Echinoids show up in the fossil record from the Ordovician (about 450 million years ago), but their fossils are sparse compared to other organisms like bryozoans and crinoids. They may not fossilize well because the skeleton of the first echinoids was made of thin plates loosely held together by soft flesh (in contrast to modern echinoid plates that interlock to make a rigid skeleton). Many modern echinoids live in places with wave action, but the ancient ones probably lived in calm, sheltered waters. Like modern echinoids, some early echinoids had long spines that they could regrow, and tube feet for feeding or locomotion. Their teeth were weaker than modern echinoid teeth, and they may have lived by scavenging food from the ocean bottom. Echinoids diversified during the Triassic and Jurassic, giving rise to the types we see today, including predatory echinoids.