Un antico studio sull’estinzione di massa rivela che i dinosauri hanno conquistato la Terra in mezzo al ghiaccio, non al calore

Con flussi di lava in lontananza, un primitivo dinosauro teropode piumato trasporta una vittima di mammifero durante un inverno vulcanico nevoso causato da massicce eruzioni durante l’estinzione del Triassico-Giurassico. Un nuovo studio afferma che i dinosauri sono sopravvissuti perché si sono già adattati alle condizioni di gelo alle alte latitudini. Credito: dipinto di Larry Fielder

Prospero in una serie di improvvisi brividi globali che hanno ucciso i concorrenti

Molti di noi hanno familiarità con la teoria popolare di come i dinosauri morirono 66 milioni di anni fa: nella violenta collisione della Terra con un meteorite, ne seguì un inverno globale causato da polvere e detriti che soffocarono l’atmosfera. Ma c’è stata un’estinzione precedente che è più misteriosa e meno discussa: un’estinzione 202 milioni di anni fa che spazzò via i grandi rettili che fino a quel momento governavano il pianeta e aprì la strada alla conquista dei dinosauri. Che cosa ha causato il cosiddetto[{” 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] Molti dei loro concorrenti morirono nel periodo Triassico”.

Randall Ermes, curatore di paleontologia al Museo di Storia Naturale dello Utah, specializzato in primi dinosauri, è d’accordo. “Questa è la prima prova dettagliata dell’antichità, la prima prova negli ultimi 10 milioni di anni del periodo Triassico e la prima prova di condizioni veramente glaciali”, ha detto. “La gente pensava a questo come a un momento in cui il mondo intero era caldo e umido, ma non era così”.

Olsen afferma che il passo successivo per comprendere meglio questo periodo è che più ricercatori cerchino fossili nelle ex regioni polari come il bacino di Jonggar. “I reperti fossili sono così pessimi, nessuno sta scavando”, ha detto. Queste rocce sono grigie e nere ed è molto difficile scavarle [for fossils] in questi strati. La maggior parte dei paleontologi è attratta dal tardo Giurassico, quando ci sono molti grandi scheletri da collezionare. L’antica regione polare è sostanzialmente ignorata”.

Riferimento: “Ghiaccio artico e l’ascesa ecologica dei dinosauri” di Paul Olsen, Jingjing Sha, Yan’an Fang, Clara Zhang, Jessica H. 2022, progresso della scienza.
DOI: 10.1126 / sciadv.abo6342

Lo studio è stato co-autore dello studio Jingeng Sha e Yanan Fang dell’Istituto di Geologia e Paleontologia di Nanchino. Clara Chang e Sean Kenny dell’Osservatorio terrestre di Lamont-Doherty; Jessica Whiteside dell’Università di Southampton. Hans Dieter ha citato in giudizio lo Smithsonian Institution; Morgan Schaller del Rensselaer Polytechnic Institute; e Vivi Vajda del Museo Svedese di Storia Naturale.

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