Dinosaurs likely died when life was at its best -- the springtime

These fish didn't know that they were alive at the tail-end of a geological era.

In an evolutionary sense, these fish were lucky. Their two species were in the minority on the planet and managed to survive the cataclysm that unfolded in the minutes and years after an asteroid made Earth its crash pad. Today, we know these fish as sturgeon and paddlefish, which people call "living fossils" because they haven't changed all that much in the last tens of millions of years.

In an individual sense, though, these fish weren't so lucky. No animal whose fossilized remains are embedded in the Hell Creek formation near Jordan, Montana (pop. 356) was lucky.

That's why they call it Hell Creek.

"It literally looks like the worst car crash you have ever seen but frozen in place — like a block of ice suddenly formed around it," During says.

During and a group of colleagues recently combed through that pristine record of prehistoric chaos for clues about when, exactly, the asteroid struck.

They report their findings in a study published on Wednesday in the journal Nature.

Their conclusion? The asteroid that killed the dinosaurs came crashing down in the spring.

Rocks that rained down from space were a big clue

Five years ago, in the spring of 2017, renowned paleontologist Jan Smit, a co-author on the new paper, gave a lecture spanning his career, including the discoveries of much of what we now know about the impact that killed the dinosaurs.

He talked about the layer of iridium (an element rare in Earth's crust but common in some asteroids) that coats the planet and, in some places, leaves a detailed record of when the asteroid hit. He talked about drilling in the impact crater itself and geologic evidence for tsunamis in Mexico.

Then Smit talked about one of his more recent projects: a site in the Hell Creek Formation called Tanis. He described fossilized fish that had died within the first 15 to 30 minutes after the asteroid hit. He knew that's what happened because their gills were filled with tiny, glass-like beads, called impact spherules, formed by rock that was kicked up by the asteroid when it hit.

During, then a master's student in search of a thesis topic, was amazed.

"I actually started typing him an email [on] my phone from the back of the room, saying, 'Hey, if you have these fishes, can we please do isotopic analysis on their bones? Because then you['ll] have a latest Cretaceous record,'" she says.

In other words, During wanted to use those fossils to figure out precisely what was happening in the fish bodies just before they were killed. Since paleontologists have become quite adept at interpreting subtle details in the fossils to understand when they died, the fossils Smit described probably contained evidence to give a fine-grained understanding of when, exactly, the most famous event in geological history happened.

By late summer, During was in the field.

The final minutes of the Cretaceous period 

Fish fossils in North Dakota contain records of something that happened 2,000 miles away in the Gulf of Mexico because the impact was so mighty. It released more than a billion times more energy than the atomic bomb dropped on Hiroshima in 1945. It sent three out of every four species into extinction and opened the door for a plucky little class of vertebrates now called mammals to become, well, more prominent.

Its immediate effect on the planet itself was remarkable. The impact sent shock waves rippling through the Earth's crust. In some places where a body of water sat on top of the crust, this shock wave caused what are called seiche waves to develop. This unusual phenomenon is hard to visualize, but can also happen in a swimming pool during an earthquake. The harmonic movement of the energy flowing through a contained body of water creates powerful waves.

Unlike tsunamis, which take hours to travel long distances, the shock wave traveling through Earth's crust arrived at what's now North Dakota within minutes. Seiche waves developed in a prehistoric sea next to what's now the Hell Creek Formation, sending a wall of muddy sediment crashing over that peaceful, slow-moving stream.

As it happened, a long list of variables aligned to turn this particular site into something like a North American Pompeii.

Just before that wall of mud stopped time, the site saw another rare and remarkable phenomenon. A shower of tiny, glassy beads — the impact spherules Smit had mentioned in his talk — started raining down.

"Spherules are pieces of Earth rock that got fired into space because of the impact," During says. "You can best compare [it] to throw in a bowling ball in a sandbox. Some sand will jump out."

These spherules, which Smit described more than two decades ago in a paper on the ways material kicked up by the impact had spread around the world, are very useful for paleontologists. The rare process that leaves spherules behind isn't replicated by anything that usually happens on Earth.

The pieces of rock were ejected into space, where they crystalized with the lightest elements in the center. "Some of them may have even circled the Moon" before raining back down onto the Earth, During says.

The fossils in the Hell Creek Formation have spherules in their gills but not further in their digestive tracts, suggesting that the fish had just started encountering them when everything went black.

As dinosaurs fell, mammals began their rise

Like most living things, the fish excavated in 2017 by During and her colleagues grew unevenly throughout the years of their life. When the researchers closely inspected thin slices of bone from the fossils, it was plain to see that the fish had died — or, more precisely, stopped growing — early in the growing season. They also used a cutting-edge technique called synchrotron scanning that uses X-rays to create 3D images.

"The reign of the dinosaurs ended in spring."

That data supported one conclusion: "These fishes died in spring; the reign of the dinosaurs ended in spring," During says.

This new information is interesting for paleontologists for several reasons. For one thing, it's a valuable data point in understanding how, exactly, the cataclysm affected life on the planet.

For instance, the six fish that During excavated were far from the only unlucky creatures that day. Animals far away from the impact site or burrowed underground were far safer from the immediate effects of the asteroid. Unfortunately for animals in the Northern Hemisphere, spring is when many animals emerge from burrows or dens or other relatively safe places. That information will probably help researchers understand why some species survived the initial days and weeks when so many didn't. 

"If you were on the surface when the meteorite struck, you were likely to be killed," During says. Before a species had the chance to test its mettle in the dark, dusty nuclear winter following the collision, it "first need[ed] to survive the actual impact." 

It turned out that our ancestors, which looked far more like mice than humans, had what it took to survive both challenges. For dinosaurs and thousands of other species cleared from niches they had dominated for millions of years, times were about to change.  

Abstract

The Cretaceous–Palaeogene mass extinction around 66 million years ago was triggered by the Chicxulub asteroid impact on the present-day Yucatán Peninsula. This event caused the highly selective extinction that eliminated about 76% of species, 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 fishes 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 fishes from the Tanis impact-induced seiche deposits reveal annual cyclicity across the final 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 influence on selective biotic survival across the Cretaceous–Palaeogene boundary.