In the Devonian Period of roughly 400 million years ago, fish were the masters of Earth, filling the seas and rivers with enormous diversity from the 30-foot-long Dunkleosteus to lobe-finned fishes like the modern lungfish. Then, 359 million years ago, something happened. The fossil record tells a clear before-and-after story: Dunkleosteus and its family members, the placoderms, disappear, and the ray-finned fishes that we currently think of when we think about fish went from underdog to dominant. Shortly thereafter, a few survivors figured out how to crawl onto land, and the rest was vertebrate history.
For paleontologists, extinction events are the chapter markers in the story of life, marking profound shifts in the course of evolution. Scientists have long focused on one such marker occurring near the end of the Devonian Period: the Kellwasser Event, considered one of the “Big Five” extinctions in Earth’s history. But evidence for the Kellwasser Event 374 million years ago was based on invertebrate fossils such as clams and mussels, which are abundant and easy to find. The vertebrate fossil record of the time, consisting largely of fishes, is less complete, making it difficult to do the detective work necessary to pinpoint a critical extinction event.
That is, until now, when Lauren Cole Sallan and Michael Coates applied analytic techniques from the field of ecology to a richer dataset of vertebrates fed by recent fossil discoveries. The research, published today by the Proceedings of the National Academy of Sciences, looked at the pattern of vertebrates in the late stages of the Devonian Period, and found a different chapter break – not at the Kellwasser Event, but at the “Hangenberg Event,” 15 million years later.
What’s more, the global species shakeup that resulted from the Hangenberg Event would set the stage for the evolution of all modern vertebrates, including humans. By significantly reducing the species pool, the extinction created a bottleneck effect that allowed a small set of survivors to re-diversify and re-populate.
“There’s some sort of pinch at the end of the Devonian,” said Coates, professor of organismal biology and anatomy at the University of Chicago. “It’s as if the roles persist, but the players change: the cast is transformed dramatically. Something happened that almost wiped the slate clean, and, of the few stragglers that made it through, a handful then re-radiate spectacularly.”
“Everything was hit, the extinction was global,” said Sallan, a graduate student and lead author of the paper. “It reset vertebrate diversity in every single environment, both freshwater and marine, and created a completely different world.”
Though there would be subsequent mass extinctions (such as the famous K-T event, believed to have wiped out the dinosaurs), the mold for modern vertebrates was cast in the Hangenberg event. Spared were the tetrapods, considered to be the ancestors of most land vertebrates, who first explored land habitats around rivers and swamps. Though fossil evidence suggests that some tetrapods, such as Neil Shubin’s Tiktaalik, were already crawling onto land before the Hangenberg event, they almost entirely disappear from the fossil record for 15 million years after the Devonian, a period known as Romer’s Gap. Sallan and Coates’ research confirms that Romer’s Gap was a tough stretch for our fishy great-great-grandfathers.
“The gap is real. Something that is classically seen after an extinction event is a gap in the records of survivors,” Sallan said. “You have a very low diversity fauna, because most things have been killed off.”
“Extinction events remove a huge amount of biodiversity,” Coates said. “That shapes in a very significant way the patchiness of biodiversity that persists to the present day.”
One long-lingering effect of the extinction still visible today may be the shared characteristic of five-digit limbs among humans, birds, mammals, and reptiles, at least in utero. A lucky tetrapod survivor of the Hangenberg event who happened to have evolved five digits may have randomly set the five-finger archetype in place for all subsequent species, the authors theorized.
But mystery remains at the heart of the Hangenberg event: what happened? Identifying the effects of an extinction event from fossils is much easier than determining the cause of said extinction, though paleontologists have been joined by geologists and atmospheric scientists in their curiosity about this particular period. Clues abound, including glacier formation, lower sea levels, shifting continents, crashing carbon dioxide levels, the appearance of the first forests, and more. But finding the original source of all these catastrophic changes is difficult from afar, Coates said.
Equally mysterious is why certain species were invigorated by the Hangenberg event, going on to diversify and repopulate different habitats, while others were fatally damaged – “dead clades walking,” as Coates’ colleague David Jablonski coined. But whatever the details, the extinction event would set the stage for the further evolution of life on Earth, a consequence we can all be thankful for.
“It is a pivotal episode that shaped modern vertebrate biodiversity,” Coates said. “We are only now beginning to place that important event in the history of life and the history of the planet, which we weren’t able to do before.”