Scientists working at the University of Chicago have shown a mass extinction that occurred 360 million years ago helped reset Earth’s evolutionary clock and set the stage for the evolution of modern animals.
This massive extinction known as the Hangenberg event occurred at the end of the Devonian Period of Earth’s history, 416 to 359 million years ago. What followed was a massive gap in the fossil record known as ‘Romer’s Gap’, which lasted 15 million years. Researchers Lauren Sallan and Michael Coates discovered this extinction effectively created a new evolutionary starting point at a time when the first vertebrates were making the transition from the sea to the land.
“The extinction was global” says Sallan, “It reset vertebrate diversity in every single environment, both freshwater and marine and it created a completely different world.” In the words of Coates, it is as if “something happened that almost wiped the slate clean.”
Prior to this event, life on earth was dominated by gigantic armoured fish such as Dunkleosteus in what was known as the Age of Fish. More modern creatures such as ray-finned fish and sharks existed but were very much in the minority. Some tentative attempts had also been made by creatures such as Tiktaalik and Ichthyostega towards life on land. Following the extinction, however, the armoured fish and the early land animals disappeared and were eventually replaced by the ancient ancestors of the majority of land animals today. The authors propose that modern vertebrate traits such as the five digit limbs shared by all mammals, bird and reptiles in the womb may have been set by an early ancestor which occurred at this time.
This work was made possible by using advances in modern analytical techniques to look at events that occurred millions of years ago. It explains why an event so catastrophic went undetected for so long.
As Sallan says, “It took the right methods to reveal its magnitude”. What is still unknown though is precisely what happened to wipe out the vast majority of life on earth. Other researchers have suggested that substantial glacial formation at the end of the Devonian may have lowered sea levels affecting life. Another mystery is why did those groups of animals that dominated before the extinction not recover? What is certain is that this event had major consequences for life on earth as we know it and remains as a pivotal moment in the history of the planet.
The work was published in the journal Proceedings of the National Academy of Sciences (PNAS).