...His findings: Overall, approximately 35 percent of the 982 trilobite species exhibited some variation in some aspect of their appearance that was evolving. But more than 70 percent of early and middle Cambrian species exhibited variation, while only 13 percent of later trilobite species did so. "There's hardly any variation in the post-Cambrian," he said. "Even the presence or absence or the kind of ornamentation on the head shield varies within these Cambrian trilobites and doesn't vary in the post-Cambrian trilobites."...