For a long time, dinosaurs were considered cold-blooded, warmth-loving creatures that primarily inhabited the tropical belt of the supercontinent Pangea. But this picture has changed in recent years: finds of footprints, fossils and even petrified dinosaur nests in Alaska and Greenland show that the dinosaurs even survived the polar night and cold without any problems. The climate was very warm during the Triassic and Cretaceous periods and there was neither permafrost nor glaciers at the poles. It could still get cold there in winter.
Findings in China now provide evidence that the dinosaurs’ cold tolerance also played a decisive role in their rise to become the dominant animal group. For their study, Paul Olsen from Columbia University in New York and his colleagues examined layers of rock in the Junggar Basin in northwest China that were deposited a good 200 million years ago. At that time – at the transition from the Triassic to the Jurassic – this region was in the far north of Pangea, well beyond the Arctic Circle.
Numerous footprints in these deposits show that early dinosaurs lived in this polar region at that time, as the researchers report. Fossils also show that there were species-rich forests in the high latitudes of primeval Pangea at this time. “The forests in this area contained numerous deciduous, broad-leaved conifers, gingko and other deciduous plants,” report Olsen and his team. These arctic forests were more species-rich than those of tropical and temperate latitudes.
The actual discovery, however, was not the dinosaur footprints, but small, inconspicuous pebbles about one and a half centimeters in diameter. These pebbles lay in otherwise fine-grained sediment that formed the muddy bottom of a lake around 206 million years ago, the team reports. The strange thing is that while such coarser pebbles are quite normal on river or lake shores, their presence in the middle of larger lakes is very uncommon.
More detailed analyzes revealed that these primeval pebbles correspond to the pebbles that are typically carried into water by ice sheets and ice floes: “The origin of these ice raft boulders is seasonal ice that freezes along the banks and encloses grains from the bottom,” the researchers explain . “This ice then drifts out into open water, where it later breaks up and then eventually melts.”
But that means: At the time when the early dinosaurs settled this polar region, there must have been frosty, very cold periods there. Although a greenhouse climate without permanent ice caps at the poles prevailed at the end of the Triassic, the winters in the far north could be cold. “Traditionally, the dinosaurs have been thought of as living in consistently warm climates, but our results suggest that they also endured frosty winters,” write Olsen and his colleagues.
According to the paleontologists, the dinosaurs were already well-armed against polar conditions a good 200 million years ago: “In principle, they must have been adapted to the cold.” On the one hand, the fine, hair-like plumage probably offered these dinosaurs protection from the cold. On the other hand, new findings suggest that the first dinosaurs could have been warm-blooded – unlike their cold-blooded reptilian contemporaries.
This could also explain why the dinosaurs survived the great mass extinction at the end of the Triassic so well: when massive volcanic eruptions darkened the sky 202 million years ago and repeatedly caused volcanic winters that lasted for decades, many dinosaurs were prepared: their adaptation to the cold allowed them to survive them to withstand frost phases reaching into the tropics.
“The high-latitude dinosaurs were sort of already in their winter coats when this happened. However, many of their Triassic rivals died out,” explains Stephen Brusatte of the University of Edinburgh, who was not involved in the study. The reptiles that previously dominated the tropics and temperate latitudes, including early relatives of the crocodiles, became extinct because they lacked warming hair and robust thermoregulation. This allowed the dinos to fill the vacant niches.
Quelle: Columbia Climate School
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The original for this article “New study shows: Cold tolerance of dinosaurs protected them from mass extinction” comes from scinexx.