End-Botomian mass extinction
The Botomian age of the Early Cambrian epoch lasted from ca 524 to ca 517 Mya. At the end of the Botomian age there was a mass extinction that wiped out a high percentage of the organisms for which fossils were found. Organisms which produced small shelly fossils were almost exterminated.
- Porter, S.M. (May 2004). "Halkieriids in Middle Cambrian Phosphatic Limestones from Australia". Journal of Paleontology 78 (3): 574–590. doi:10.1666/0022-3360(2004)078<0574:HIMCPL>2.0.CO;2. Retrieved 2008-08-01.