Antik kitlesel yok olma çalışması, dinozorların Dünya’yı sıcaklıktan değil, buzun ortasında ele geçirdiğini ortaya koyuyor

Uzakta lav akıntıları varken, ilkel tüylü bir theropod dinozoru, Triyas-Jurassic yok oluşu sırasında büyük patlamaların neden olduğu karlı bir volkanik kış sırasında bir memeli kurbanı taşır. Yeni bir araştırma, dinozorların yüksek enlemlerde donma koşullarına zaten uyum sağladıkları için hayatta kaldığını söylüyor. Kredi: Larry Felder tablosu

Rakipleri öldüren bir dizi ani küresel tüy diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken diken oldu.

Birçoğumuz dinozorların 66 milyon yıl önce nasıl öldüğüne dair popüler teoriye aşinayız: Dünya’nın bir göktaşıyla şiddetli çarpışmasında, atmosferi boğan toz ve döküntülerin neden olduğu küresel bir kış yaşandı. Ancak daha önce çok daha gizemli ve daha az tartışılan bir yok oluş vardı: 202 milyon yıl önce, o zamana kadar gezegeni yöneten büyük sürüngenleri yok eden ve dinozorların devralmasının yolunu açan bir yok oluş. Ne sözde neden oldu[{” 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.”

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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.

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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.

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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] Rakiplerinin çoğu Triyas döneminde öldü.”

Erken dinozorlar konusunda uzmanlaşmış Utah Doğa Tarihi Müzesi’ndeki paleontoloji küratörü Randall Ermes de aynı fikirde. “Bu, yüksek antik çağa ait ilk ayrıntılı kanıt, Triyas döneminin son 10 milyon yıllık ilk kanıtı ve gerçekten buzul koşullarının ilk kanıtı” dedi. “İnsanlar bunu tüm dünyanın sıcak ve nemli olduğu bir zaman olarak düşünürdü, ama durum böyle değildi.”

Olsen, bu dönemi daha iyi anlamak için bir sonraki adımın daha fazla araştırmacının Jonggar Havzası gibi eski kutup bölgelerindeki fosilleri araması olduğunu söylüyor. “Fosil kayıtları çok kötü, kimse kazı yapmıyor” dedi. Bu kayalar gri ve siyahtır ve onları kazmak çok zordur. [for fossils] bu katmanlarda. Çoğu paleontolog, toplanacak çok sayıda büyük iskeletin olduğu geç Jura dönemine çekilir. Antik kutup bölgesi temelde göz ardı ediliyor.”

Referans: Paul Olsen, Jingjing Sha, Yan’an Fang, Clara Zhang, Jessica H. 2022, bilim ilerlemesi.
DOI: 10.1126 / sciadv.abo6342

Çalışma, Nanjing Jeoloji ve Paleontoloji Enstitüsü’nden Jingeng Sha ve Yanan Fang tarafından ortaklaşa yazılmıştır. Lamont-Doherty Dünya Gözlemevi’nden Clara Chang ve Sean Kenny; Southampton Üniversitesi’nden Jessica Whiteside. Hans Dieter, Smithsonian Enstitüsü’ne dava açıyor; Rensselaer Politeknik Enstitüsü’nden Morgan Schaller; ve İsveç Doğa Tarihi Müzesi’nden Vivi Vajda.

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