August 11 , 2016

by Carolyn Gramling

Some 70 million years ago, a duck-billed dinosaur lived alongside a shallow sea covering what is now New Jersey—and it had a real pain in the arm. At long last, paleontologists are giving it a diagnosis: septic arthritis, a bone disease that often develops when an injury is followed by infection. Even though the scientists had nothing more than two fossilized arm bones to go on, the fossils contain enough evidence to make the postmortem analysis possible. Such an analysis, they add, offers a unique, if painful, window into dinosaurs’ daily lives.

Even to the naked eye, the arm bones don’t look quite right, says Jennifer Anné, a paleopathologist at the University of Manchester in the United Kingdom and the lead author of the new study. Their oddness piqued her interest—dinosaur fossils aren’t common along the East Coast to begin with, and it’s even rarer to find bones with a clear pathology. “I’m interested in anything with an injury.”

The bones—a radius and an ulna—were highly fragile and rapidly decaying. They had been discovered in the 19th century in New Jersey’s Navesink formation, a greenish, calcium carbonate–rich layer of sand, clay, and mud. Early paleontologists found a wealth of fossils in the formation that reflect the region’s watery past: swimming reptiles like plesiosaurs and mosasaurs, crocodiles and turtles, plus a handful of land-dwelling hadrosaurs. The hadrosaur fossils ushered in the study of paleontology in the United States.

Now, Anné and her team wanted to get a closer look at the bones. They arranged to borrow them from the New Jersey State Museum in Trenton. But diagnosing ancient diseases from fossils is tricky for a number of reasons. For one thing, the fragility of the fossils made it imperative not to further damage them. Cutting them open to get more information was out of the question. For another, looking at still-living related species like reptiles and birds—something paleontologists regularly do to intuit facts about dinosaur metabolism and physiology—can’t answer every question, because birds and reptiles heal in very different ways. And then there’s the obvious, Anné says: “Our patient died 70 million years ago. A doctor doesn’t just look at bones to diagnose an infection.”

So the researchers turned to a noninvasive medical technique to examine the interior of the bones as well as their exteriors. They scanned the two arm bones with x-ray microtomography, looking both along the length and then taking cross-sections within the bones.

On the inside, both bones showed regions of erosion, with a porous Styrofoam-like texture instead of healthy, dense bone tissue. On the outside, both showed areas of bone growth, bulges and spurs of new bone that formed in response to an injury. Based on the images, the researchers then began a process of elimination, considering various possible diseases—including gout, osteomyelitis (in which the bone itself is infected), and septic arthritis (in which infected cartilage can also affect bone tissue).

The best possible diagnosis from the available visual data was septic arthritis, they report online today in Royal Society Open Science. The patterns of erosion within the bone and the patterns of bone growth that likely acted to strengthen the joint all point to that particular condition, they suggest. Paleopathologists have previously spotted signs of everything from cancer to combat in dinosaur bones, but this is the first dinosaur diagnosis of septic arthritis.

What caused the initial injury isn’t possible to say, of course, but it could have been any number of things, according to Anné. “It could have started out that it did have arthritis. It could have gotten a cut, or broken that joint, and then had an infection,” she adds. “It’s a hard-knock life for any wild animal.” But, she notes, the extent of the bone growth postinjury does suggest that the dinosaur likely survived for some time.

It’s a reasonable diagnosis, says Bruce Rothschild, a paleosteopathologist and radiologist at Northeast Ohio Medical University in Rootstown, who was not involved in the study. He notes that the two bones were apparently originally fused together when excavated—connected by some of the reactive bone growth—but had apparently become separated over the years. That’s unfortunate, he says, because that area might have provided additional useful information. Still, he says, septic arthritis makes sense from the available information.

Rothschild has previously helped diagnose gout in a Tyrannosaurus rex, facial tumors in dwarf dinosaurs, and cancer in hadrosaurs; he acknowledges that if you look just at the literature, hadrosaurs did seem to be particularly susceptible to disease. But, although hadrosaurs may have been susceptible to cancer for congenital reasons, that doesn’t figure into the current diagnosis, he notes. The answer is likely much simpler: “They were a prey animal.”

Anné says the use of nondestructive techniques such as x-ray microtomography is having a significant impact on paleopathology. “As a result, how we’re approaching diagnosing is changing—it’s letting us look at more individuals, so we have a higher chance of finding things.”