Lesions mar the fossil bones. “With falling partial pressure the dissolved nitrogen comes out of the solution,” says Dr. Hayman. He is head of the Department of Pathology at the University of Melbourne and the bones he is talking of belong to an icthyosaur. It was an ancient sea-going reptile, about 150 million years old. Osteonecrosis, bone death, is visible at the joints. “Bubble formation occurs if this process is too rapid and it is this condition that produces ‘the bends’.” The surrounding tissue would have become swollen with air bubbles, squeezing muscles and nerves and choking them of oxygen.
Hayman is following up on a paper published by Rothschild, Xiaoting, and Martin of the University of Kansas. Their field is known as paleopathology, the study of ancient injuries and diseases. Like modern pathologists, the researchers examine bones and tissue, searching for clues to the animal’s life, habitat, and death. It’s Dr. House, hundreds of millions of years old, and the smallest signs of decay or growth can describe volumes.
Even the creature’s evolution opens to examination. As Rothchild et al notes this kind of visible necrosis is common in marine fossils, but the pattern is not always consistent. “We have examined a large Triassic sample of icthyosaurs and compared it with an equally large post-Triassic sample,” says Rothschild and company, “Necrosis was observed in over 15% of Late Middle Jurassic to Cretaceous icthyosaurs… but was rare or absent in geologically older specimens.” Something changed between the Triassic and Jurassic peroid. “Triassic reptiles that dive were either physiologically protected,” continues Rothschild, “or rapid changes of their position in the water column rare.”
What happened? Perhaps the physiological protection was too expensive. Or perhaps behaviors changed. As the Rothschild et al note, “Diving in the Triassic [may] have been a ‘leisurely’ behavior until the evolution of large predators… forced sudden depth alterations.” New dangers forced the icthyosaurs to take deeper and more dangerous dives to survive.
In the long-term, the animals probably benefited from these risks more than they suffered. As Hayman notes, the icthyosaur’s fitness may not have declined until later in life. Healthy tissue could have hidden the symptoms of osteonecrosis until the joints started to decay from arthritis. Avoiding death by predator while young would have easily outweighed pain while old, especially if it gave the animal time to grow, breed, and pass its genes on to the next generation. “[The reptiles] hunting ability and reproductive success would be unaffected,” says Hayman.