What are Lazarus taxa?

The many definitions and interpretations associated with the 'Lazarus effect' have considerably confused this notion. While several authors regard the Lazarus effect as the temporary disappearance of taxa from the fossil record in any given time interval, many others consider the Lazarus effect as a pattern restricted to mass extinction episodes. The adequacy of the fossil record is the key for interpreting the Lazarus pattern: either Lazarus taxa reflect the incompleteness of the fossil record (the 'stratigraphic alternative'), or they illustrate genuine extinction-linked phenomena (the 'biological alternative'). The latter option includes two traditional hypotheses implying existence of refugia and post-extinction low population diversity. Differences between these two biological explanations seem to be more rhetorical than substantiated. In addition, the sampling intensity is an external factor commonly linked with both the stratigraphic and the biological alternatives. Interpreting the Lazarus effect is an asymmetrical procedure because the biological alternative is only favoured when the stratigraphic one cannot be documented. Consequently, analytical techniques assessing the completeness of the fossil record are critical to understand the meaning of Lazarus taxa. In particular, gap distributions are better compared with extinction and origination rates rather than with total diversity estimates. Finally, the Lazarus effect should be defined broadly to include range gaps from any given time interval. Copyright (C) 2001 John Wiley & Sons, Ltd.