THE DAY THE DINOSAURS DIED
New BBC documentary, Dinosaurs: The Final Day, presented by Sir David Attenborough, digs into a fossil site that may have recorded the moment an asteroid wiped out almost all life on Earth. Holly Spanner met with Robert DePalma, one of the palaeontologists at the site, to find out more about the significance of this discovery…
YOU’VE BEEN WORKING AT TANIS, A TOP-SECRET FOSSIL LOCATION IN NORTH DAKOTA. WHEN WAS THIS SITE LAID DOWN?
We worked out the site was deposited during the fallout of coarse ejecta material [debris] from an asteroid impact. To give it probably the broadest span possible, we can say the site is constrained [dated] to the first one to two hours [after asteroid impact], maximum, because that’s how long ejecta would have been falling for.
HAVE YOU FOUND ANY OTHER EVIDENCE THAT THIS SITE WAS CREATED RIGHT AFTER THE ASTEROID IMPACT?
We have constrained the site in multiple different ways, using plants, pollen, marine dinoflagellates and different organisms. In addition to that, we have physics. When the asteroid hit, material was thrown out of the crater [as vaporised and molten rock] and out of the atmosphere, where it cooled and solidified, then fell back to Earth in a rain of glowing glass droplets, or spherules. These spherules would have started to arrive at Tanis tens of minutes after impact, raining down for around an hour, two at most. The impact glass has been radiometrically dated, and it dates to the very end of the Cretaceous. Basically, it correlates exactly with the Chicxulub impact event [the moment the asteroid that wiped out the dinosaurs hit Earth in Mexico].
WHAT IS THE SIGNIFICANCE OF THE SPHERULES?
You’re dealing with stuff that was molten by the impact, and they therefore contain contamination from the impactor itself. When glass goes into the ground, it absorbs water. Over time, glass turns to clay. But some of these spherules were trapped in amber before getting deposited, so they were preserved [as glass]. And, in some, we’re able to identify little fragments of stone. Most are calcium-rich, probably part of the carbonate platform from the Yucatán Peninsula. But a couple of those fragments were wildly different. They were enriched in nickel, iron, chromium, and a few other elements not common here on Earth. Our preliminary analyses are supporting the fact that those fragments probably came from the impactor itself!
HOW BIG ARE THE FRAGMENTS?
The spherules themselves are one to two millimetres in diameter. And the fragments that we’re dealing with are around half a millimetre. Look at the period [fullstop] at the end of a sentence. That’s basically the size.
HOW WERE THE FOSSILS PRESERVED SO WELL?
It appears much of the site was deposited by a massive surge of water, not long after the Chicxulub impact. This massive wall of water and sediment came into the river valley, and basically ‘locked in’ that period of time, like a snapshot into prehistory.
HOW DID AN IMPACT 2,000 MILES AWAY CAUSE A SURGE AT TANIS?
All the flow indicators showed this massive surge that essentially went backwards up a river, inland. We looked at all the different diagnostic clues, and it looked like a tsunami. But the numbers didn’t match up. A tsunami coming from the crater [in Mexico] could not have reached Tanis [in North Dakota] during that two-hour window. It would have taken around 18 to 20 hours. Well, what did arrive there within that time frame? My colleague Mark Richards figured out that it was seismic waves. When we have the arrival of seismic waves, and a massive surge of water at the same time, the evidence indicates the surge was triggered, in some way, by seismic waves. And that’s not without precedent. It’s happened multiple times in recorded history.
ONE OF THE FOSSILS IS A DINOSAUR LEG. HOW DID YOU FEEL WHEN YOU SAW THE PATCH OF SCALES EMERGING FROM THE MUD?
That surprise was absolutely genuine. I was excavating a fossil palm frond, the trowel flipped up a piece of matrix, and there were scales underneath, gleaming up at me! Whenever we find soft tissue out there, we’re always on alert. Because that’s your golden moment to learn, and it’s not always available to palaeontologists – we’re used to dealing with bones, mostly. Soft tissue preservation opens a whole new door for research. So, we were really excited. And as more and more became exposed, we were able to see this was not just a patch of skin. There was a joint and a bone sticking out. And then eventually, a whole dinosaur leg!
WHAT CAN WE LEARN FROM THIS DINOSAUR LEG?
Firstly, it contains soft tissue features of the scales, unknown for that type of dinosaur. So we’ve got a much better idea of what that dinosaur looked like. Furthermore, that dinosaur existed right at that moment. Even if it died, for the sake of argument, months before; those dinosaurs existed right up until the end of the Cretaceous. In addition to that, the leg had some tearing to the skin, which gives us an idea, probably, as to the forcefulness of the surge. We already know it was incredibly forceful, based on other animals and plants that were preserved. But this is just one more line of evidence; it was a turbulent surge that mirrors what we’ve seen in river floods today. Where you’ve got a turbulent flood, you’ve got logs and large objects in that flood, and animals get caught up in that, it’s almost like a meat grinder. They’re literally ripped open in those events. And it appears something much like that could very well have happened here.
