Buried in the rocks of North Dakota is evidence of the exact day the dinosaurs were wiped off the planet, some 66 million years ago. That’s the claim of palaeontologist Robert DePalma and his colleagues, whose work was captured by the BBC in its recent landmark documentary Dinosaurs: The Final Day with David Attenborough.
For the past ten years, DePalma has focused his work on a fossil-rich site, which he has named “Tanis,” in North Dakota’s Hell Creek Formation. And since 2019, he and his colleagues have come up with some very strong claims about what Tanis tells us about the end of the Cretaceous period.
DePalma believes that Tanis is a massive graveyard for creatures killed during the asteroid impact.
There is no doubt that an asteroid caused the mass extinction of non-avian dinosaurs, and at least 50% of other species, 66 million years ago. But there has been some controversy surrounding DePalma’s claim that the site documents the very day the asteroid hit and reveals direct evidence of the last dinosaurs on Earth.
So, let’s take a look at what we know about this very important moment in our planet’s history, and what remains uncertain.
The Great Asteroid Collision
When the asteroid impact theory was first proposed in 1980, there was no crater. The only evidence was two sites with substantial enrichment of iridium – an element that reaches the Earth’s surface from outer space – in rocks exactly at the level of the end of the Cretaceous.
There are now hundreds of locations around the world showing the tip of iridium, at what is known as the K-Pg (Cretaceous-Paleogene) boundary, a geological signature in the sediment.
And then, in 1991, the big breakthrough happened: The Chicxulub crater was found in what is now the Yucatan Peninsula in southern Mexico.
At 180 km (110 miles) wide and 20 km (12 miles) deep, the crater shows that a huge 10 km (six miles) wide asteroid crashed into the sea. Its force was so great that it unleashed huge tsunami waves, as well as massive amounts of rock debris and dust containing iridium into the atmosphere, and also caused a powerful heat wave.
Most experts agree that all life within 1,700 km (1,000 miles) of the collision would have been instantly wiped out.
But Tanis was more than 2,800 km (or 1,800 miles) away. And until now, there was no evidence of the last dinosaurs. So what is the basis for DePalma’s groundbreaking revelation that Tanis finally provides the elusive evidence for the last day of the dinosaurs?
Evidence of asteroids in Tanis
There is little doubt that the Tanis site is near the end of the Cretaceous Period, because DePalma has identified the iridium layer immediately above the fossil bed, placing it on the K-Pg boundary.
He also presented some compelling evidence that the site marks the exact day the asteroid hit.
Read more: Dinosaur-killing asteroid hit at worst angle to do maximum damage: new research
First of all, there are the ancient channels in the sedimentary rocks of Tanis: these are evidence of the huge waves of stagnant water (or “seiche”) that engulfed Tanis. At the time, North America was divided by a major seaway that passed near the Tanis site: seiche waves would have risen up streams and out again several times, mixing fresh and sea water to create the waves.
Ground-borne shock waves from the asteroid’s impact, which caused the devastating surges of water, could easily travel through the Earth’s crust from the impact site to Tanis.
When the asteroid slammed into Earth, tiny ejector spheres, glassy pearls about 1mm across, formed from molten rock, and were able to travel up to around 3,200 km (2,000 miles) through the atmosphere because they were very light.
Surprisingly, DePalma found these glassy spherules at the site, and also in the gills of fossil sturgeons that occupied the streams of Tanis. He believes the spherules were produced by the Chicxulub impact due to their shared chemistry, and some even encapsulated “fragments of the asteroid.” If this is true, their occurrence at Tanis would certainly confirm that they mark the actual day of impact, because the spherules would have fallen to the ground within a few hours of impact.
Tanis Fossil Finds
From decades of studying the rocks and fossils in the Hell Creek Formation, we know that Tanis was a hot, humid forested environment, with a thriving ecosystem teeming with dinosaurs, pterosaurs (flying reptiles), turtles, and early mammals. Although not yet described in detail, DePalma and his colleagues reveal some incredible new animal fossils, and he believes they may well have died on impact day, due to their location on the doomed Tanis sandbar.
First, there is an exceptionally preserved leg from the herbivorous dinosaur. thecelosaurusshowing not only the bones, but also the skin and other soft tissues.
But that is not all. There’s a baby pterosaur, about to hatch from its egg, and some amazingly well-preserved ones. triceratops skin, which is an extremely unusual finding.
Even more surprising, there is a turtle impaled on a stick, which DePalma believes could be evidence of a tragic death in the turbulent seiche waves triggered by the impact.
DePalma’s final claim is that the impact and final day occurred in May, based on microscopic and geochemical analysis of growth rings in the fin spines of the fossil sturgeon. The bones show seasonal banding, where the bone grows rapidly when food is plentiful and slowly when conditions are poorer, so summers are often shown with a broad pale band and winters with a narrow dark band. The last banding cycle in the sturgeon confirms that it died in May. And another study this year has confirmed it.
So why the uncertainty?
There’s no question that DePalma’s claims have been controversial since they were first introduced to the world in 2019, likely because the ad ran in the New Yorker magazine rather than a peer-reviewed magazine.
Read more: Fishbones and water lilies help pinpoint the month the dinosaurs died
But the seiche wave findings were published in an academic paper just a month later, and most geologists were convinced.
It is true that the fossils, which were first revealed in the BBC documentary – along with evidence that the Tanis glass spheres are linked to the Chicxulub impact – have yet to be published in scientific journals, where they would be the subject to peer review.
But experience shows that most of what DePalma has revealed in the past has subsequently been backed up by peer-reviewed articles.
For the last two years I worked as one of the BBC’s independent scientific consultants, verifying the claims, while they made the documentary. I and my colleagues, and many other experts, are satisfied that the Tanis site likely reveals the last day of non-avian dinosaurs.
And of course, as we all know, the asteroid impact went way beyond that day. It led to a dark, icy planet, on a global scale, that lasted for days or maybe weeks, and out of this worldwide mass extinction came the age of mammals.