Ediacaran biota
As Narbonne explains, the Ediacaran biota occupies a critical position in the fossil record. Before them, for a long period of the Archaean and most of the Protoerozoic, the only fossils were those of bacteria and bacterial mats, a form of life which continues to the present day. The Ediacaran biota is found worldwide, and the most typical forms disappeared just before the Cambrian radiation, which begins with the 'small shelly fossils'. That is probably enough to justify the description of the change as an extinction event, though what caused it is not clear. One suggestion is that the evolution of animals which grazed upon the soft-bodied forms would have been sufficient to cause a collapse of the biota.