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Prehistoric bones show creatures coped with common ailments

Monday, September 06, 1999

By Byron Spice, Science Editor, Post-Gazette

Amid rising seas, packs of scythe-clawed carnivores and the hard rain of death-dealing meteors, dinosaurs also coped with the same health complaints as modern folk -- aching teeth, sore joints, creeping cancers.

 
  A cross-section of a 150-million-year-old leg bone reveals the earliest known example of a metastatic cancer. (Annie O'Neill, Post-Gazette)

Throw in some adultery and the odd case of amnesia and the drama of the Mesozoic Era could rival the scope of "General Hospital" or "The Young and The Restless."

Skeletal characteristics, such as a jaw of fossilized teeth, a trunk-like leg bone, or a great, swooping tail, might define an animal, but scientists have found that they can also learn about a creature by studying its diseases, a field called paleopathology.

"Pathologies in fossils can yield insights into how these animals lived," said Christopher Beard, associate curator for vertebrate paleontology at the Carnegie Museum of Natural History.

And fossils can also provide clues about the nature of diseases that still plague humans.

Mary Dawson, the Carnegie's curator of vertebrate paleontology, said fossils that display signs of disease "are moderately rare, which is surprising because we're looking at animals that have died."

Then again, even if a disease is discovered by a paleontologist, it didn't necessarily have anything to do with the animal's death.

For example, Dawson hefts a 150-million-year-old hunk of leg bone that once supported a hadrosaur -- a duck-billed dinosaur -- that probably stood 8 feet tall and stretched 15 or 20 feet long. The fossilized bone has been cut crosswise to reveal an oval-shaped tumor that was bigger than a softball.

The tumor was malignant, Dawson said as she traced its margins with her finger. "It is a good indication that cancer has been around a long time." The cancer might have killed this animal, or it might have contributed to its death by turning it lame.

"If an animal is lame," she explained, "chances are it's going to get caught by someone."

It may have been a very sick animal, however. Dr. Bruce Rothschild of the Northeast Ohio Universities College of Medicine and colleagues from Iowa and Israel suggest that the tumor may be the earliest known case of metastatic cancer -- a cancer that has spread from elsewhere in the body. They reported on the fossil, originally collected in western Colorado and now displayed at the Carnegie, in the July 31 issue of the British medical journal The Lancet.

Sore joints also are nothing new. A 70-million-year-old toe from a smaller cousin of Tyrannosaurus rex is pitted and deformed on one end, a sign that the 20-foot-tall meat-eater was suffering from gout. An inflammation of the joints caused by the accumulation of uric acid, gout can be diagnosed in humans by checking for uric acid in joint fluid.

The Tyrannosaurid's uric acid is long gone, but the pitting it caused on the joint-end of the bone resembles the damage caused by gout in modern animals and is unlike any dissolution that groundwater might have caused.

Arthritis, however, may have been less common among dinosaurs than people have assumed, said Rothschild, who treats patients at the Arthitis Center of Northeast Ohio in Youngstown and is a research associate at the Carnegie.

Osteoarthritis, for instance, is the type of arthritis that occurs as joint surfaces wear out. It has been assumed that excess weight would increase a person's risk of osteoarthritis.

But dinosaur fossils suggest that it's not that simple. If it were, osteoarthritis would have been fairly common among big sauropod dinosaurs such as Apatosaurus (once known as Brontosaurus) and Diplodocus. It wasn't.

Rothschild said it seems the type of joint has more to do with osteoarthritis than weight. Sauropods have simple hinge joints at the knee that allow the leg only to bend. Humans, on the other hand, are also able to twist the leg bones.

"We get osteoarthritis not because of weight, but because of instability of the joints," he said.

One place sauropods were thought to develop arthritis is in the tail. Many sauropod skeletons have several tail vertebrae fused together.

"We've been saying for years, 'Oh, these dinosaurs have arthritis where their tails hit the ground,' " Dawson said. The problem is, paleontologists are now convinced that these dinosaurs did not drag their tails; though numerous dinosaur foot prints have been found, no one has found the track of a dinosaur tail. Instead, modern illustrators and skeleton assemblers now show these dinosaurs carrying their tails high off the ground.

Both the Apatosaurus and Diplodocus fossil skeletons on display at the Carnegie feature sections of fused vertebrae. Though both are configured in the old style, with tails dragging behind them, the fused section in each skeleton is above the portion of the tail in contact with the ground.

Dawson said some scientists had argued that these dinosaurs used their tails like bullwhips, snapping them to make a loud noise. But again, she noted, the fused sections are nowhere near the tips, where stress would be greatest.

What is clear, Rothschild said, is that the fusion does not occur because of a disease, such as arthritis. Rather, cross-sectioning of a fossil from the American Museum of Natural History shows that ligaments surrounding the vertebrae hardened into bone. Another notable fact, he said, is that about half of all Diplodocus and Apatosaurus skeletons have this fused section. This suggests that the fused vertebrae were a characteristic peculiar to only one sex.

No one can detect the sex of these fossilized skeletons, but Rothschild argues that the fusions were a female trait. "There is a biomechanical logic to it," he maintained. Those long, massive tails would seem a major obstacle to mating. The fused section is in a location of the tail where it might serve as a cantilever to help pull the tail out of the way, he said.

The biomechanics of chewing have Beard wondering if scientists may be off-base in their assumptions about the evolution of early mammals known as archaic primates. These squirrel-like animals with small brains and side-facing eyes flourished after the dinosaurs went extinct 65 million years ago.

One notable feature of these animals is a blade-like tooth next to the molar. Used to bite into fruits and nuts with tough outer coatings, this tooth extends further along the jaw with each successive generation of these critters. The growth of this tooth is so consistent, Beard said, "you can tell time by finding these fossils."

The classical interpretation, he said, is that the lengthening of the tooth is an evolutionary response that allowed the animal to eat more efficiently and therefore eat more types of food.

But a fossil that Beard has been studying lately has caused him to question that assumption. The fossil has been in the Carnegie's collection since it was discovered in 1981 in the Bison Basin of southern Wyoming; only recently did Beard recognize that the chipmunk-size, tree-loving animal was a previously unknown species. One other thing caught his eye, however -- an area underneath the blade-like tooth where the jaw has resorbed and the tooth roots are visible.

This injury makes him wonder if perhaps the blade-like tooth evolved not so much to improve the eating efficiency of these animals as to protect the animals' jaws from the food they were already eating.

It's possible that the sore tooth is a fluke. "My sample size is one," Beard acknowledged, "so what can you say?" But it's also possible that paleopathology is providing a new insight on evolution.

"It may be more appropriate to call the field 'paleohealth,' " Rothschild said, because most specimens display good health. One of his current projects, for instance, is studying the fossils of marine reptiles to see how they evolved protection against caisson disease, the decompression sickness known to divers as the bends.

The bends occur when an animal moves from a high-pressure environment to one of low pressure, as when a diver surfaces. Gas bubbles can form in the blood if the change is too rapid, resulting in pain. Rothschild said it could also result in bone death in areas where gas bubbles have cut off the flow of blood.

He has been studying these symptoms in sea turtles, about half of which seem to have suffered the bends during the age of the dinosaurs, compared with 1 percent today.

Discovering what happened in these animals to reduce those rates could benefit divers. "The dive tables [used to guide the speed of ascent after a deep dive] are not as protective as we would like to see," Rothschild explained.

Could a role in "Sunset Beach" be far behind?



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