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 are captivated by trilobites' diversity and adaptability, enthralled by the possibility of catching a glimpse of a transcendently strange past.