Paleopathology: Ancient CSI

Author: Beverly Eschberger
Published on: Aug 6, 2004

You might think that once an animal has become fossilized, there is little that can be learned about the health of that animal, or the cause of its death. To some degree, this is true. Most diseases and injuries do not leave any lasting effect on the skeleton that would be fossilized. Some diseases and injuries, however, do leave evidence in the bones. While this evidence cannot always tell us exactly what killed an animal, it can tell us what sort of diseases and injuries the animal may have suffered from while it was alive.

Paleopathology is the study of malformations in the bones that may have been caused by disease or injury, and these malformations are often referred to as paleopathologies. There are basically two causes of malformations: trauma (injury) and disease.

Trauma or injury generally shows up as puncture wounds, unhealed breaks (which are difficult to distinguish from a break caused after death), and a healed break. Puncture wounds can be caused by the teeth of a predator. Skull bones tend to be thin, and can be easily punctured by teeth. The big cats and other predators often carry their prey by the head, and can leave tooth marks in the skull bones. Victims of sabre-toothed cats can be easily recognized by the large wounds left by their strong teeth. Paleoanthropologists (paleontologists who study early humans) sometimes find puncture wounds that might have been caused by arrow- or spear-heads.

Often, paleontologists will find bones that have been broken. These unhealed breaks could have been caused when the animal was alive, perhaps during a fight or because of a fall, or they might have been caused long after the animal was already dead, and perhaps even after the bones were fossilized. It is very difficult to determine whether the breaks happened while the animal was alive or after death (post mortem), so unhealed breaks really cannot tell us much about how the animal lived.

Paleontologists can also find bones that were broken, then partially or completely healed. This can tell us something about the animal’s life: we know that it survived long enough after the initial break for the break to heal. Without prehistoric doctors to set broken bones, these breaks would generally heal unevenly, which could cause a limp if the break was in a foot or leg bone, and could result in other malformations in the bone. Most of these healed breaks would have occurred when the animal was fairly young, and could recover from a broken bone more easily; broken bones in older animals were more likely to lead to death.

Healed breaks can take many forms. Exotoses are bone asymmetries or outgrowths that are caused due to an injury to the periosteum, the thick membrane covering living bone. The periosteum guides the growth of the bone, and if it is injured, the bone can grow into shapes that it is not supposed to. Wounds that result in a haematoma (blood clot) will heal with a callus and the necrosis (death) of the bone surrounding the fracture. This haematoma would normally be replaced with fibrocartilage, but can become ossified and preserved as bone. Fractures can also heal with the fusion of two or more bones, known as a bony callus or ankylosis, which can completely surround and enclose the region of the fracture. Bony outgrowths replace the fibrocartilage callus, new bone is produced, and the necrotic (dead) bone is resorbed. If the callus is not resorbed before the animal’s death, the bones will remain fused.

Most diseases do not leave any evidence in the skeleton, but paleontologists can sometimes find signs of disease. Many strange bony outgrowths have been interpreted as signs of bone cancer, but many paleontologists believe that they were actually caused by infections. Periostitis is an infection of the periosteum (the membrane covering the bone), and can leave behind signs of pitting or bony outgrowths (exotoses). If the infection spreads to the bone cavity (a rare occurrence), this is known as osteomyelitis. When an infection spreads this far, the bone can become filled with and surrounded by bacteria and pus, and results in necrotic (dead) bone and bony outgrowths. Infections to teeth can result in dental abscesses that can even cause holes in the jaw or back of the skull where the bone has been eaten away by infection.

In addition to infections, bones can also be marked by osteoarthritis, an inflammation of the joints. Osteoarthritis is a degenerative disease that erodes the joint surfaces, causing bone growth around the affected surfaces. This growth has a characteristic “lipping” which is known as osteophytosis, and this osteophytosis can result in the complete fusion of adjacent bones (ankylosis).

Oddly enough, paleontologists have found evidence of gout in the skeletons of two tyrannosaurid dinosaurs! In humans, gout is caused by a build up of uric acid in the muscles, and associated with a diet high in purines, which are found in red meat. So, I guess these tyrannosaurids were eating too much red sauropod meat, and not enough meat from bipeds, which may have had “white meat” like modern fowls.

The two major degenerative skeletal diseases are osteoporosis and osteomalacia. Osteoporosis is commonly known as “brittle bone disease” and is generally seen in older humans who do not consume enough calcium, but can also be seen in younger individuals with nutritional deficiencies. Osteomalacia is essentially the opposite of osteoporosis, and presents itself as a “softening” of the bone. At this point, no-one has conducted any studies involving either osteoporosis or osteomalacia in the fossil record.

A condition that appears often in dinosaurs is Diffuse Idiopathic Skeletal Hypertosis (DISH), in which the vertebrae become fused. Several vertebrae in a row can become fused to one another. In humans, this condition seems to be a result of age and stress on the bones, and may be connected with diabetes. In dinosaurs, DISH most commonly appears in the caudal (tail) vertebrae, and the fusion of vertebrae may have strengthened the tails of dinosaurs, allowing them to keep their tails stiff and straight and lift them off the ground (which would have acted as a counter-balance for a bipedal dinosaur) with little effort.

Paleopathologies can also be observed in invertebrates with hard shells. Snails often bore into the hard shells of invertebrates in order to attack the animals inside, and these bore holes can be preserved. Bite marks can also be left by vertebrate predators trying to catch the animal, or crunch into the invertebrate shell.

Crime Scene Investigation in the modern world can lead to the capture of criminals, for paleontologists it can give us information about the life of an animal, and maybe also its death.

For more information about paleopathology: Paleobiology: A Synthesis Chapter 4.15 “Paleopathology” by L. B. Halstead and The Complete Dinosaur Chapter 31 “Dinosaurian Paleopathology” by Bruce M. Rothschild.

Special Thanks to Darren Tanke and Lorrie McWhinney for the use of paleopathology specimens for the photos!