2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 144-6
Presentation Time: 2:30 PM


MCMENAMIN, Mark A.S., Geology and Geography, Mount Holyoke College, South Hadley, MA 01075

The Signor-Lipps Effect explains the tendency of mass extinctions to appear gradual due to differential preservation among organisms of various sizes and abundances. Signor‑Lipps also applies to estimates of the origination dates of taxa appearing as fossils long after their actual first appearances. Swapping extinction for origination when considering individual lineages, some authors have facetiously called this second aspect the Lipps‑Signor Effect. Lipps‑Signor has long been used in attempts to neutralize the threat that an abrupt Cambrian Explosion poses to Darwinian gradualism. Darwin, Walcott and others argued that incompleteness of the fossil record obscured the assumed long pre‑Cambrian histories for animal phyla. I propose here a third permutation: the Lignor‑Sipps Effect. With Lignor‑Sipps, less easily preserved organisms will appear to have originated long after a rapid diversification event because it may take some time for the appropriate depositional environments to arise that can preserve delicate forms representing lineages in the early stages of acquiring hard parts. The temporal smearing of the paleontological record by Lignor‑Sipps is a near mirror image of Signor‑Lipps, but the smearing pattern may be somewhat different in Lignor‑Sipps considering the delays in the acquisition of skeletons. Lipps‑Signor may apply to a few metazoa of the Cambrian Explosion, chitons for example, but the records of many Cambrian higher taxa clearly represent cases of Lignor‑Sipps. The Cambrian Explosion was therefore even more abrupt than suggested by the Burgess Shale and Chengjiang lagerstätten. Estimates of the duration of the Cambrian Explosion must be revised accordingly.
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