buried in the rocks in North Dakota are evidence of the exact day the dinosaurs were obliterated from the planet, about 66 million years ago. That’s the claim of paleontologist Robert DePalma and colleagues, whose work was captured by the BBC in its recent historical documentary. Dinosaurs: The Last Day with David Attenborough.
For the past ten years, DePalma has focused his work on a fossil-rich site—which he called “Tanis”—in the Hell Creek Formation of North Dakota. 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 mass graveyard of creatures killed during the asteroid strike.
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 surrounding DePalma’s claim that the site documents the day the asteroid struck and reveals direct evidence of the last dinosaurs on Earth.
So let’s take a look at what we know about this most important moment in our planet’s history — and what remains unclear.
The giant impact hypothesis
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 late Cretaceous.
Now, there are hundreds of places around the world showing the peak of iridium at what is known as the K-Pg (Cretaceous-Paleogene) boundary, a geological signature in the sediment.
And then, in 1991, came the big breakthrough – the Chicxulub crater was found in what is now the Yucatan Peninsula in southern Mexico.
At 110 miles wide and 12 miles deep, the crater shows that a massive six-mile-wide asteroid crashed into the sea. Its force was so great that it unleashed massive tsunami waves as well as massive amounts of rock debris and iridium-containing dust into the atmosphere and also unleashed a powerful heat wave.
Most experts agree that all life within about 1,000 miles of the collision would have been wiped out instantly.
But Tanis was more than 2,800 kilometers away. And until now, there was no evidence of the last dinosaurs. So what’s the basis for DePalma’s groundbreaking revelation that Tanis finally provides the elusive evidence of the dinosaurs’ last day?
Evidence of asteroids on Tanis
There is little doubt that the Tanis site is near the end of the Cretaceous Period because DePalma identified the iridium layer immediately above the fossil bed, which places it on the K-Pg boundary.
He also presented some compelling evidence that the site marks the exact day the asteroid struck.
First, there are the ancient channels in the sedimentary rocks of Tanis – they are evidence of the huge waves of still water (or “seiche”) that engulfed Tanis. At that time, North America was divided by a great waterway that passed near the site of Tanis. The waves of the seiche would have come up the streams and out again, several times, mixing fresh water and sea water to create the waves.
The terrestrial shock waves from the asteroid impact – which caused the devastating waves of water – can easily travel through the Earth’s crust from the impact site to Tanis.
When the asteroid collided with Earth, tiny ejector spherules, glassy beads about 1 mm wide, were formed from molten rock — and were able to travel up to about 2,000 miles through the atmosphere because they were so light.
Surprisingly, DePalma found these glassy spherules at the site and also in the gills of sturgeon fossils that 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 hours after impact.
Fossil discoveries of Tanis
From decades of studying the rocks and fossils in the Hell Creek Formation, we know that Tanis was a warm and humid forest environment with a thriving ecosystem full of dinosaurs, pterosaurs (flying reptiles), turtles and early mammals. While not yet described in detail, DePalma and colleagues reveal some incredible new animal fossils — and he believes they could have died on the day of the impact itself, given their location on the doomed Tanis sandbar.
First, there is an exceptionally preserved leg of the herbivorous dinosaur. Thescelosauruswhich shows 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 incredibly well-preserved Triceratops skin, which is an extremely unusual finding.
Even more surprising is a turtle impaled by a pole, which DePalma believes could be evidence of a tragic death in the turbulent seiche waves unleashed by the impact.
DePalma’s final claim is that the impact, and final day, occurred in May, based on microscopic and geochemical analyzes of growth rings on the spines of fossil sturgeon fins. Bones show seasonal bands – where bone grows rapidly when food is plentiful and slowly when conditions are poorer, often summers are shown by a wide pale band and winters by a narrow dark band. The last cycle of banding on the sturgeon confirms that it died in May. And a further study this year confirmed that.
Why paleontologists are unsure about Tanis
There’s no doubt that DePalma’s claims have been controversial since they were first presented to the world in 2019 – likely because the announcement was in the New Yorker magazine rather than a peer-reviewed journal.
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 glass spherules at Tanis are linked to the Chicxulub impact – have yet to be published in scientific journals, where they would be subject. for peer review.
But experience shows that most of what DePalma has revealed in the past has been later corroborated by peer-reviewed articles.
For the past two years, I’ve worked as one of the BBC’s independent scientific advisors, verifying claims while they were making the documentary. My colleagues and I, and many other experts, are convinced that the Tanis site likely reveals the last day of non-avian dinosaurs.
And of course, as we all know, the asteroid impact went far beyond that day. This led to a dark and icy planet, on a global scale, lasting for days or perhaps weeks – and out of this worldwide mass extinction came the age of mammals.
This article was originally published on The conversation in Michael J. Benton at the University of Bristol. Read the original article here.