Begravd i klipporna i North Dakota ligger bevis för den exakta dagen då dinosaurierna utplånades från planeten, för cirka 66 miljoner år sedan. Det hävdar paleontologen Robert DePalma och hans kollegor, vars arbete fångades av BBC i sin senaste landmärkedokumentär "Dinosaurs:The Final Day with David Attenborough".

Under de senaste tio åren har DePalma fokuserat sitt arbete på en fossilrik plats – som han har döpt till "Tanis" – i North Dakotas Hell Creek Formation. Och sedan 2019 har han och hans kollegor framfört några mycket starka påståenden om vad Tanis berättar om slutet av kritaperioden.

DePalma tror att Tanis är en masskyrkogård av varelser som dödades under asteroidanfallet.

Det råder ingen tvekan om att en asteroid ledde till massutrotningen av icke-fågeldinosaurier – och åtminstone 50 % av andra arter – för 66 miljoner år sedan. Men det har förekommit en del kontroverser kring DePalmas påstående att platsen dokumenterar samma dag som asteroiden slog till – och avslöjar direkta bevis på de allra sista dinosaurierna på jorden.

Så låt oss ta en titt på vad vi vet om denna viktigaste tid i vår planets historia – och vad som förblir osäkert.

Den enorma asteroidkollisionen

När asteroidnedslagsteorin först föreslogs 1980 fanns det ingen krater. Det enda beviset var två platser med betydande anrikning av iridium - ett grundämne som anländer till jordens yta från yttre rymden - i klipporna exakt i nivå med slutet av krita.

Nu finns det hundratals platser över hela världen som visar iridiumspetsen, vid vad som kallas K-Pg-gränsen (Krita-Paleogen), en geologisk signatur i sedimentet.

Och så 1991 kom det enorma genombrottet – Chicxulub-kratern hittades i det som nu är Yucatán-halvön i södra Mexiko.

På 180 km (110 miles) bred och 20 km (12 miles) djup visar kratern att en enorm 10 km (sex miles) bred asteroid kraschade i havet. Dess kraft var så stor att den utlöste enorma tsunamivågor, såväl som enorma mängder stenskräp och damm som innehöll iridium i atmosfären – och utlöste även en kraftig värmebölja.

De flesta experter är överens om att allt liv inom cirka 1 700 km (1 000 miles) från kollisionen skulle ha utplånats omedelbart.

Men Tanis var mer än 2 800 km (eller 1 800 miles) bort. Och fram till nu fanns det inga bevis på de allra sista dinosaurierna. Så, vad är grunden för DePalmas banbrytande avslöjande att Tanis äntligen ger det svårfångade beviset på dinosauriernas sista dag?

Asteroidbevis vid Tanis

Det råder ingen tvekan om att Tanis-platsen ligger nära slutet av kritaperioden, eftersom DePalma har identifierat iridiumskiktet omedelbart ovanför fossilbädden, vilket placerar det vid K-Pg-gränsen.

Han har också presenterat några övertygande bevis för att platsen markerar den exakta dagen då asteroiden träffade.

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.