PNAS – Proceedings of the National Academy of Sciences of the United States of America
http://www.pnas.org/content/early/
(Accessed 23 July 2016)

.
Commentary
Ecological disruption precedes mass extinction
Steven M. Hollanda,1
[Initial text]
Mass extinctions are dramatic features of the fossil record in which extinction risk is substantially elevated above background levels. Although extinction risk varies markedly over geologic time, as well as geographically, it was particularly elevated and global in extent during the so-called Big Five events: the Late Ordovician, Late Devonian, end-Permian, end-Triassic, and end-Cretaceous (1). These events were originally recognized by variations in extinction rate in marine animal families, and their importance remains in analyses at the genus level and that account for variable preservation over geologic time (2, 3). Increasing attention has concentrated on understanding the ecological effects of mass extinction and other lesser but still significant extinction episodes (4⇓⇓⇓⇓⇓–10). In PNAS, Sheets et al. (11) document the ecological changes in marine planktonic communities not only during, but preceding the Late Ordovician (447–444 Ma) mass extinction.

Examining the ecological changes during a mass extinction would seem to be straightforward: go to a stratigraphic column spanning the mass extinction and describe the changing ecological composition of successive sedimentary layers through the extinction episode. This would be a direct history of ecological changes related to the extinction if those layers recorded the same habitat through time, such as the distance from shore or water depth for marine benthic organisms, or the overlying water masses for marine plankton. Unfortunately, this simple scenario is rarely the case, as numerous studies of sedimentation over the past 40 y have shown (12). Processes of sediment accumulation create two challenges for studying ancient mass extinctions (13).