The last day of the dinosaurs

       

Has there been, at last, fossil evidence of the end of the Cretaceous?

History is well known. 66 million years ago, a huge asteroid hit the Yucatan coast, near what is now the city of Chicxulub, in the Gulf of Mexico. The violent shock lifted a mountain of molten material higher than Everest and warmer than the surface of the Sun, releasing more energy than a billion bombs like Hiroshima. The ground caught fire for thousands of kilometers around, and a shower of molten rock fell over the western hemisphere, burning 70% of the forests. The dust accumulated in the atmosphere prevented for months the photosynthesis of terrestrial plants and marine phytoplankton, collapsing the trophic chains and the carbon cycle in land and sea, and the lack of light froze the planet. The oxygen practically disappeared from the atmosphere, replaced by carbon monoxide and dioxide, methane and sulfur, which caused acid rains for months. As a result, 75% of the species was destroyed, 99.9% of all living organisms perished, and the planetary ecosystem was destroyed and had to be reinvented. After the impact, the Earth changed geological era: it finished the Cretaceous, and with it the Mesozoic era, and began the Cenozoic.

But this story has always had a weak point. If in a single day, which is a moment in geological terms, thousands of millions of living beings died, including all the dinosaurs, should not there be a geological stratum absolutely saturated with the remains of these animals and plants? And should not it be located all over the world? In fact, the stratigraphic boundary between the Cretaceous and the Cenozoic – limit K-T, of the German KreideTertiär – is well determined, among other things because there is abundant iridium, a scarce metal in the earth’s crust but very abundant in meteorites. Therefore, the impact is well recorded and dated, and the time in which it occurred as well, as well as its stratigraphic location throughout the planet. But in this stratum no fossils have been found, much less any evidence of a dinosaur planetary death. Until now.

Robert DePalma of the University of Kansas (and yes, cousin of the film director), published his discoveries on April 23 at a site in North Dakota, dated exactly at the K-T boundary. The deposit, called Tanis, is located 3,000 kilometers from Chicxulub. 66 million years ago, Tanis was on the shore of an inland sea that drowned what is now the Midwest. The impact caused a series of large seismic waves, which caused a huge tsunami in this inland sea, with waves of up to 10 meters, which piled many animals and plants in a chaotic sediment and full of fossil remains. Is this the first evidence that the impact of Chicxulub decimated the terrestrial biosphere?

The fossils of Tanis are spectacular: mixed freshwater and saltwater fish, tree trunks, dinosaurs, flower petals, echinoderms, small mammals, feathers … including more than a dozen plants and animals unknown until now. In addition, the fossils of Tanis are preserved in a multitude of random orientations, including fish in vertical position, which proves that their deposition was the result of a violent and very fast process, which captured them in a mud mass almost instantaneously. Some of the remains of dinosaurs are at the top of the stratigraphic sequence, which suggests that their bodies were floating in the water before the terrain dried out again. The meter and a half thick of the deposit is saturated with glass microspheres, called tectites, a product known from the rain of molten rock that causes a great meteoric impact. And the deposit is covered by a layer of two centimeters rich in iridium. Therefore, Tanis includes all the necessary elements to become a dream come true for any paleontologist.

DePalma has been working at Tanis since 2012, and some colleagues he has invited to participate in his excavations have described his work as “extremely meticulous”. However, other researchers question some of their conclusions. For example, although DePalma has assured the press that there are fossils of dinosaurs in Tanis, this end has not yet been published in a scientific journal, and therefore this possible finding has not been submitted to the scrutiny of other scientists. DePalma argues that those results will be published very soon.

In addition, DePalma still has a reputation to amend: in 2015 it found a new species of dinosaur, the Dakotaraptor, but when rebuilding its skeleton it included a bone that turned out to be a turtle. When other colleagues pointed out the error, DePalma was forced to publish a correction to his work, and since then has not enjoyed special recognition among the paleontological community.

Do we finally have a picture of the day the dinosaurs died? The reality is that there are some minor details to be confirmed. Is Tanis one of the most important discoveries in the history of paleontology, or has the relevance of a discovery that is relatively substantial again been exaggerated? If everything that DePalma claims to be correct, it will be the first time we have incontrovertible evidence that dinosaurs still wandered on Earth at the end of the Cretaceous. Thus, although there are other hypotheses to explain the extinction in the KT (for example, several tens of thousands of years of unbridled volcanism in what is now India, and which would have caused a huge decline in the world population of dinosaurs during the Late Cretaceous ), Tanis would provide the necessary evidence to affirm without a doubt that at least some dinosaurs were still alive to witness the last day of the Cretaceous.

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