Wednesday, November 12, 2008

Trace Fossils and the End Permian Extinction

The biggest extinction event in Earth History is the Permian/Triassic Extinction (~251 Mya), where almost 95% of marine species went extinct (that’s a lot!). In fact, it was such a serious extinction, that stromatolites made a dramatic resurgence in many shallow-water settings immediately following the big die-off. But, since it’s a Mass Extinction ™, folks are always wrangling over what the mechanism for the Big Kill-off was.

One of the popular explanations has been the oxygen-stress hypothesis, which posits that extensive anerobic and dyseroic conditions (perhaps related to unstable ocean water masses interacting with the unique paleogeography of the Permian world) may have played an important role in the extinction. A good summary of the End Permian Extinction mechanism can be found in Knoll et al. (2007), available here (for FREE!).

A recent paper in Geology, Beatty et al. (2008), points out that anoxic/dysoxic conditions present testable hypotheses, particularly in the paleontological realm. Permian body fossils, however, can’t really be used due to taphonomic complications; they’re just too chewed up. Beatty et al. (2008) instead use the trace fossil record from these intervals, and identify a complex suite of post-extinction ichnofossil assemblages preserved in high energy, shallow-marine deposits.

According Beatty et al (2008), these bioturbated shallow marine rocks represented oxygenated refugia in the otherwise anoxic Panthalassa Ocean. The figure below is Figure 3 from page 773 of Beatty et al (2008), and shows their explanation for why these oxygen rich shoreface successions existed:

In effect, wave agitation, enhanced by storm-activity, drove aeration in shoreface sands, while the deeper water settings suffered under anoxic conditions (which are borne out by the restricted trace fossils assemblages there). In these oxygen-rich shoreface zones, the effects of the oxygen-deficiency were greatly reduced, perhaps allowing for reduced recovery time following the end Permian event. It’s a pretty neat paper, and perhaps helps explain the occurrence of some Early Triassic ichnofaunas that didn’t seem to follow the apocalyptic pattern in other environments.

Man, is there anything those trace fossils can’t do!?!

References Cited

Knoll, A.H., Bambach, R.K., Payne, J.L., Pruss, S., Fischer, W.W., 2007, Paleophysiology and end-Permian mass extinctions: Earth and Planetary Science Letter, v. 256, p. 295-313.

Beatty, T.W., Zonnenveld, J-P, Henderson, C.M., 2008, Anomalously diverse Early Triassic ichnofossil assemblages in Northwest Pangea: A case for a shallow-marine habitable zone: Geology, v. 36, p. 771-774.

EDIT: Turns out Shanan Peters, of Mass Extinction Nature Paper fame, has commented on this paper in Nature's Journal Club; check it out here (if you have access to Nature).

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