How did the Siberian Dinosaurs Die?

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Imagine, for a moment, an ideal habitat for a dinosaur. What does it look like? Many people think of them crashing through tropical forests and wallowing in swamps, but in truth dinosaurs inhabited a wide range of ecological settings. That includes the temperate forests of the cold northern latitudes, and as a new paper published in the journal Naturwissenschaften shows, dinosaurs were thriving there right to the end.

Last year I wrote about a PBS special that focused on the polar dinosaurs of Alaska, and Smithsonian magazine ran a story on dinosaurs in Alaska and the South Pole. But the new journal paper is concerned with a different chilly location just on the other side of the Bering Strait. At Kakanaut, in northeastern Russia, paleontologists have found a fossil assemblage dating to the very end of Cretaceous, 65 to 68 million years old. Like the sites in Alaska, it is within the Arctic Circle, and even in the time of the dinosaurs it was apparently so cold that no small reptiles or amphibians lived there. This is because these smaller animals were ectothermic, meaning that their body temperature was fluctuated with the surrounding environment. That dinosaurs not only lived in such a place, but seemed to thrive there, adds evidence to the growing understanding that they were not cold-blooded creatures.

Much like sites in Alaska, at Kakanaut paleontologists have found many remains from hadrosaurs, horned dinosaurs, ankylosaurs, tyrannosaurs and dromeosaurs. There were herbivores and carnivores both large and small, and it seems that a large variety of dinosaurs were able to survive the cooler temperatures. The real surprise, however, was fragments of dinosaur eggshell. It has long been debated whether polar dinosaurs lived in the cold year round or migrated at particular seasons. The eggshell indicates that at least some dinosaurs reproduced in this habitat, which means they were remaining there for a long period of time. Some might have even stayed year-round, particularly if their young required parental care.

This has important implications for the extinction of the dinosaurs, too. There is an ongoing debate whether the dinosaurs died out gradually due to some unknown cause or whether they became extinct suddenly, perhaps because of the impact of a meteor 65 million years ago. The Russian site affects both ideas. If dinosaurs were this diverse at the very end of the Cretaceous, it is unlikely that their global extinction was gradual. At the same time, it has been suggested that a meteor strike would have caused cooler global temperatures which would have ultimately killed off the dinosaurs. The existence of so many polar dinosaurs, however, shows that some dinosaurs were capable of occupying cold regions. Thus cooling temperatures alone cannot explain why all the dinosaurs disappeared (at least, the ones that had not evolved into birds).

The extinction of the dinosaurs is still one of the most complex murder mysteries ever known.

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