Slipping Through the Cracks

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Science  06 Mar 2009:
Vol. 323, Issue 5919, pp. 1266
DOI: 10.1126/science.323.5919.1266b

The fossil record shows that, at least in the marine realm, unusually small taxa predominate in the aftermath of mass extinctions. In extreme cases such as after the end-Cretaceous extinction, 65 million years ago, it took several millions of years for diversity to recover. This pattern poses a potential bias in assessing the impact of the extinction, as small species tend to be more difficult to preserve in the fossil record. Sessa et al. evaluate this bias by comparing fossils in lithified and unlithified sediments across the Cretaceous-Paleocene boundary from thick sections in the Gulf of Mexico. Their data show that small fossils are indeed lost (perhaps by dissolution) from the fossil record as sediments are compacted and form rocks—by a factor of up to 2.4. This process, however is not systematic in time, and lithified sediments tend to predominate in the Paleocene after the mass extinction. Thus, the pattern of a delayed recovery may be partly exaggerated by the sediment record, as might enhanced diversity before the extinction. This bias decreases further back in time, as unlithified sediments become scarce, but illustrates the inherent selection of the fossil record. — BH

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