The Precambrian-Cambrian transition (just prior to the Cambrian Explosion) is characterized by the appearance of small shelly fossils that are called the Tommotian fauna after the are of Siberia where they were first discovered. These small shelly animals were millimeters in size and represent the first appearance of diverse skeletal material in the fossil record, some 10 million years before the first trilobites appear in the fossil record. This fauna that existed some 530 million years ago is of essential evolutionary importance in that these oldest known metazoans (animals) had mineralized (skeletal) hard parts, and thus are the probable ascendants to the many phyla of the Cambrian Explosion.

Many of the 525 million year old Chengjiang Biota, for example arthropods brachiopods, archaeocyathids, molluscs, echinoderms, and some enigmatic animals, possessed mineralized body parts. Though well studied, the ancestral affinities of many of these Tommotion metazoans remain unknown. Prior to the Tommotian fauna, the Ediacaran metazoans were all soft-bodied. Some of the Tommotion small shells appear to be worm teeth; such toothed worms might have been effective predators of the soft-bodied Ediacaran life forms. The Tommotion age is believed by some scientists to be separated from the rest of the early Cambrian by the Botomian extinction event that eliminated many Tommotian forms, which were then followed by more familiar early taxa of the Cambrian Explosion.