The less spectacular period boundary of the Mesozoic…

I guess most people interested in geology and the history of life on Earth have heard about the dramatic period boundaries of the Mesozoic.

The most well known is of course the upper boundary of the Mesozoic, the Cretaceous-Paleogene boundary which marks the demise of the dinosaurs ca 65 million years ago. This boundary is marked not only by a catastrophic extraterrestrial impact, but is also contemporaneous to massive volcanism (Deccan Traps in India) and subsequent climatic and environmental  changes of these events.

Almost equally famous and marked by an even bigger mass extinction is the lower boundary of the Mesozoic, the Permian-Triassic boundary ca 251 million years ago, associated with the end-Permian mass extinction event when ca 95% of all life died out. The end-Permian event is linked to massive volcanism in Siberia and the longterm climatic and environmental consequences where so severe that the major part of the Triassic is characterised by a harsh arid hothouse climate.

Some have probably also heard of the Triassic-Jurassic boundary, ca 201 million years ago, also associated with a major mass extinction event linked to the perhaps largest large igneous province known, the Central Atlantic Magmatic Province – massive volcanism during the initial opening of the Central Atlantic – with severe climatic and environmental consequences.

Am I repeating myself? 😉

But how many have heard of the Jurassic-Cretaceous boundary ca 145 million years ago?

This less well-known period boundary is also marked by major climatic changes. At least on the northern Hemisphere. Palynological, sedimentological and geochemical studies bear witness of climatic change from predominantly arid to semi arid conditions in the latest Jurassic to more amicable humid conditions in the earliest Cretaceous of NW Europe (see e.g. our paper Lindström and Erlström, 2011 for references).

So far, this less spectacular period boundary has not been linked to any major volcanic events or extra terrestrial impacts. And now Valentin Fischer and his colleagues have published a study in PLoS ONE (read paper here) about one specific ischtyosaur that survived the less famous Jurassic-Cretaceous (J/K) mass extinction and in fact their study suggests that the J/K event hardly affected ischtyosaurs at all 😀

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Extraterrestrial impact helped cause the end-Triassic mass extinction?

In a news feature in the latest issue of Nature a team of geologists lead by Paul Olsen and Dennis Kent are in search for evidence that connect the end-Triassic mass extinction with the Rochechouart impact crater in France, which recently was dated to 201.2 ± 2.0 million years ago (Schmieder et al., 2010). Triassic-Jurassic boundary rocks in the UK are known to contain disturbed sediments close to the level of extinction, and Simms (2003) suggested that these “seismites” were in fact impact related.

The Rochechouart is a farly small impact crater, measuring only 20-25 km in diameter compared to the 180 km width of the Chixculub impact crater of the Cretaceous/Paleogene event.

Could the Rochechouart impact have helped cause the end-Triassic mass extinction event?

Well, Olsen makes sure all angles are covered:

“Perhaps it was one of a series of asteroids that hit around the same time. Alternatively, a lone French crash might have been the final straw for a world already reeling from volcanic eruptions. Or the impact may have come first, weakening ecosystems enough that when the eruptions started, life took a nosedive.”