For more than 250 million years, the primeval oceans of the Paleozoic teemed with trilobites. These hardy invertebrates evolved into an astonishing array of separate species--more than 25,000 at last count--and much remains unknown about these once-ubiquitous creatures. Fossil enthusiasts have become captivated by trilobites' diversity and adaptability, enthralled by the possibility of catching a glimpse of a seemingly alien past.