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Which fossils mark the boundary between the Cambrian and Ordovician periods?

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Ordovician paleogeography — Ordovician Period The distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the Ordovician Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The boundary between the Cambrian Period and the Ordovician Period is formally defined by the first appearance of the conodont species Iapetognathus fluctivagus.12

Historically, the definition of this boundary has been subject to regional variation. In Britain, the base of the Tremadocian Stage—which marks the beginning of the Ordovician—was traditionally defined by the first occurrence of the graptolite Rhabdinopora flabelliformis socialis. Because this graptolite is difficult to identify globally, an international panel recommended the use of Iapetognathus fluctivagus as the standard marker for the boundary.1

It is important to note that while there is a formal global stratotype for this boundary, the process of defining it was complicated by regional differences in classification; for many years, British geologists assigned rocks and fossils of Tremadoc age to the Cambrian, whereas many other geologists assigned them to the Ordovician.23