Who did it? Who pulled the trigger, or rather, what pulled the trigger at the Triassic-Jurassic boundary that wiped out 20% of all marine families in Earths oceans, and, on land, most non-dinosaurian archosaurs, most therapsids, and the last of the large amphibians? Whatever it was, it shot down much of the competition so dinosaurs could later dominate the Earth.

"The Triassic-Jurassic boundary extinction event is one of the 'big five'mass extinctions of the Phanerozoic Eon, profoundly affecting life on land and in the oceans," explained Lawrence Tanner, a Professor of Geography and Geoscience at Bloomsburg University in Pennsylvania. Tanner will shed light on the various extinction theories on Wednesday, June 27, at the Earth System Processes meeting in Edinburgh Scotland.

"Ultimately, this presentation concerns our ability to test various hypotheses for the cause of large-scale extinction events of the past," he said.

Tanner will address possible explanations for this event. One is gradual climate change or sea-level change during the Late Triassic, but these explanations fail to explain the suddenness of the extinctions in the marine realm. Then there's; asteroid impacthowever, no impact structure can be tied directly to the Triassic-Jurassic boundary. And the current favorite hypothesis is that flood basalts that constitute the Central Atlantic Magmatic Province erupted. And the release of CO2 or SO2 aerosols during these eruptions are thought to have caused intense global warming (from the former) or cooling (from the latter). But Tanner's new data on the isotopic composition of fossil soils of Late Triassic and Early Jurassic age demonstrates that there is no evidence of any change in the CO2 composition of the atmosphere.

So the smoking gun still eludes scientists and the cause of this extinction is still at large. "Other possibilities need to be investigated more fully," Tanner explained.

The Geological
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Contact: Ann Cairns
acairns@geosociety.org
303-447-2020 ext. 11
Geological Society of America
27-Jun-2001