How did the dinosaurs become the rulers of the Jurassic and Cretaceous ages? Evidence of early dinosaurs that were able to survive in frigid arctic conditions now suggests that cold tolerance may have become the key to this group of animals’ success. The dinosaurs, which are probably already insulated by feathers, survived the global cold snaps that eliminated many of their naked reptilian competitors at the end of the Triassic era, the scientists explain.
The focus is usually more on the demise of the dinosaurs: a catastrophic meteorite impact devastated the earth around 66 million years ago and led to the extinction of the former rulers of the world. The dinosaurs paved the way for the spread of birds and mammals. Interestingly, a similar story seems to have unfolded before that: Around 202 million years ago, fossil records indicate a mass extinction event that wiped out many species of reptiles that had previously dominated the planet. At the same time, the career of the dinosaurs ignited – apparently they were able to seize the opportunity and take over power on the planet. The study by researchers led by Paul Olsen at Columbia University in Palisades now sheds light on why dinosaurs survived the Triassic-Jurassic extinction while other animals went extinct.
Why did the dinosaurs become so successful?
Until the mass extinction event 202 million years ago, reptiles dominated the vast tropical and subtropical regions of the world, most of which then consisted of the vast continent of Pangea. Among them were relatives of the crocodiles and other creatures, some of them bizarre. It is assumed that the first representatives of the dinosaurs emerged 231 million years ago – but initially led a shadowy existence. From the time of 214 million years ago there is evidence that they settled in what was then the far north of the supercontinent. However, it was much warmer there than it is today: During the Triassic and most of the Jurassic, the concentration of carbon dioxide in the atmosphere was high and the correspondingly strong greenhouse effect ensured ice-free poles.
According to fossil finds, forests also grew in the far north at that time. However, some climate models indicate that it could still get quite cold at times in the high latitudes. So far, however, there has been no clear evidence that ice formation has occurred in the habitat of the dinosaur species there. Olsen and his colleagues are now reporting traces of such frosty conditions. Their results are based on investigations of deposits in the Junggar Basin in Northwest China. The sediments formed there about 206 million years ago in a lake that was then well north of the Arctic Circle. Footprints found there by the authors and others document that dinosaurs lived along the banks.
Findings from the former polar region
In the lake’s deposits, the researchers have also now found abundant pebbles, up to 1.5 centimeters in diameter, in the normally fine sediments – far from areas they identified as the former shorelines. The scientists explain that these findings are due to the effect of ice: the pebbles in the sediments from the central lake areas come from ice that formed on the coastal landmass in winter and absorbed parts of the underlying material in the process. Then, as it got warmer, the ice broke off and drifted away, letting the pebbles trickle to the bottom of the lake as it melted 206 million years ago. The researchers conclude: “There are signs that these areas are regularly frozen and that the dinosaurs coped well with them,” summarizes co-author Dennis Kent from Columbia University.
According to the scientists, this in turn sheds light on why dinosaurs emerged so successfully from the Triassic-Jurassic extinction event. As they explain, there was increased volcanism on Earth at that time, as is known from studies. During the most violent phases of the eruptions, large releases of sulfur aerosols into the atmosphere may have repeatedly led to global volcanic winters that lasted a decade or more, the researchers say. Even in the tropics, this could have led to prolonged periods of frost. This could have killed the heat-loving, poorly insulated reptiles there. “The dinosaurs led a shadowy existence during the Triassic. The key to their later dominance was probably that they were inherently cold-adapted: when it got cold everywhere, they were prepared – other animals weren’t,” says Olsen.
Cold resistance as the key to success
But what kept the dinosaurs of the north warm? Possibly feathers and an intensive metabolism, the researchers explain: Since the 1990s, evidence has accumulated that many dinosaur species were feathered. Early in their evolutionary history, these structures may have kept them warm, as they still do today with their descendants – the birds. The dinosaurs may have had another feature that they also have early on: it is now considered well documented that, in contrast to the reptiles, many dinosaur species had a warm-blooded, metabolically strong body. Both of these properties may have allowed dinosaurs to exist in cold conditions from an early age.
Finally, the researchers emphasize that further investigations should now shed more light on the interesting era at the beginning of the reign of the dinosaurs. The decisive step towards a better understanding consists in specifically looking for fossils in the former polar regions. “Most paleontologists are interested in sites from the late Jurassic period, which is known to be the home of many large skeletons. The Paleo-Arctic has basically been ignored so far,” laments Olsen.
Source: Columbia Climate School, Article: Science Advances, doi: 10.1126/sciadv.abo6342