One fateful day about 66 million years ago, a large object crashed into the Yucatan Peninsula. The resultant firestorm and long, cold winter wiped out 75 percent of life on Earth, including most dinosaurs. Scientists have studied the so-called Chicxulub impact for decades but only on millennia timescales. For the first time, researchers have zeroed in on when the asteroid or comet walloped Earth, and they say it happened on an otherwise pleasant spring day. This could help explain why some species survived and others perished.
The impact extinction was first proposed in 1980 by geologists Luis and Walter Alvarez, and is therefore known as the Alvarez hypothesis. The best evidence for this idea is the Chicxulub crater, discovered in Mexico several years before the Alvarez family had their say. The crater is the result of a large asteroid or comet impact, and its location in the geological record matches the Cretaceous–Palaeogene mass extinction that ended the reign of dinosaurs. This is the scientific consensus today, although the evidence is trending toward asteroid rather than comet.
One problem for the Alvarez hypothesis has been why the extinctions were so selective. All the non-avian dinosaurs died out, as did the majority of marine mammals and ammonites. And yet, many species of mammals would survive to eventually give rise to humanity. The mass extinction also spared crocodiles, birds, and smaller reptiles. The authors set out to determine if the pattern of extinctions had anything to do with the time of year when the impact occurred. The first order of business was to identify the season, and to do that, they went to North Dakota.
The Chicxulub impactor is believed to have struck the surface at a very steep angle, imparting maximum energy upon landing. While it was only a few miles in diameter, it released a billion times more energy than the atomic bombs dropped in World War 2. The collision was so catastrophic that it shook the continental plate, sending a wave of water and sediment upstream (all the way to North Dakota and beyond) from what is today the Gulf of Mexico. The surge enveloped and instantly killed a huge number of fish, turning them into artifacts from the final day of the Cretaceous era. At the same time, glass beads of melted rock rained down on the landscape, ending up preserved alongside the fossilized fish. This is known as the Tanis event.
North Dakota has one of the best Tanis deposits, allowing researchers to examine the conditions on Earth that fateful day. The team excavated paddlefish and sturgeon from the deposit (see above), ending up with several lower jaws, teeth, and pectoral fins. The remains were scanned using X-ray tomography and rendered in 3D for analysis. There are minute cellular changes in many fish species throughout the seasons, and that can tell us the point in their growth cycle they died. In this case, all six representative specimens died in the spring (northern hemisphere). The glass beads were present only in the gills, which showed the fish were alive and foraging at the end of the Cretaceous.
The study speculates that many of the species that survived may have been prevalent in the southern hemisphere, where rather than gearing up to mate, they would have been hibernating in burrows or caves. That might have shielded them from the immediate fallout and helped them eke out a life in the ashes.