Erma Bombeck called motherhood the world's second oldest profession, but she probably wasn't familiar with trilobites. These extinct marine arthropods may have been taking care of their young over 500 million years ago. Paleontologists Richard Fortey of the Natural History Museum in London and Nigel Hughes of the University of California at Riverside propose that a mysterious swelling, seen in front of the head of many trilobite fossils, is in fact a brood pouch, in which females sequestered their tiny larvae. The Cambrian seas were no place for unprotected youngsters. "There were lots of predators around," says Fortey. "There was a horrible big thing called Anomalocaris, a very large sort of shrimp with grasping appendages."
By guarding her larvae, a trilobite could help more offspring survive. Fortey got the idea while dining on horseshoe crab, the trilobites' closest living relative, in Thailand. "I couldn't for the life of me work out what there was to eat on it. They're rather scratchy creatures," he says. "To my amazement, when they brought the thing, I found it was the eggs you eat. They're stored in the front of the animal, in the same position you get these swellings in trilobites."
Hughes and Fortey compiled a list of about 20 trilobite pairs that appeared at the same time and place, with swellings and without. These may represent males and females of the same species. "It's the first suggestion of sexual differentiation in trilobites," says Fortey. "It's also the oldest geologic example of parental care."