During their long history on Earth (about 500 million years), gastropods have evolved various ways to feed. Flat snails (e.g., Maclurites) likely lived like clams, sitting in one place and eating small food suspended in the water. They were common in the Paleozoic, but are now extremely rare. Most gastropods actively find detritus or algae to eat, scraping food off rocks or other surfaces using a special mouthpart (the radula). Other gastropods are carnivores: shells with holes in them are evidence of gastropods using their radula to drill a hole and eat the animal inside. The earliest gastropods were detritivores, but carnivory has evolved independently several times (convergent evolution). During the shift from herbivory to carnivory, the teeth on gastropod's radulas were lost or modified for use as predatory tools. Early gastropods lived in the sea, but by the Carboniferous (360 - 299 million years ago) some freshwater gastropods invaded land. During the transition to land, gastropod shells either remained rather thick (in dry climates) or were reduced or even lost altogether (in humid climates).