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Mystery #1: What killed the phytosaurs?

Jennifer has been using fossilized leaf stomata to investigate some of the mysteries of the past — including what caused different extinction events. Though the word sounds disastrous, extinction is a normal process in the history of life. Lineages go extinct for different reasons — perhaps a random event (like a hurricane) hit them particularly hard or perhaps they were out-competed by another species. Regardless, extinction is an important aspect of evolution. Extinctions can free resources and niche-space of which another lineage might be able to take advantage, leading to a new evolutionary trajectory. However, at particular points during the history of life, extinctions of massive proportions have occurred, killing off a large percentage of species alive at the time and altering the course of evolution. These events are called mass extinctions. Mass extinctions are easy to identify in the fossil record — but figuring out what caused them is much more difficult.

Jennifer is particularly interested in the cause of the end-Triassic mass extinction some 200 million years ago. This extinction decimated species in marine reefs and killed around 50% of North American vertebrates, including the phytosaur shown below. But what caused this extinction? Many hypotheses had been proposed - meteorites, global cooling, global warming, sea level rise, and sea level fall - but no conclusive evidence had yet supported one hypothesis in particular. Jennifer decided to study the climate at the time of this event to see what clues it might offer.

Phytosaurs, which had many of the same adaptations as crocodiles, went extinct at the end of the Triassic.

Jennifer collected fossil leaves deposited before, during, and after the mass extinction event. She discovered a major drop in the number of stomata on leaves coinciding with the mass extinction — and that meant major increases in carbon dioxide and global temperatures right at the time of the mass extinction. Her calculations suggested that global temperatures would have risen 5° C - such a global warming translates into a regional warming of up to 16° C!

Stomata density through the end-Triassic extinction dropped, and that meant increased atmospheric CO2 and global temperature

Jennifer's data on fossil stomata supported the hypothesis that global warming played a role in the end-Triassic mass extinction. To further investigate this hypothesis, she completed two more studies:
  • First, she used mathematical models of plant physiology to simulate how a plant would respond to the warmer climate at the end-Triassic. These models suggested that end-Triassic plants — especially plants with large leaves — would have overheated at those temperatures.

  • Second, she catalogued which terrestrial plant lineages went extinct during this extinction event and found that large-leafed lineages were particularly likely to go extinct — exactly what one would expect to find if global warming had contributed to the mass extinction! As Jennifer puts it, "This is a really nice result. It shows that our predictions from modern ecological understandings of plants were borne out in the fossil record."

    Small-leafed lineages survived the end-Triassic extinction event, while large-leafed lineages did not.

Together, these various lines of evidence (changes in fossil stomata, mathematical models of plant physiology, and studies of diversity before and after the extinction event) strongly support the hypothesis that global warming played a role in the end-Triassic mass extinction. Such intense global warming would likely have had a cascade of consequences for end-Triassic ecosystems. Some lineages might have gone extinct as a direct result of this warming — for example, large-leafed plant species that overheated. Those extinctions could have had a ripple effect on other species that depended on them, potentially even causing extinctions of carnivores like the phytosaur. And the increased carbon dioxide levels along with higher temperatures could have had similar effects on ocean life. Although no one is yet certain of the exact sequence of events, it is clear that global warming of this magnitude has the potential to cause major changes in ecosystems.

Global greenhouse

Mystery #2: What caused oceanic mass extinctions?

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