Древнее исследование массового вымирания показывает, что динозавры захватили Землю во льду, а не в тепле

С потоками лавы вдалеке примитивный пернатый динозавр-теропод переносит жертву млекопитающего во время снежной вулканической зимы, вызванной массовыми извержениями во время триасово-юрского вымирания. Новое исследование говорит, что динозавры выжили, потому что они уже адаптировались к условиям замерзания в высоких широтах. кредит: картина Ларри Фелдера

Процветание в серии внезапных глобальных мурашек по коже, которые убили конкурентов

Многие из нас знакомы с популярной теорией о том, как динозавры погибли 66 миллионов лет назад: после сильного столкновения Земли с метеоритом последовала глобальная зима, вызванная пылью и обломками, удушающими атмосферу. Но было предыдущее вымирание, гораздо более загадочное и менее обсуждаемое: вымирание 202 миллиона лет назад, которое уничтожило крупных рептилий, правивших планетой до того времени, и проложило путь к захвату динозаврами. Чем вызвано так называемое[{» attribute=»»>Triassic—Jurassic Extinction, and why did dinosaurs thrive when other creatures perished?

We know that the world was generally hot and steamy during the Triassic Period, which preceded the extinction, and there were similar conditions during the following Jurassic, which kicked off the age of dinosaurs. However, new research turns the idea of heat-loving dinosaurs on its head: It presents the first physical evidence that Triassic dinosaur species, which were a minor group largely relegated to the polar regions at the time, regularly endured freezing conditions there.

The telltale indicators are dinosaur footprints along with odd rock fragments that only could have been deposited by ice. The authors of the study explain that during the extinction, cold snaps already happening at the poles spread to lower latitudes, killing off the cold-blooded reptiles. Dinosaurs, which had already adapted, survived the evolutionary bottleneck and spread out. The rest is ancient history.

“Dinosaurs were there during the Triassic under the radar all the time,” said Paul Olsen, a geologist at Columbia University’s Lamont-Doherty Earth Observatory, and lead author of the study. “The key to their eventual dominance was very simple. They were fundamentally cold-adapted animals. When it got cold everywhere, they were ready, and other animals weren’t.”

The study, based on recent excavations in the remote desert of northwest China’s Junggar Basin, was published today (July 1, 2022) in the journal Science Advances.

Pangaea Supercontinent 202 Million Years Ago

The supercontinent of Pangaea 202 million years ago, shortly before the Triassic-Jurassic Extinction. Evidence of early dinosaurs has been found in the indicated areas; most species were confined to the high latitudes, and those few nearer the tropics tended to be smaller. Red area at the top is the Junggar Basin, now in northwest China. Credit: Olsen et al., Science Advances, 2022

Dinosaurs are thought to have first appeared during the Triassic Period in temperate southerly latitudes about 231 million years ago, when most of the planet’s land was joined together in one giant continent geologists call Pangaea. They made it to the far north by about 214 million years ago. Until the mass extinction at 202 million years, the more expansive tropical and subtropical regions in between were dominated by reptiles including relatives of crocodiles and other fearsome creatures.

During the Triassic, and for most of the Jurassic, atmospheric concentrations of carbon dioxide ranged at or above 2,000 parts per million—five times today’s levels—so temperatures must have been intense. There is no evidence of polar ice caps then, and excavations have shown that deciduous forests grew in polar regions. However, some climate models suggest that the high latitudes were chilly some of the time; even with all that CO2, they would have received little sunlight much of the year, and temperatures would decline at least seasonally. But until now, no one has produced any physical evidence that they froze.

At the end of the Triassic, a geologically brief period of perhaps a million years saw the extinction of more than three-quarters of all terrestrial and marine species on the planet, including shelled creatures, corals and all sizable reptiles. Some animals living in burrows, such as turtles, made it through, as did a few early mammals. It is unclear exactly what happened, but many scientists connect it to a series of massive volcanic eruptions that could have lasted hundreds of years at a stretch. At this time, Pangaea started to split apart, opening what is now the Atlantic Ocean, and separating what are now the Americas from Europe, Africa and Asia. Among other things, the eruptions would have caused atmospheric carbon dioxide to skyrocket beyond its already high levels, causing deadly temperatures spikes on land, and turning ocean waters too acid for many creatures to survive.

