Tommotian

The Tommotian, named after the mollusc Tommotia, which began approx. 530 million years ago is an early part of the Cambrian period and lasted for only about 3 million years.

Paleogeography
World climate during the Tommotian was mild; there was no glaciation. Most of North America lay in warm southern tropical and temperate latitudes, which supported the growth of extensive shallow-water archaeocyathid reefs all through the Lower Cambrian. Siberia, which also supported abundant reefs, was a separate continent due east of North America. Baltica - what is now Scandinavia, eastern Europe, and European Russia - lay to the south. Most of the rest of the continents were joined in a supercontinent known as proto-Gondwana. What is now China and east Asia was fragmented at the time. Western Europe was also in pieces, with most of the pieces lying northwest of what is now the north African coastline.

Fauna
The Tommotian saw the rise of diversified metazoans with skeletons, small shelly fauna, the first archaeocyathids, primitive molluscs - monoplacophorans, Lapworthella, and inarticulate brachiopods. Archaeocyaths are sponges with a simple morphology. Their calcareous skeleton consists of an inner and an outer wall that are variably connected. The small shelly fauna consists of various calcareous (also some silica, some calcium phosphate) fossils some 1-5 mm long. They represented a variety of organisms: sponges, molluscs, annelids, lobopods, and other forms that do not seem to belong to any recent phylum. Many of these organisms were recognized either as of unknown affinity or as representatives or groups that became extinct before the end of the Cambrian. The most primitive stage is marked by characteristic elements, such as anabaritids, tommotiids, and hyolithellids, also known as the "Tommotian fauna". This period had the first major radiations of animals.

The origin of the many kinds of skeletons during this time was a major evolutionary development. The rapid evolution of a variety of external skeletons was probably in response to the evolution of advanced predators.