Fossils of soft-bodied Cnidarians are rare, although they may be some of the earliest fossils of complex animals. Paleontologists have debated whether fossils from the Ediacaran (more than 540 million years ago) that look like jellyfish and sea pens are Cnidarians. The first definite Cnidarian fossils are from the Cambrian (about 500 million years ago). Some Cnidarians begin to make mineralized skeletons then, which were more likely to fossilize than the soft body parts underneath. During the Paleozoic, Cnidarians underwent a diversification into many forms. Jellyfish, hydrozoans, and corals emerged as distinct lineages. While various extinction events impacted Cnidarians along the way, these lineages survived to the present day. Colonial corals, whose skeletons accumulate into large reefs, ended up leaving a substantial fossil record. Because corals are sensitive to environmental variables, such as temperature and water quality, they have been used as indicators of past climate and other conditions.