about what dinosaur hearts must have been like. Indeed, there has been lively discussion. Why all the interest?

To understand the problem, it is necessary to stand back and look how hearts evolved as vertebrates invaded land from the sea. A fish heart has two central chambers, a single atrium to collect blood and a single ventricle to pump it. Blood is pumped from the heart through the gills, where it loads up on oxygen, then through the tissues of the body until it finally returns to the heart. Most of the umph from the heart’s pumping is lost as the blood passes through the fine passageways of the gills, so a fish’s blood circulation is sluggish.

The first land vertebrates overcame this problem of whimpy blood circulation by returning oxygen-rich blood from the lungs to the heart for repumping, so that it would shoot forcefully through the tissues of the body. In the amphibian heart there is the beginning of a wall within the heart, separating its interior into two halves, one for the blood going to the lungs, and the other for the blood coming back from the lungs that will be pumped to the body’s tissues.

In amphibians the wall, called a septum, separates the artium into two separate chambers, but does not extend into the ventricle. As a result, there is considerable mixing of blood from the two circulation paths in the ventricle.

In reptiles, the septum passes about half way through the ventricle as well, cutting down on the mixing within the heart. In birds and mammals, the septum through the ventricle is complete, producing a four-chambered heart with two atriums and two ventricles.

Now hear’s the problem: Dinosaurs are reptiles. If dinosaurs had the same sort of heart found in reptiles today — one with an incomplete septum within the ventricle — then it is very hard to understand how they could have been as big as they were.

As an example, take Brachiosaurus. This huge 90 ton plant eater had a barrel shaped body with a very long neck. A brachiosaur stands over 35 feet tall, with its brain approximately 25 feet higher than its heart. A column of blood that tall would create an enormous head of pressure within the heart. Imagine trying to pick up a stack of 25 trash cans full of water, and you will get some idea of the force pushing into the ventricle through the body circulation. If dinosaurs had a typical reptilian heart, then this great pressure would slam the body’s blood across the opening within the ventricle, and into the lung circulation path. The lungs would flood, and the brachiosaur literally drown in its own blood.

Because brachiosaurs seem to have gotten along just fine, paleontologist have assumed that they must have had a complete septum across the ventricle, as birds and mammals do. But no one knew for sure. It just seemed a logical suggestion.

Now we know. What appears to be a genuine dinosaur heart is contained within a fossil skeleton of Thescelosaurus, a small, plant-eating dinosaur that died 66 million years ago. A professional fossil hunter discovered this fossil in the badlands of South Dakota in 1993. Nestled within its chest cavity, surrounded by its ribs, was a rust-colored concretion, the sort of bothersome rocky material that is almost always chiseled away when cleaning a fossil for mounting.

Instead of discarding the fist-sized rock, researchers subjected it to a little high-tech wizardry. Using computerized tomography machines from a local hospital (a sort of super X-ray machine that gives 3D images), they looked into the concretion’s interior.

The concretion was a fossil heart. The chambers of the stony heart can be clearly seen, with left and right ventricles cleanly separated by a complete septum. The image of the heart’s interior is so clear, says Andrew Kuzmitz, the physician who carried out the analysis, that it looks like “a carcass that should be hanging from a meat hook.”

So the arm chair speculation about dinosaur hearts was right on the money. Its not often that science rewards speculation with evidence so forthrightly. So much of our view of dinosaurs rests on logical speculation that I find this particular outcome comforting.

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