Phanerozoic Stromatolites: Decline during the Radiation of Multicellur Animals with Brief Revivals after Extinction Events

Microbialite abundance and morphological disparity reached a highpoint during the neo-Proterozoic. With the advent of large, multicellular animals, the abundance and morphological disparity of stromatolites and microbialites declined rapidly as animals began disrupting and feeding on the microbial mats. The Cambrian Evolutionary Fauna (EF) had relatively low diversity compared to later EFs and this is reflected by relatively modest bioturbation of sediments. Although reduced in importance, stromatolites were still common in Cambrian and Early Ordovician shallow marine environments. The expansion of the Paleozoic EF in the middle Ordovician coincided with reduction of abundance and morphological disparity of stromatolites to near modern levels accompanied by sequestering to extreme environments lacking animals. The end-Ordovician, Late Devonian and end-Permian extinction events marked temporary declines of animals, and also marked the beginning of intervals several MY in length when stromatolites increased in abundance and when morphologies not seen since the Cambrian reappeared. Localized environmental disasters also produced local increases in stromatolite abundance. However, following the end Cretaceous extinction event no stromatolite resurgence has been recorded. One of the characteristics of the Modern EF (which expanded following the end Permian extinction) is that infaunal animals increased in abundance, and many of these infaunal animals were detritus feeders. Primary production was lost for several months during darkness caused by the K/T asteroid impact. But detritus feeders survived the K/T by relying on detritus that was already stored in the sediments, and these animals may have prevented a resurgence of stromatolites. In the future, increasing solar luminosity will bring temperatures above levels that support life within ~0.5 to ~1.5 BY. Animals should thrive in the oceans for only part of this time, after which a second interval of microbialite dominance similar to that prior to the Phanerozoic will be followed by a decline until oceans evaporate.