Many animal fossils appear in a head-thrown-back position called the “dead-bird” pose, which paleontologists traditionally attribute to rigor mortis, desiccation of the carcass, or the shifting of bones by water currents. Now, scientists report that the posture probably came about because dinosaurs or other animals died of central nervous system damage. Fossils of nearly all birdlike Archaeopteryx, as well as some Tyrannosaurus rex and other ancient creatures, exhibit the curious pose.
Study coauthor Cynthia Marshall Faux, a paleontologist at the Museum of the Rockies in Bozeman, Mont., who is also a veterinarian, has seen the same pose in many modern animals with central nervous system damage. She says that the posture, called opisthotonus, is common in animals suffering from brain injury or from oxygen deprivation, known as hypoxia. Bacterial infections such as meningitis and toxins from certain algal blooms (SN: 5/4/02, p. 275) can also send animals into opisthotonus.
“The pattern we see here with dinosaurs, pterosaurs, and mammals, is consistent with the problem of hypoxia causing trauma to the nervous system,” says study coauthor Kevin Padian, curator of the Museum of Paleontology at the University of California, Berkeley.
Faux and Padian argue that in a skeleton moved by water currents, limbs should lie in the same direction as the head and tail. To test whether rigor mortis could have deformed ancient remains, the researchers examined the movement of large, dying birds at a raptor-care center. The birds exhibited no change in position as the postmortem muscle and skeletal changes set in.
Contortion of the skeleton from muscles and tendons drying and pulling at the joints also failed to explain the pose. The carcasses of red-tailed hawks left to dry in Styrofoam “peanuts” showed no movement after 3 months. Beef tendons pinned to a piece of Styrofoam didn’t shrink enough to dislodge the pins.
“The same principles applied back then as they do now,” Faux says. Since desiccation couldn’t produce the same posture in modern animals, she notes, “it didn’t seem to fit the scenario for drying out a T. rex.”
The researchers argue instead that animals found in the dead-bird pose died that way and were buried rapidly, so the posture remained undisturbed.
Scientists use the position of fossilized bones to infer the environment in which the animal perished. The new findings, which appear in the spring Paleobiology, give paleontologists a snapshot of the creatures’ last moments.
“What’s useful about this particular study is they’ve countered [traditional explanations] with real information, not just arguments,” says Matthew Carrano, curator of dinosauria at the Smithsonian Institution’s National Museum of Natural History in Washington, D.C. “It allows us more of a window into understanding how the dinosaurs died.”