Tommotian fauna

Also see: Ediacaran Fossils of the Vendian White Sea

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 area 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 are the oldest known metazoans (animals) that had mineralized (skeletal) hard parts, and thus are the probable ascendants to the many phyla of the Cambrian Explosion. They appear in the late Ediacaran Period, close to 550 million years ago, and some 20 million years before the soft tissue Ediacaran Biota. Their fossil remains are found throughout the world, so their radiation must have been extensive. They persisted into the early Cambrian, and have been discovered in trilobite fossil beds.

Many of the 525 million year old Chengjiang Biota, for example, including 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 (like the annelids) might have been effective predators of the soft-bodied Ediacaran life forms. The small shelly animals appear to be related to diverse organisms such as primitive annelids, lobopodians, sponges, mollusks, and other forms that played a role in the early Cambrian and did not persist. The Tommotion age is believed by some scientists to be separated from the rest of the early Cambrian by the End-Botomian extinction event (from about 524 to 517 million years ago in the early Cambrian) that eliminated many Tommotian forms, which were then followed by more familiar early taxa of the Cambrian Explosion.