Stage 2 of the Cambrian is often referred to as the Tommotian. The proposed lower boundary is ~529 million years ago coinsiding with the appearance of small shelly fossils of the archaeocyathid species or the mollusc Watsonella crosbyi. The proposed upper boundary is ~521 million years ago with perhaps the first appearance of trilobites.
Cambrian Stage 2, Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc. 22 July 2004. Web. 30 November, 2012. http://en.wikipedia.org/wiki/Cambrian_Stage_2