I HAVE TO ASK ABOUT THE TURTLE FOSSIL…
Oh, the turtle, yes! It was totally unexpected. There was this little bit of shell sticking out and it turned out to be a whole turtle. And then, we find that it’s skewered on the end of a branch. The minute we saw the branch going through the shell, we thought, “Oh, this poor thing!” The branch goes in one end, and out the other, out the leg opening. We can see both ends of this branch; this thing was thoroughly impaled. Our hearts really went out to the turtle. There is a slight chance it might have died within the weeks before the surge – that’s probably our longest timespan to consider, but it’s more likely it died in the surge because of that impaling. But it’s a glorious little fossil. The shell is perfectly preserved, and that turtle is now able to tell us its own, personalised story.
AND THE BABY PTEROSAUR…
The baby pterosaur was unexpected, because it was found a little higher up in the surge deposit. We found this little object, the size of a hen’s egg. And at first, we thought, could it be a nut, or a seed? But as it was prepared and put under magnification, we noticed these little, delicate, hollow bones. We consulted pterosaur experts, including David Unwin, and it became clear to us that this was an unhatched pterosaur embryo. Pterosaur embryos are unknown from North America, they’re unknown from the Late Cretaceous, and they’re unknown from that particular type of pterosaur.
It was identified as an azhdarchid pterosaur and would have hatched at about the size of a bat, so it would have been this adorable, bat-sized little creature. We call them ‘flaplings’ when they come out of the egg, because they, probably, could have flown right out of the egg. I love the word.
DID YOU FIND EVIDENCE OF ANIMALS THAT SURVIVED THE SURGE?
Yes – and we want to find out why. There’s a preserved burrow at the site, where a mammal dug through the surge deposit, after the surge. And we have bones of two mammals, the same species, same age, in that burrow. It’s an interesting snapshot of how mammals were coping at that moment. Without this impact event, if the dinosaurs were not taken out, it’s possible that mammals would never have diversified to the extent they have, and humans might never have come to be. This is a really pivotal moment in our history.
IF YOU COULD FIND ONE THING AT TANIS, IN YOUR WILDEST DREAMS, WHAT WOULD IT BE?
My gosh. We found a handful of fossil feathers, so if we were able to find any sort of a feathered organism there, either a bird, or one of the feathered dinosaurs, I would be absolutely blown away! We know the preservation potential is there, so it’s in the back of our minds that it could be just waiting there.
WHAT DO WE KNOW ABOUT THIS DAY SO FAR?
This day was very, very bad. You have a 10km-wide object hitting Earth at perhaps 20km/s. And, once that happened and all the ejecta was flying through the air, not long after, you would have felt ground shaking in North Dakota. The animals would have known something was wrong. Soon after that, the spherules fell from the sky in a glowing rain of hot molten glass. And not long after that, if the animals weren’t perturbed enough, they would have seen a massive surge of water coming up the river valley. But it wasn’t beautiful tropical water. It was a muddy, roiling mass of death, with logs, branches, animals and fish, all tumbling together. If you were unlucky enough to have been caught in it, you probably would have been killed instantly. But if you survived the surge, you essentially would have had a front-row seat to the end of the Cretaceous. Debris would have been falling out of the sky, the skies would have turned dark with the dust, ash and everything else, and you would have had three to 10 years of impact winter and ecological collapse.
IN THE CONTEXT OF WHAT IS HAPPENING TODAY, CAN YOU EXPLAIN WHY THIS NARRATIVE THAT YOU’VE UNCOVERED IS SO IMPORTANT?
That brings the story full circle and highlights why the work at Tanis is so important. When you talk about mass extinction events, the Chicxulub impact was one of the top five. The other mass extinctions happened over tens of thousands, to millions of years. And geologically speaking, that’s a mass extinction. But the Chicxulub impact happened rapidly. And this highlights the importance of hindsight, offered by the fossil record, to understand the reaction of our modern biomes to global-scale hazards. We can use that window to see how these animals are going to respond, to better understand our current crisis. This could be very important to navigating the waters of our current environmental crisis. And as a species that’s able to comprehend this, it’s up to us to actually use the data to do that.
ROBERT DEPALMA
Robert is a palaeontologist and postgraduate researcher at the University of Manchester, and an adjunct professor at Florida Atlantic University.
Dinosaurs: The Final Day, with Sir David Attenborough, is available now on BBC iPlayer.