How scientists pinpointed the death of the dinosaurs to a warm spring day

Spring is the season for new life and new love; a time of renewal both physical and spiritual. And on one fateful day millions of years ago, it was also the season of death.

The asteroid that wiped out the dinosaurs and much of the rest of life on Earth 66 million years ago likely hit our planet in the springtime, says a team of European researchers in a new paper in Nature.

They took a look at growth rings in the fossils of fish killed during the impact, and say they match up with patterns of growth typically seen during the spring. It’s a find that helps to explain a few puzzling trends in the aftermath of the destruction, they say, including which animals survived and which didn’t.

Here’s the background — In the Earth’s geologic record, there’s a curiosity: A thin layer of iridium, found in rock deposits all over the world dating to 66 million years ago. Iridium isn’t often found near Earth’s surface, but it’s pretty common in asteroids. That layer would eventually lead father-son duo Luis and Walter Alvarez to theorize that a massive asteroid hit Earth right around then — a time that also corresponded to a huge extinction event.

Today we know much more about that hellish day. A six-mile-wide asteroid slammed into Earth’s surface just off the Yucatan peninsula, triggering megatsunamis and wildfires for hundreds or even thousands of miles around. Droplets of glass from rock melted by the impact and cast high into the atmosphere rained down around the world. The cloud of dust kicked up by the impact blocked out the Sun, starving plants and creating a global winter that may have lasted decades. Some three-fourths of all species on Earth died, including the non-avian dinosaurs.

The impact was a global catastrophe. But in their new paper, the scientists take a close look at one very specific part of the world in the minutes after impact.

What’s new — Sixty-six million years ago, in a small creek or river in what is now North Dakota, paddlefish and sturgeon were foraging for food. It was springtime, and they’d emerged from a long winter ready to eat and perhaps to mate. But their feeding was interrupted by a sudden shift in the currents, and a strong rush of water carried them away and upstream. If they looked up, they may have noticed a darkening sky, and maybe even the flicker of fire. But they were fish, and they probably didn’t.

That surge of water was borne on currents created by the asteroid impact thousands of miles away. It gathered up the plants and animals living in inland streams and swept them away, to be buried alive under tons of silt and debris. Millions of years later, they formed what’s called a thanatocoenosis — a mass death deposit.

Paleontologists recently unearthed many of these fossilized fish from the Hell Creek formation in North Dakota, and they noticed small bits of rock embedded in their gills. These were drops of molten rock and glass, called impact spherules, that rained down after the asteroid hit. They were falling even as the fish were being swept away.

The impact spherules tied the fishes’ death to the asteroid. But the researchers were able to go a step further. Some fish bones grow by layering new material on top of old material. It leaves a series of growth rings not unlike those found in trees. Some layers, like those laid down in spring, are thicker, showing that the fish were growing more quickly. Others, like those in winter, are thinner, which translates to slower growth.

In six fossil fish from the Hell Creek deposit, the scientists were able to see that the last layer of their bones, which was created right before they died, was from the beginning of a high growth season, and right after a slow growth season. That corresponds to spring, they say.

Why it matters — So, the dinosaurs died in the spring. It’s a detail that feels, in its near-excess of specificity, almost poetic. But that knowledge is useful to scientists. We know, for example, that plant, insect, and animals populations in the southern hemisphere rebounded almost twice as fast as those in the northern hemisphere. That could be because those populations, including the ancestors of modern-day mammals, were entering a time of hibernation, which could have left them much better prepared to weather months of wildfires and other natural disasters, the authors say.

An impact during the northern hemisphere’s springtime may have also contributed to the demise of the dinosaurs, they propose. If baby dinosaurs had just hatched, or were about to hatch, they would have been much more vulnerable to the cataclysmic disruptions the asteroid caused. The same goes for others species who rely on the springtime to feed and grow after a dormant winter.

Of course, the asteroid impact also led to what was likely decades of winter, and irrevocably changed our planet. Even species that happened to be hibernating would have woken up to a world much changed from the one they knew. And life in the oceans, which may have been protected from some of the initial impact, also went through a massive extinction in the aftermath.

But for some species, perhaps it was the small things that made the difference — like happening to be asleep underground while the planet was torn apart.

Abstract — The Cretaceous–Palaeogene mass extinction around 66 million years ago was triggered by the Chicxulub asteroid impact on the present-day Yucatán Peninsula1,2 . This event caused the highly selective extinction that eliminated about 76% of species3,4 , including all non-avian dinosaurs, pterosaurs, ammonites, rudists and most marine reptiles. The timing of the impact and its aftermath have been studied mainly on millennial timescales, leaving the season of the impact unconstrained. Here, by studying fshes that died on the day the Mesozoic era ended, we demonstrate that the impact that caused the Cretaceous–Palaeogene mass extinction took place during boreal spring. Osteohistology together with stable isotope records of exceptionally preserved perichondral and dermal bones in acipenseriform fshes from the Tanis impact-induced seiche deposits5 reveal annual cyclicity across the fnal years of the Cretaceous period. Annual life cycles, including seasonal timing and duration of reproduction, feeding, hibernation and aestivation, vary strongly across latest Cretaceous biotic clades. We postulate that the timing of the Chicxulub impact in boreal spring and austral autumn was a major infuence on selective biotic survival across the Cretaceous–Palaeogene boundary