Trilobita is a class of arthropods which first appeared approximately 540 million years ago. Their closest living relatives are horseshoe crabs. It is believed by some paleontologists that they were the first animals with color vision. Most were detritus feeders, living off of dead matter on the ocean floor, although some were predatory. They were most diverse and common around 500 million years ago. After that, they entered a slow decline, becoming extinct 245 million years ago.