Abstract
The evidence that extinctions since the mid-Permian show periodicity of ˜26 million years rests primarily on the stratigraphic ranges of marine families. We have checked the echinoderm and fish families which together make up about 20% of the data. Only 25% of these fish and echinoderm extinctions are real (disappearance of a monophyletic group). The remaining 75% is noise, chiefly 'extinctions' of non-monophyletic groups, mistaken dating, and 'families' containing one species only. The signal-to-noise ratio is very similar in echinoderms (27:73) and fishes (23:77). Periodicity in our sample is a feature of the noise component, not of the signal.
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