Mammals and birds survived asteroid that wiped out dinosaurs because it hit during this season

By Mark Waghorn via SWNS

The city sized space rock that wiped out the dinosaurs crashed to Earth in spring, which gives scientists a clue as to why mammals and birds survived.

It triggered a massive tsunami, wiping out many breeding species in the northern hemisphere but giving southern hemisphere animals a much better chance as it was autumn there.

Remains of filter-feeding creatures such as sturgeons and paddlefishes have shed fresh light on the most selective extinction in history.

The seven mile wide asteroid strike happened 66 million years ago in what's now the Gulf of Mexico.

Three quarters of species disappeared - including non-avian dinosaurs, pterosaurs, ammonites and most marine reptiles.

Yet mammals, birds, crocodiles and turtles survived. Knowing the season helps explain the phenomenon.

Lead author Melanie During said: "The timing of this catastrophic impact would have coincided with a particularly sensitive stage for many Northern Hemisphere species that reproduced and had developing offspring during the spring.

"Southern Hemisphere ecosystems - which were struck during autumn - appear to have recovered up to twice as fast."

It happened during particularly sensitive life stages of Latest Cretaceous organisms - including the onset of reproduction cycles.

The preparation for winter may have protected those in the Southern Hemisphere.

Ms. During, a PhD student at Uppsala University in Sweden, said: "This crucial finding will help to uncover why most of the dinosaurs died out while birds and early mammals managed to evade extinction."

It's based on analysis of fish bones unearthed at a Late Cretaceous river bed named Tanis in North Dakota.

Scans identified distinct growth patterns inside the pristinely-preserved fossils - providing a record of seasonal change.

Combined with skeletal carbon acquired through diets, the results show the animals died in the Northern Hemisphere spring.

The tsunami surged into the river bringing in everything in its path - including dinosaurs and trees.

Senior author Dr. Jeroen van der Lubbe, of Vrije University (VU) in Amsterdam, said: "The retrieved growth rings not only captured the life histories of the fishes but also recorded the latest Cretaceous seasonality and thus the season in which the catastrophic extinction occurred."

The fish were buried alive - within 60 minutes. The impact also caused glass particles, or spherules, to rain down from the sky.

They can still be seen filtered out and stuck in their gills in state-of-the-art synchrotron X-rays - along with soft tissues.

Co-author Professor Sophie Sanchez, of Uppsala University in Sweden, said: "These bones registered seasonal growth very much like trees do."

Further evidence was provided by the distribution, shapes and sizes of the bone cells - which seasonally fluctuate.

Co-author Dr. Dennis Voeten, also from Uppsala, said: "In all studied fishes, bone cell density and volumes can be traced over multiple years.

"These were on the rise but had not yet peaked during the year of death."

The annual feeding pattern of one of the paddlefishes found its favorite prey zooplankton peaked between spring and summer.

Co-author Dr. Suzan Verdegaal-Warmerdam, from VU, said: "This temporary increase of ingested zooplankton enriched the skeleton of its predator with the heavier 13C carbon isotope relative to the lighter 12C carbon isotope."

Ms. During said: "The carbon isotope signal across the growth record of this unfortunate paddlefish confirms the feeding season had not yet climaxed – death came in spring."

The groundbreaking study has been published in the world's number one science journal Nature.