Junggar Basin Shale Cliff

A shale cliff in the Junggar Basin in northwest China, where scientists found ice-rafted pebbles amid otherwise fine-grained sediments. Credit: Paul Olsen/Lamont-Doherty Earth Observatory

The authors of the new study cite a third factor: During the eruptions’ fiercest phases, they would have belched sulfur aerosols that deflected so much sunlight, they caused repeated global volcanic winters that overpowered high greenhouse-gas levels. These winters might have lasted a decade or more; even the tropics may have seen sustained freezing conditions. This killed uninsulated reptiles, but cold-adapted, insulated dinosaurs were able to hang on, say the scientists.

The researchers’ evidence: fine-grained sandstone and siltstone formations left by sediments in shallow ancient lake bottoms in the Junggar Basin. The sediments formed 206 million years ago during the Late Triassic, through the mass extinction and beyond. At that time, before landmasses rearranged themselves, the basin lay at about 71 degrees north, well above the Arctic Circle. Footprints found by the authors and others show that dinosaurs were present along shorelines. Meanwhile, in the lakes themselves, the researchers found abundant pebbles up to about 1.5 centimeters across within the normally fine sediments. Far from any apparent shoreline, the pebbles had no business being there. The only plausible explanation for their presence: they were ice-rafted debris (IRD).

Briefly, IRD is created when ice forms against a coastal landmass and incorporates bits of underlying rock. At some point, the ice becomes unmoored and drifts away into the adjoining water body. When it melts, the rocks drop to the bottom, mixing with normal fine sediments. Geologists have extensively studied ancient IRD in the oceans, where it is delivered by glacial icebergs, but rarely in lake beds; the Junggar Basin discovery adds to the scant record. The authors say the pebbles were likely picked up during winter, when lake waters froze along pebbly shorelines. When warm weather returned, chunks of that ice floated off with samples of the pebbles in tow, and later dropped them.

“This shows that these areas froze regularly, and the dinosaurs did just fine,” said study co-author Dennis Kent, a geologist at Lamont-Doherty.

How did they do it? Evidence has been building since the 1990s that many if not all non-avian dinosaurs including tyrannosaurs had primitive feathers. If not for flight, some coverings could have used for mating display purposes, but the researchers say their main purpose was insulation. There is also good evidence that, unlike the cold-blooded reptiles, many dinosaurs possessed warm-blooded, high-metabolism systems. Both qualities would have helped dinosaurs in chilly conditions.

“Severe wintery episodes during volcanic eruptions may have brought freezing temperatures to the tropics, which is where many of the extinctions of big, naked, unfeathered vertebrates seem to have occurred,” said Kent. “Whereas our fine feathered friends acclimated to colder temperatures in higher latitudes did OK.”

The findings defy the conventional imagery of dinosaurs, but some prominent specialists say they are convinced. “There is a stereotype that dinosaurs always lived in lush tropical jungles, but this new research shows that the higher latitudes would have been freezing and even covered in ice during parts of the year,” said Stephen Brusatte, a professor of paleontology and evolution at the University of Edinburgh. “Dinosaurs living at high latitudes just so happened to already have winter coats [while] Многие из их соперников умерли в триасовый период».

С ним согласен Рэндалл Эрмес, куратор отдела палеонтологии Музея естественной истории в Юте, специализирующийся на ранних динозаврах. «Это первое подробное свидетельство глубокой древности, первое свидетельство за последние 10 миллионов лет триасового периода и первое свидетельство действительно ледниковых условий», — сказал он. «Раньше люди думали об этом как о времени, когда весь мир был жарким и влажным, но это было не так».

Олсен говорит, что следующим шагом к лучшему пониманию этого периода станет поиск окаменелостей в бывших полярных регионах, таких как бассейн Джонггар. «Летопись окаменелостей настолько плоха, что никто не занимается раскопками», — сказал он. Эти породы серо-черные, и раскопать их очень сложно. [for fossils] в этих слоях. Большинство палеонтологов привлекает поздний юрский период, когда нужно собрать много крупных скелетов. Древний полярный регион в основном игнорируется».

Ссылка: «Арктические льды и экологическое восстание динозавров» Пола Олсена, Цзинцзин Ша, Яньань Фанг, Клара Чжан, Джессика Х. 2022, научный прогресс.
DOI: 10.1126 / sciadv.abo6342

Исследование было проведено в соавторстве с Jingeng Sha и Yanan Fang из Нанкинского института геологии и палеонтологии. Клара Чанг и Шон Кенни из Земной обсерватории Ламонт-Доэрти; Джессика Уайтсайд из Университета Саутгемптона; Ганс Дитер судится со Смитсоновским институтом; Морган Шаллер из Политехнического института Ренсселера; и Виви Вайда из Шведского музея естественной истории.

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