Why Small Plankton Survived the K-Pg Extinction

Study lead author Dr Rui Ying showing an example of the Cretaceous paleogeography/bathymetry model in the paper. On the right is the simulated ocean current with small arrows representing the direction of water movement.
Photo Credit: University of Bristol

Scientific Frontline: Extended "At a Glance" Summary: Extinction Patterns of Prehistoric Marine Life

The Core Concept: A recent study reveals that microscopic marine organisms survived the mass extinction that wiped out non-avian dinosaurs because their smaller body size required less energy and allowed them to tolerate extreme darkness and turbulent waters.

Key Distinction/Mechanism: Survival was primarily dictated by metabolic needs and environmental adaptability. Small plankton thrived in post-asteroid darkness due to lower energy demands, while larger marine species adapted to high light and warmer waters perished.

Origin/History: The research investigates the Cretaceous-Paleogene (K-Pg) boundary, a mass extinction event that occurred approximately 66 million years ago following the catastrophic Chicxulub asteroid impact.

Major Frameworks/Components:

• Deployment of a unique numerical model designed to map marine ecosystem traits on a global scale.
• Analysis of the base of the food chain (plankton) using survival trade-offs, predator-prey dynamics, and specific physical attributes like temperature, light levels, and body size.
• Utilization of century-timescale environmental proxy data to isolate the primary causes of selective species survival.

Branch of Science: Paleontology, Marine Biology, Oceanography, and Earth Sciences.

Future Application: The trait-based modeling used to evaluate this ancient biodiversity crisis can be applied to forecast how modern marine ecosystems will respond to ongoing climate change, including hotter environments and altering ocean conditions.

Why It Matters: By conclusively linking environmental shifts to selective fossil record patterns, this research solves a longstanding evolutionary enigma, offering unprecedented insights into the mechanics of global mass extinctions and ecosystem resilience.

Image shows microscopic marine plankton, which are larger in size and went extinct. 
Image Credit: Brian Huber / Smithsonian

Scientists have shown conclusively for the first time that tiny marine organisms in polar oceans survived the mass extinction event that wiped out prehistoric dinosaurs because they needed less energy and were more tolerant of darkness.

The study, led by the University of Bristol and published in the journal Nature, sought to solve a longstanding evolutionary enigma: what factors determined whether marine species would survive the mass extinction event at the Cretaceous–Paleogene (K–Pg) boundary some 66 million years ago? The findings revealed that being small and accustomed to darkness proved to be the vital attributes.

Study lead author Dr. Rui Ying said, “It’s an exciting breakthrough. For so many years, scientists have been unable to test what actually determined whether a species prevailed or perished, because the extinction event involved multiple environmental changes, such as ocean acidification and darkness.

“It is difficult to understand the causality because of the lack of fossil data and environmental proxy data, especially at the century timescale. Using a numerical model, I looked at the base of the food chain—plankton—which helped us identify the most likely cause and the best survival strategies for plankton.”

The Cretaceous–Paleogene (K–Pg) boundary is an ancient and much-studied geological signature marking the mass extinction that wiped out non-avian dinosaurs, separating the Mesozoic Era (the age of reptiles) from the Cenozoic Era (the age of mammals). It is thought that the impact of an asteroid, called Chicxulub, caused the extinction of around 75% of species in the fossil record by triggering catastrophic environmental changes.

Despite decades of research, the mechanisms linking the environmental changes to the selective extinction patterns observed in the fossil record have, until now, been unresolved. By creating and deploying a unique model that maps ecosystem traits globally, the scientists have been able to establish what attributes resulted in the marine plankton community’s survival.

Dr. Ying, who is now a senior research associate at the University of East Anglia, said, “The model is based on traits and the trade-offs of how often they are eaten by predators and what they can eat against specific attributes, such as temperature, light level, and body size.”

Study coauthor Dr. Fanny Monteiro, an associate professor of ocean sciences at the University of Bristol, explained, “The body size and abundance of small plankton mean the organisms rely on less energy, increasing their likelihood of survival. An ability to deal with lower light, darkness, and turbulent waters in higher latitudes also makes them more adaptable to polar regions. In contrast, species adapted to higher light and warmer waters were more vulnerable to this type of mass extinction.”

The model allowed the traits of millions of organisms to be analyzed and quantified with unprecedented accuracy, providing important insights into the physical and chemical changes linked to diversity. Besides shining a light on the distant past of marine life, the research can also help inform forecasts of how ecosystems might respond in the future.

Study coauthor Professor Daniela Schmidt, a professor of earth sciences at the University of Bristol, said, “This study not only demonstrates how trait-based models can help us better understand biodiversity crises in ancient history, but it also has the potential to indicate how less light and hotter environments, as a result of global warming, might impact current and future ecosystems.”

Funding: The research was funded by a China Scholarship Council (CSC)–Bristol Ph.D. Scholarship and NERC grants.

Published in journal: Nature

Title: Darkness and body size shaped end-Cretaceous marine extinction patterns

Authors: Rui Ying  (应锐), Fanny M. Monteiro, James D. Witts, and Daniela N. Schmidt

Source/Credit: University of Bristol

Edited by: Scientific Frontline

Reference Number: pal052826_01

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