EGU Blogs

Marine Reptiles

Why I think the Jurassic/Cretaceous boundary is super important

This was originally posted here.

Mass extinctions are insanely catastrophic, but important, events that punctuate the history of life on Earth. The Jurassic/Cretaceous boundary, around 145 million years ago, was originally thought of to represent a mass extinction, but has subsequently been ‘down-graded’ to a minor extinction event based on new discoveries.

However, compared to other important stratigraphic boundaries, like the end-Triassic or the end-Cretaceous, both time periods representing mass extinction events, the Jurassic/Cretaceous (J/K) boundary actually remains really poorly understood. This is both in terms of what was going on with different animal groups at the time, and what environmental changes were occurring alongside this.

Well, I have a new research paper out now that synthesises more than 600 research articles, bringing them together to try and build a single picture of what was going on around this time! It’s free to read here, and is essentially the literature review from my thesis, or as I like to think of it, the justification for my existence as a researcher!

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Crocodiles feeling the heat of extinction

New research on crocodiles shows that a combination of changing sea levels and temperatures were responsible for driving their biodiversity over millions of years.

Living crocodiles are threatened by climate change, with 10 out of 23 species at a high risk of extinction. As ectotherms, animals which require external sources of heat to function, they are sensitive to changes in temperature. With 2-4 degrees of increasing global temperatures predicted over the next century, these ruling reptiles have never been more threatened.

The fossil record is our gateway into understanding the evolutionary history of crocodiles. They actually have a long and complex ancestry, going back around 250 million years. During this time, crocodiles and their ancestors, known collectively as crocodyliforms, diversified into an array of forms, including 12-15 foot giants, dwarfed species no bigger than a cat. Some groups even took to the seas, evolving sleek and ‘dolphin-like’ forms, including flippers! The origin of modern crocodiles, known as crocodylians, was around 80 million years ago, during the Cretaceous period. During this time, most of the Earth was experiencing high temperatures, and known as a ‘greenhouse world’.

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It’s kind of like a turtle-fish-dolphin…

Close your eyes. Go back in time 250 million years, and the world would seems as strange to you as a different planet. On land, there was a whole host of bizarre and now extinct animals: strange, crocodile-like things, and the precursors of dinosaurs; weird mammal-like beasts, that looked like the lost offspring of a hippo and a monitor lizard.

In the seas, marine reptiles dominated. A whole range of unusual animals lived, such as the long-necked plesiosaurs, popularised with reference to the mythical Loch Ness monster. Alongside these were the equally unusual ichthyosaurs. At first glance, a typical one might look like to you much like a dolphin.

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New ‘fish lizard’ used to prowl the Scottish seas

New ‘fish lizard’ used to prowl the Scottish seas

A cool new ichthyosaur – a type of marine reptile – has just been named in the Scottish Journal of Geology. I’ve written about it here, with some great comments from the lead author (Steve Brusatte). Most reports on the new beastie just focus on the finding, but we’ve gone for a different angle by delving into what it means for the evolution of ichthyosaurs during the Jurassic period. Enjoy!

In the mean time, enjoy this maybe-photoshopped reconstruction by James Lewis. Original artwork by Todd Marshall.

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