Enterrada en las rocas de Dakota del Norte se encuentra la evidencia del día exacto en que los dinosaurios fueron borrados del planeta, hace unos 66 millones de años. Esa es la afirmación del paleontólogo Robert DePalma y sus colegas, cuyo trabajo fue capturado por la BBC en su reciente e histórico documental "Dinosaurs:The Final Day with David Attenborough".

Durante los últimos diez años, DePalma ha centrado su trabajo en un sitio rico en fósiles, al que ha llamado "Tanis", en la Formación Hell Creek de Dakota del Norte. Y desde 2019, él y sus colegas han presentado afirmaciones muy sólidas sobre lo que Tanis nos dice sobre el final del período Cretácico.

DePalma cree que Tanis es un cementerio masivo de criaturas muertas durante el impacto del asteroide.

No hay duda de que un asteroide condujo a la extinción masiva de dinosaurios no aviares, y al menos el 50% de otras especies, hace 66 millones de años. Pero ha habido cierta controversia en torno a la afirmación de DePalma de que el sitio documenta el mismo día en que impactó el asteroide y revela evidencia directa de los últimos dinosaurios en la Tierra.

Entonces, echemos un vistazo a lo que sabemos sobre este momento tan importante en la historia de nuestro planeta, y lo que sigue siendo incierto.

La gran colisión de asteroides

Cuando se propuso por primera vez la teoría del impacto de un asteroide en 1980, no había ningún cráter. La única evidencia fueron dos sitios con un enriquecimiento sustancial de iridio, un elemento que llega a la superficie de la Tierra desde el espacio exterior, en las rocas exactamente al nivel del final del Cretácico.

Ahora hay cientos de lugares en todo el mundo que muestran la punta de iridio, en lo que se conoce como el límite K-Pg (Cretácico-Paleógeno), una firma geológica en el sedimento.

Y luego, en 1991, se produjo el gran avance:se encontró el cráter Chicxulub en lo que ahora es la península de Yucatán, en el sur de México.

Con 180 km (110 millas) de ancho y 20 km (12 millas) de profundidad, el cráter muestra que un enorme asteroide de 10 km (seis millas) de ancho se estrelló contra el mar. Su fuerza fue tan grande que desató enormes olas de tsunami, así como enormes cantidades de escombros de roca y polvo que contenían iridio en la atmósfera, y también provocó una poderosa ola de calor.

La mayoría de los expertos están de acuerdo en que toda la vida dentro de los 1.700 km (1.000 millas) de la colisión habría desaparecido instantáneamente.

Pero Tanis estaba a más de 2.800 km (o 1.800 millas) de distancia. Y hasta ahora, no había evidencia de los últimos dinosaurios. Entonces, ¿cuál es la base de la revolucionaria revelación de DePalma de que Tanis finalmente proporciona la esquiva evidencia del último día de los dinosaurios?

Evidencia de asteroide en Tanis

Hay pocas dudas de que el sitio de Tanis se encuentra cerca del final del Período Cretácico, porque DePalma ha identificado la capa de iridio inmediatamente encima del lecho fósil, lo que lo ubica en el límite K-Pg.

También ha presentado algunas pruebas convincentes de que el sitio marca el día exacto en que impactó el asteroide.

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.

Pero eso no es todo. 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.