Why bite the right of a trilobite?

Why bite the right of a trilobite?

If old wounds heal, what might they reveal 500 million years later? According to paleontologist Loren E. Babcock, injuries that old can offer surprising information about long-extinct animals. Take, for instance, the trilobite, an ancient, crustacean-like creature. While studying bite marks on fossilized trilobites, Babcock and Richard A. Robinson of the University of Kansas in Lawrence discovered that bite scars most often appear on the right-hand side of the fossil -- suggesting the intriguing possibility that trilobite predators preferred attacking from the right side, the researchers report.

Trilobites, which lives mostly on the seafloor, are among the earliest known organisms to grow hard shells. In their study of 158 trilobite fossils with obvious wounds, Babcock and Robinson adopted a scheme to distinguish regular injuries from healed bite marks. They discounted any damage to fragile parts of the body that might have broken easily. They also looked for large curve-shaped wounds, as these should match the mouth shape of trilobite predators, which were arthropods, fish and nautilus-like animals.

Of the 81 trilobite fossils bearing bite wounds, 69 percent have chunks missing from their right side only, while 27 percent show marks on their left side only. The rest of the individuals have scars on both sides. For comparison, non-bite wounds are almost evenly distributed between right and left sides.

Babcock calls this relationship important because it suggests trilobite predators most often attacked from the right. If so, this find would represent the earliest known example of behavioral asymmetry -- the tendency to use right and left sides of the body differently. Many animals, both invertebrates and vertebrates, exhibit this trait, which often reflects asymmetries in the brain. The Kansas team proposes brain asymmetries date back to the time of the trilobite predators.

Some scientists, however, snap at such suggestions about biting and brains. "I am terribly skeptical about this stuff. I think they are totally overstepping their bounds in the interpretations," says Geerat Vermeij, an evolutionary biologist at the University of California, Davis. Vermeij says it is impossible to decipher anything about predatory behavior from the propensity toward right-side scars. Others suggest trilobites may have caused the side bias. Perhaps the prey curled up with their right side always exposed. Alternately, trilobites may have had a vital organ on their left side. If so, a predator attacking from the left would stand a better chance of killing the trilobite and eating the whole thing, eliminating the trilobite's chance of becoming a fossil.