The Cretaceous-Paleogene mass extinction – severity, recovery, and biogeography

Is evolution driven primarily by the slow action of everyday processes, or do the rare, extreme events – disasters, catastrophes, and sudden environmental shifts – play the defining role in evolution? Darwin saw evolution playing out gradually, with the slow and steady action of competition, predation, and environmental changes driving evolution. Much as the rain slowly wears away a stone, he saw such changes gradually altering species, generation by generation, across millions of years. Darwin’s view was shaped by the idea of uniformitarianism – the idea that the present is the key to the past. The problem with this view is that the present is a very short time. Darwin lived for less than a century, a geological eyeblink. Over the course of millennia and eons, extreme, highly improbable events outside the realm of human experience – massive volcanic eruptions, runaway climate change, and other disasters – will become probable. Cuvier, the father of paleontology, saw the history of life differently. Studying the fossil record, he saw how species disappeared and were replaced by completely different species. He saw catastrophes as wiping out entire faunas and paving the way for new species. Cuvier’s perspective may also have been a product of his times. While Darwin lived in a time of relative peace and stability, Cuvier lived through the French Revolution and the Napoleonic Wars, and saw first-hand how the established order could be suddenly, violently overturned.

Darwin’s view long dominated evolutionary biology, but the fossil record has increasingly lent support to Cuvier’s view. The fossils show that at key points in Earth’s history, vast numbers of species – over half of all the species on the planet – disappeared suddenly, worldwide. At least five major mass extinction events are now recognised, along with many lesser events. The most recent and perhaps the most famous is the extinction of the dinosaurs 66 million years ago. Evidence increasingly ties this extinction to the impact of a giant asteroid in the Yucatan peninsula, which kicked dust and debris into the air, darkening the skies for years, and causing photosynthesis to shut down, slowly starving the planet.

Some of the smaller dinosaur species that were wiped out by the asteroid.

Yet there is still much we don’t know about the extinction. How severe was it – what percentage of species were wiped out? Some estimates suggest extinction rates of 75%, but recent work by my colleagues and I suggests that up to 90% of terrestrial species were wiped out in western North America, suggesting that the extinction was even more severe. What characteristics made species vulnerable? Surprisingly, highly diverse groups often became extinct, suggesting that the specialist species that make up diverse groups fared poorly compared to generalists. How did the extinction affect biodiversity? Mammals rapidly moved in to replace the dinosaurs, but they did not simply replace dinosaurs but evolved entirely new kinds of animals – bats, with their ability to fly, whales, able to dive deep in the ocean, primates, characterised by remarkable intelligence. The innovation of mammals and other groups in the aftermath suggests that counterintuitively, the long-term effects of mass extinction may be to drive an increase in diversity. Is it possible that extinctions drove the distribution of species as well, that colonisations of plants and animals in the wake of extinction- and not slow continental drift – help explain the distribution of species.

To understand how extinction not only creates but destroys diversity, our lab is studying Cretaceous-Paleogene transition. Over the next five years, we will study fossils of small dinosaurs from western North America and giant marine reptiles from Morocco, comb through databases and run simulations to understand patterns of extinction, and look at the evolutionary relationships of modern species to  understand whether we can trace their origins to bursts of evolution in the wake of the asteroid impact. By studying the biological effects of the asteroid impact, our goal is to study the fundamental processes that drive evolution, and explain the origin of the diversity all around us.  

Dr Nicholas Longrich
University of Bath
Research Leadership Award