The extinction of ammonites, which coincided with that of the dinosaurs, is proving a puzzle, with problems emerging from the favored theory. As the world rushes towards a repetition of some of the environmental conditions seen at the time, the question is now of interest far beyond paleontologists and shell collectors.
The ammonites in question were free-swimming molluscs, not the biblical people from what is now Jordan. For more than 300 million years, they were enormously successful, to the extent that they represent some of the most common fossil finds.
Where the Ammonite people supposedly had their origins in the destruction of Sodom and Gomorrah, the molluscs to which they gave their name went the other way, vanishing from the fossil record as fire rained from the sky in the aftermath of the asteroid collision that led to the extinction of the (non-avian) dinosaurs.
However, the reason for their disappearance is uncertain. Ammonites had, after all, survived at least three previous mass extinctions. A clue may lie in the fact that at the same time more than 90% of calcium carbonate-forming plankton, coccolithophores and foraminifera, died out, although both classes eventually managed to recover. Meanwhile, the toll was far smaller for non-calcifying species.
The most obvious explanation is ocean acidification. When carbon and sulfur dioxide levels in the atmosphere rise, some of it is absorbed by the oceans. Both oxides are acidic when dissolved in water, reducing the slightly alkaline nature of the ocean, without which calcium carbonate formation is impossible.
Where did these gasses come from though? Burning forests would have released a lot of carbon dioxide into the air, and gypsum rocks vaporized in the collision would produce acid rain. However, there is a difference between knowing that these factors would change the pH of the oceans and establishing that the effect would have been dramatic enough to have the drastic effects observed.
This is what Professor Toby Tyrrell of the University of Southampton set out to test. Tyrrell's results are published in The Proceedings of the National Academy of Science. "While the consequences of the various impact mechanisms could have made the surface ocean more acidic, our results do not point to enough ocean acidification to cause global extinctions,” he says. "It throws up the question, if it wasn't ocean acidification what was it?"
Tyrrell says we need to keep investigating other possible explanations, such as soot causing a “nuclear winter.”
The research has modern relevance because carbon dioxide from the burning of fossil fuels is already thought to be interfering with the capacity of some modern species dependent on building calcium carbonate shells or skeletons. Tyrrell and his co-authors note, “Most paleo-episodes of ocean acidification (OA) were either too slow or too small to be instructive in predicting near-future impacts.” If the marine extinction at the end of the Cretaceous really was caused by a sudden release of carbon dioxide, it will be the best guide we can find to what we face if emissions are not rapidly curtailed.