We don’t Know Exactly When the Dinosaurs Died, but Now We Know it was in the Springtime

In a geological instant, 75% of the plants and animals on Earth were wiped out. We now know that the disaster happened during the springtime, even though we don't know exactly what year it happened.

Most scientists agree that the disaster was caused by an asteroid that hit the region in Mexico. The impact released 2 million times more energy than the bomb.

Melanie During and colleagues from the University of Sweden studied the seasonal information of fish found in a site in North Dakota that died as a result of the devastating impact.

The K-T boundary is a layer of ash sandwiched between layers of rock. iridium is an element common in asteroids but rare on Earth. More than three decades ago, the extinction event was revealed to be an asteroid strike.

The well-preserved bones of filter-feeding sturgeons and paddlefishes found in the Tanis fossil site in North Dakota had impact debris lodged in their gills, but nowhere further down the digestive system, suggesting an almost-instantaneous death occurred when an impact-triggered seiche occurred.

The growth patterns in the fossils show when the fish reproduced and had developing offspring. This would have been in the spring in the northern hemisphere.

The authors of a paper theorize that the timing of the Chicxulub impact was a major influence on biotic survival.

The timing of the collision, at least for the Northern Hemisphere, would have been very sensitive for many plants and animals.

A large group of the late Cretaceous biota were out and about looking for food, tending to offspring and trying to build up resources after the harsh winter.

The researchers said that the ecosystems in the Southern Hemisphere appear to have bounced back more quickly than those in the Northern Hemisphere.

The large fish, which are up to a meter (3 feet) long, died quickly after the asteroid strike even though the fossils were found thousands of kilometers away from the impact crater. They were buried alive by the massive body of water unleashed by the asteroid strike.

Small bits of molten rock ejected from the crater went high into the atmosphere or even to space where they formed a glass-like material in the fish's gills.

The impact spherules rained back down on Earth. It looks like the most violent thing I have ever seen.

There are previous studies that show fossils in the same condition, which suggest that the extinction event happened in June.

Melanie was excavating a paddlefish in the Tanis deposit.

Natu re press info and paper.