How real are biological extinctions in the fossil record?

Much has been made by catastrophists of the apparent wholesale extinctions of many forms of life from one geological period to another. Some uniformitarians have been arguing all along that these so-called extinctions are more apparent than real. The present study supports this view with its study of world-wide records of the Triassic-Jurassic transition period. Correlation of U.S. and South African rocks imply that this key geological interface was actually a period of gradual faunal replacement rather than sudden, simultaneous extinction of live forms.

(Olsen, Paul E., and Galton, Peter M.; "Triassic-Jurassic Tetrapod Extinctions: Are They Real?" Science, 197:983, 1977.)