Does this dinosaur ‘graveyard’ reveal their final day on Earth? An expert explores the evidence

By Michael J Benton for The Conversation,

Buried in the rocks in North Dakota lies evidence of the exact day the dinosaurs were obliterated from the planet, some 66 million years ago. That’s the claim of palaeontologist Robert DePalma and colleagues, whose work was captured by the BBC in its recent landmark documentary Dinosaurs: The Final Day with David Attenborough.

For the last 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 put forward some very strong claims about what Tanis tells us about the end of the Cretaceous period.

DePalma believes that Tanis is a mass graveyard of creatures killed during the asteroid strike.

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There is no doubt that an asteroid led to the mass extinction of non-avian dinosaurs – and at least 50% of other species – 66 million years ago. But there has been some controversy around DePalma’s claim that the site documents the very day that the asteroid struck – and reveals direct evidence of the very last dinosaurs on Earth.

So, let’s take a look at what we know about this most important time in our planet’s history – and what remains uncertain.

The huge 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 arrives on the Earth’s surface from outer space – in the rocks exactly at the level of the end of the Cretaceous.

Now there are hundreds of places worldwide showing the iridium spike, at what is known as the K-Pg (Cretaceous-Paleogene) boundary, a geological signature in the sediment.

And then in 1991 came the huge breakthrough - the Chicxulub crater was found in what is now the Yucatan Peninsula in southern Mexico.

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At 180 km (110 miles) wide, and 20 km (12 miles) deep, the crater shows that a huge 10 km (six mile) 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 triggered a powerful heat wave.

Most experts agree that all life within around 1,700 km (1,000 miles) of the collision would have been wiped out instantly.

But Tanis was more than 2,800 km (or 1,800 miles) away. And up until now, there was no evidence of the very last dinosaurs. So, what’s the basis for DePalma’s groundbreaking revelation that Tanis finally provides the elusive evidence of the dinosaurs’ last day?

Asteroid evidence at Tanis

There is little doubt that the Tanis site lies close to the end of the Cretaceous Period, because DePalma has identified the iridium layer immediately above the fossil bed, which places it at the K-Pg boundary.

He has also presented some compelling pieces of evidence that the site marks the exact day the asteroid struck.

First, there are the ancient channels in the sedimentary rocks at Tanis – these are evidence of the huge standing water (or “seiche”) waves which engulfed Tanis. At that time North America was divided by a great seaway that passed close to the Tanis site: the seiche waves would have run up the creeks, and out again, several times, mixing fresh and sea waters to create the waves.

The ground-borne shock waves from the asteriod impact – which caused the devastating water surges – could readily travel through the Earth’s crust from the impact site to Tanis.

When the asteroid crashed into Earth, tiny ejector spherules, glassy beads about 1mm wide, were formed from melted molten rock – and were able to travel up to around 3,200km (2,000 miles) through the atmosphere because they were so light.

Astonishingly, DePalma found these glassy spherules at the site, and also in the gills of sturgeon fossils which occupied the Tanis streams. He believes the spherules were produced by the Chicxulub impact because of their shared chemistry, with some even encapsulating “fragments of the asteroid” itself.

If this is true, their occurrence at Tanis would indeed confirm that they mark the actual day of impact, because the spherules would have fallen to the ground within hours of the impact.

Tanis fossil findings

From decades of study of the rocks and fossils at Hell Creek Formation, we know that Tanis was a warm and wet forest environment, with a thriving ecosystem full of dinosaurs, pterosaurs (flying reptiles), turtles and early mammals.

Although they are yet to be described in detail, DePalma and colleagues reveal some incredible new fossils of animals – and he believes they could well have died on the day of the impact itself, due to their location in the doomed Tanis sandbank.

First, there’s an exceptionally preserved leg of the herbivorous dinosaur Thescelosaurus, which shows not only the bones, but also skin and other soft tissues.

But that’s not all. There is a pterosaur baby, just about to hatch from its egg – and, some incredibly well preserved Triceratops skin, which is an extremely unusual find.

Even more astonishingly, there is a turtle impaled by a stick, which DePalma believes could be evidence of a tragic death in the turbulent seiche waves set off 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 bone grows rapidly when food is abundant and slowly when conditions are poorer, so often summers are shown by a wide pale band and winters by a narrow dark band.

The last banding cycle in the sturgeon confirms it died in May. And a further study this year has confirmed this.

So, why the uncertainty?

There is no doubt that DePalma’s claims have been controversial since they were first presented to the world in 2019 – probably because the announcement was in the New Yorker magazine rather than a peer-reviewed journal.

But the findings about seiche waves were then published in an academic paper only a month later, and most geologists were convinced.

It is true that the fossils, which were revealed for the first time in the BBC documentary – along with the evidence that the glass spherules at Tanis are linked to the Chicxulub impact – have yet to be published in scientific journals, where they would be subject to peer review.

But, experience shows that most of what DePalma has revealed in the past has been backed up subsequently by peer-reviewed papers.

Over the past two years I worked as one of the independent scientific consultants to the BBC, verifying the claims, as they made the documentary. Both I and my colleagues, and many other experts, are satisfied that the Tanis site probably does reveal the very last day of the non-avian dinosaurs.

And of course, as we all know, the impact of the asteriod went far beyond that one day. It led to a freezing dark planet, on a global scale, lasting for days or maybe weeks – and, from this mass extinction worldwide, the age of the mammals emerged.

(The author is Professor of Vertebrate Palaeontology, University of Bristol)

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