Cambrian

The Cambrian is a major division of the geologic timescale that begins about 542 million years before the present (BP) at the end of the Proterozoic eon and ended about 488.3 million years BP with the beginning of the Ordovician period (ICS, 2004). It is the first period of the Paleozoic era of the Phanerozoic eon. The Cambrian is the earliest period in whose rocks are found numerous large, distinctly-fossilizable multicellular organisms that are more complex than sponges or medusoids. During this time, roughly fifty separate major groups of organisms or "phyla" (a phylum defines the basic body plan of some group of modern or extinct animals) emerged suddenly, in most cases without evident precursors. This radiation of animal phyla is referred to as the Cambrian explosion.

External links and references

• Amthor, J. E., et al.. 2003. Extinction of Cloudinia and Namacalathus at the Precambrian-Cambrian boundary in Oman. Geology, 31: pp 431-434.
• Report on the web on Amthor et al. from Geology vol. 31.
• Paleomap Project.
• {{Web reference | title=International Commission on Stratigraphy (ICS)| work=Home Page| URL=http://www.stratigraphy.org/ | date=Sept 19 | year=2005}}