Plankton turn hunters to survive dinosaur-killing asteroid impact

New research by an international team of scientists shows how marine organisms were forced to 'reboot' to survive following the asteroid impact 66 million years ago which killed three quarters of life on earth.

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
[Credit: Paul Bown]

Researchers from the University of Southampton and UCL, along with colleagues in Paris, California, Bristol and Edinburgh used an exceptional record of plankton fossils and eco-evolutionary modelling techniques to examine how organisms behaved before and after this extinction event - and why some survived and some didn't.

The team found that prior to the asteroid impact, species of nannoplankton - microscopic algae - were exclusively reliant on harnessing energy from sunlight (photoautotrophs), but those living afterwards were capable of capturing food and eating it in addition to using photosynthesis to feed (mixotrophs). This suggests the blocking of light from the sun played an important role in killing off some species and over time, encouraging others to evolve and adapt.

The research team's breakthrough came when they found that many of the nannoplankton skeletons (coccospheres) post mass-extinction included a large hole, indicating the position of flagella - tiny tail like structures used by the algae for movement and feeding. This indicates these microscopic organisms, which survived the asteroid strike, were capable of hunting and ingesting food.

"Those species that were lost at the mass extinction show no evidence of a mixotrophic lifestyle and were likely to be completely reliant on sunlight and photosynthesis," explains Dr Samantha Gibbs of the University of Southampton. "Fossils following the Cretaceous-Paleogene (K-Pg) extinction show that mixotrophy dominated and our model indicates this is because of the exceptional abundance of small prey cells - most likely surviving bacteria - and reduced numbers of larger 'grazers' in the post-extinction oceans."

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) with
the scale bars next to each image showing the size of a micron (1/1000th mm)
[Credit: Paul Bown]

Opposing evolutionary forces led to the emergence of more diverse feeding strategies and eventually a return to greater reliance on photosynthesis in open ocean nannoplankton. Most nannoplankton today only photosynthesise. So, what caused this devastating mass extinction of photoautotrophs and other species?

The simple answer is a lack of light. The K/Pg event was triggered by an asteroid impact that formed the Chicxulub crater in Mexico, 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," says Paul Bown, Professor of Micropalaeontology at UCL. "The significant 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 the darkness caused by the asteroid impact acting as a kill mechanism."

Graphic explaining the research method and findings
[Credit: Gibbs et al., 2020]

Samantha Gibbs adds: "This 'blackout' or 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."

Findings are published in the journal Science Advances.

Source: University of South Hampton [October 30, 2020]

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Cracking the secrets of dinosaur eggshells

Since the famous discovery of dinosaur eggs in the Gobi Desert in the early 1920s, the fossilized remains have captured the imaginations of paleontologists and the public, alike. Although dinosaur eggs have now been found on every continent, it's not always clear to scientists which species laid them. Now, researchers reporting in ACS Omega have narrowed down the list for an unknown eggshell from Mexico by comparing its microstructure and composition with four known samples.

Researchers studied eggshell microstructures to help estimate whether an unknown sample
was laid by an ornithopod (herbivorous; top) or a theropod (carnivorous; bottom)
[Credit: Nerith R. Elejalde-Cadena et al. 2020]

Because many dinosaur eggs are similar in size and shape, it can be difficult to determine what type of dinosaur laid them. Clues can come from fossilized embryos (which are rare), hatchlings in the same nest or nearby adult remains. Scientists also have identified microscopic features of eggshells that differ among groups of dinosaurs. In addition, researchers have studied the elemental composition of fossil eggshells to learn more about the paleoenvironment and conditions that led to the eggs' fossilization. 

Abel Moreno and colleagues wanted to compare the microstructure and composition of five dinosaur eggshells from nests in the El Gallo Formation of Baja California, Mexico. Based on the eggs' shapes and sizes and the fossil record of the area, the researchers had concluded that three of the eggs were laid by ornithopods (bipedal herbivores) of the hadrosaur family (duck-billed dinosaurs) and one by a theropod (bipedal carnivores) of the troodontidae family (small, bird-like dinosaurs). The remaining sample was too damaged to classify by the naked eye.

Using scanning electron microscopy, the team examined the external and internal surfaces and a cross-section of each eggshell. In contrast to the smooth outer surface of the theropod shell, the shells from the ornithopods and the unknown sample had nodes at different distances across the shell. Images of shell cross-sections from the ornithopods revealed that mammillary cones -- calcite crystals on the inner surface of the shell -- formed thin, elongated columns arranged in parallel, with irregular pores. 

In contrast, the eggshell from the theropod showed thicker, shorter cones arranged in a bilayer, with wider pores. The unknown sample more closely resembled the ornithopod eggshells, leading the researchers to hypothesize that it was probably also from the hadrosaur family. In addition, the researchers conducted an elemental composition analysis, which they say is the first such analysis on dinosaur eggshells collected in Mexico. They say the findings might help reveal how the fossilization process varied among species and locales.

Source: American Chemical Society [October 28, 2020]

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