Chixculub: may not be the whole story (Introduction)

by David Turell , Monday, December 17, 2018, 01:23 (2168 days ago) @ David Turell

By studying the prior period and other places on Earth, the story as to the dino extinction may be better understood:

https://www.smithsonianmag.com/science-nature/we-still-dont-know-why-reign-dinosaurs-en...

“'In order to get an idea of what happened in the wake of the asteroid impact, we need solid baseline data on what rates of background extinction were like before the K/Pg took place,” Natural History Museum paleontologist Paul Barrett says. A moment of catastrophe can only make sense within the broader context of life before and after. “This would make the difference between the cataclysmic events at Chicxulub being either the primary cause of the extinction or merely the coup de grace that finished off an ecosystem whose resilience had been gradually worn away.”

"While the K/Pg extinction was a global crisis, how it played out at various locales around the planet is largely unknown. The amount of information at any given location depends on how well the relevant rock layers are preserved and how accessible they are to scientists. Some of the best exposures happen to be located in western North America, where there’s a continuous sequence of sedimentary layers recording the end of the Cretaceous straight through to the beginning of the Paleogene. These rocks offer before and after shots of the extinction, and it’s these exposures that has allowed Royal Saskatchewan Museum paleontologist Emily Bamforth to investigate what was happening in the 300,000 years leading up to the explosive close of the Cretaceous.

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"This picture of shifting ecosystems inverts the focus of the K/Pg disaster. While the reason non-avian dinosaurs and other organisms died off always grabs our attention, it’s been harder for scientists to determine why the survivors were able to pass through to the next chapter of life’s history.

"Species that survived the impact were typically small, semi-aquatic or made burrows, and able to subsist on a variety of foods, but there are some key contradictions. There were some small non-avian dinosaurs that had these advantages and still went extinct, and many reptiles, birds and mammals died out despite belonging to broader groups that persisted.

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"The surprise, Holroyd found, was that the difference between the survivors and the extinct of the K/Pg event mimicked a pattern that has held true for tens of millions of years before and after the asteroid impact. Species living on land, particularly large species, tend not to persist as long as those living in freshwater environments. Terrestrial species often go extinct at a greater rate than those in aquatic environments even without a massive catastrophe to take them out of the picture. Species that lived in and around freshwater habitats appear to have persisted longer even when there wasn’t a crisis, and when the extinction at the end of the Cretaceous struck in full force, these organisms had an advantage over their purely terrestrial neighbors.

"But even in their relatively safe aquatic environments, everything wasn’t peachy for water-faring animals. Holroyd notes that Cretaceous turtles, for example, lost fifty percent of their diversity globally, although only about twenty percent in the more localized area of western North America, further underscoring the importance of understanding local versus global patterns. Even lineages that can be considered “survivors” still suffered losses and may not have bounced back to their former glory. Marsupial mammals, for example, survived the mass extinction as a group but had their diversity and abundance drastically cut back.

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"How these ecological pieces fit, and what they mean for life’s recovery after the extinction, have yet to fully come into focus.

“'The western interior of North America gives us our only detailed window on what happened to life on land during the K/Pg extinction, but it’s totally unclear if this was typical,” Barrett says. “We don’t know much about how the intensity of the extinction varied around the world,” especially in locations that were geographically distant from the asteroid strike. “It seems unlikely that a one-size-fits-all model would be responsible” for cutting down organisms as different from each other as Edmontosaurus on land and coil-shelled ammonites in the seas, among so many other species lost to the Cretaceous."

Comment: This article could not be more exact in its notation of how important ecosystems (econiches) are and how their imbalance leads to extinctions, and also how important are extinctions which allow new species to appear, while we still have no idea how speciation happens. The flip side is that it is not every day survival that is important for evolution to happen but extinctions, which result in tremendous changes and challenges. And not all extinctions have to be as dramatic as Chixculub, but can be quite local. There is no evidence that the old theory that species competition drives evolution has any basis.