Sea cucumbers are hard to find in the fossil record because their skeletons are made up of many, miscroscopic pieces (ossicles) that are easily overlooked. Only a couple dozen full body fossils have been found, so most of what we know about the evolutionary history of sea cucumbers is from ossicles. Ossicles do not give you a clear picture of sea cucumber abundance or diversity because they can differ within one individual, or depending on an individual’s age. The oldest known fossil sea cucumber ossicle is from the late Silurian (about 420 million year ago). Paleontologists believe that sea cucumbers may have been around earlier than that, and continue to analyze what organisms they evolved from. Possible ancestors may are little-known “calledophiocistioids” or ancient sea urchins. Because sea cucumbers look like giant larvae, it has even been proposed that they evolved as a group of adults keeping the characteristics of larvae (paedomorphism).

Echinoderms appear in the fossil record more than 500 million years ago, during the early Cambrian. What are usually left behind are hard mouthparts or parts of their skeletons, made of calcite plates. Rarely, an entire skeleton fossilizes, for example in a situation where it was quickly buried in sand. Even a piece of skeleton can provide information, because echinoderms have specific patterns in their skeletons. The echinoderms you see today have five-point (pentaradial) symmetry, often noticeable in five arms. While some of the earliest echinoderms were pentaradial, others had unusual body shapes. The “helioplacoids” had long, oval-shaped bodies with no arms, and a spiral pattern on the surface from tube feet wrapped around a central core. Helioplacoids went extinct even before the end of the Cambrian, as did a variety of other echinoderms, including the star-shaped Somasteroidea. Some echinoderms survived and diversified, becoming dominant in the oceans of the Paleozoic era.