Small shelly fossils of late Precambrian and early Cambrian age: a review of recent work

Stratigraphic sections in the high Precambrianlow Cambrian of Siberia are described. They have yielded the variety of fossils—archaeocyathids, gastropods, hyolithids, hyolithelminthids, poriferids, tommotiids and others—that characterize the Tommotian, Atdabanian and Lenian Stages. Some shelly fossils are found high in the Precambrian (Yudomian). The Tommotian Stage is regarded here as the lowest Cambrian unit. Trilobites first appear at the base of the Atdabanian Stage.

A selection of fossils is illustrated and briefly described. Faunas from equivalent levels in Sweden (Tommotian Stage) and England (Nuneaton: Tommotian Stage; Comley: Atdabanian Stage) are cited. Comments are offered on ranges and on distribution (it is already apparent that certain Atdabanian Stage fossils are not restricted to the “provinces” identified for trilobites). The problems of recognizing primary shell-structure, the existence of symmetry-pairings, the manner of coiling of shells and the occurrence of perforations of shells (some seemingly due to boring by predators) are also discussed.