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Plankton turn hunters to survive dinosaur-killing asteroid impact.

30 October 2020

So, what caused this devastating mass extinction? The bias found in the nannoplankton extinctions – removal of open-ocean photoautotrophs but survival of mixotrophs that could hunt and feed – can only be fully explained by impact darkness as a kill mechanism.

Fossil cell coverings (coccospheres)

How did some marine organisms survive the asteroid impact event 66 million years ago and reconquer the oceans in the aftermath? A team that includes four UCL researchers (Paul Bown, Hojung Kim, Sarah Alvarez and Odysseas Archontikis) and colleagues from Southampton, Bristol, Paris, Edinburgh and California, have tackled that question using a combination of exceptional plankton fossils and a new eco-evolutionary model. In an article published in the journal Science Advances (30th October 2020), they present a remarkable record of fossil plankton skeletons that reveal a striking shift in lifestyle, shedding light on the mass extinction kill mechanism and why some organisms survived whilst others perished. The Cretaceous/Paleogene (K/Pg) mass extinction was triggered by an asteroid impact that formed the Chicxulub crater, and is well known for the extinction of dinosaurs, plesiosaurs, ammonites and many other groups.

“This huge impact flung vast amounts of debris, aerosols and soot into the atmosphere, causing darkness, cooling and acidification over days and years but we wanted to find out what caused the extinctions in the oceans, why some marine organisms survived and some didn’t, and what life traits were advantageous to the colonisers that reconquered the seas”

says Paul Bown from UCL. “We looked at the most continuous, most abundant fossil record of ocean life we could find – the remains of calcareous algal plankton known as nannoplankton (or nannofossils) – targeting exquisitely preserved fossils of the survivors and their descendants, which told us not only about abundance and diversity, but also original lifestyle and ecology. At the same time, we ran an eco-evolutionary model simulation of a recovering ocean ecosystem to better understand how the base of the food chain would reboot after a mass extinction.”

High-resolution scanning electron microscope (SEM) images of fossil cell coverings of nannoplankton (coccolithophores)
Image: High-resolution scanning electron microscope (SEM) images of fossil cell coverings of nannoplankton (coccolithophores) highlighting holes that would have allowed flagella and haptonema to emerge from the cell and draw in food particles (red dots). We have shown a reconstruction of one of these ancient cells based on living coccolithophores and related algae. On the right is a SEM view of a seafloor after the extinction showing the abundance of these cells with flagellar openings. These cells are around 7 microns in diameter (7/1000ths of a millimetre) (images P. Bown) with the scale bars next to each image showing the size of a micron (1/1000th mm).

What did this tell us? The research team’s big break came when they found that many of the nannofossil skeletons (coccospheres) after the mass extinction included a large hole (see image below), indicating the position of flagella in life and therefore that these cells were able to capture and ingest food. “This is really unusual compared with their modern counterparts that rely almost entirely on photosynthesis for nutrition – they are ‘plant-like’ plankton” say Samantha Gibbs from Southampton. “We then focused on reconstructing the ecology and distribution of the 11 or so survivor species and again concluded that they were capable of capturing and eating food, as well as photosynthesis – a strategy known as ‘mixotrophy’ (i.e., mixed food sources)”. However, those species that were lost at the mass extinction show no evidence of this mixotrophic lifestyle and were likely completely reliant on sunlight and photosynthesis. Fossils above the K/Pg boundary show that mixotrophy dominated in the aftermath of the mass extinction and our model indicates that this is because of the exceptional abundance of small prey cells – likely survivor bacteria – and reduced numbers of larger grazers in the post-extinction oceans. Opposing evolutionary forces eventually led to the emergence of more diverse feeding strategies and a return to greater reliance on photosynthesis in the open ocean nannoplankton.

So, what caused this devastating mass extinction? The bias found in the nannoplankton extinctions – removal of open-ocean photoautotrophs but survival of mixotrophs that could hunt and feed – can only be fully explained by impact darkness as a kill mechanism. This shutdown of primary productivity would have been felt across all of Earth’s ecosystems and reveals that the K/Pg event is distinct from all other mass extinctions that have shaped the history of life both in its rapidity, related to an instantaneous impact event, and its darkness kill mechanism, which shook the foundations of the food chains. “The K/Pg boundary event likely represents the only truly geologically instantaneous mass extinction event” says Paul Bown.

Links:

  • Samantha J. Gibbs, Paul R. Bown, Ben A. Ward, Sarah A. Alvarez, Hojung Kim, Odysseas A. Archontikis, Boris Sauterey, Alex J. Poulton, Jamie Wilson and Andy Ridgwell. Algal plankton turn to hunting to survive and recover from end-Cretaceous impact darkness. Oct 2020 Science Advances 6: DOI
  • Prof Paul Bown research profile
  • Micropalaeontology Research group