Why did dinosaurs escape the end-Triassic mass extinction? The answer is found

◎ Science and Technology Daily reporter Zhang Ye

In the long geological history of the Earth, there have been five mass extinction events. At the end of the Triassic period about 200 million years ago, a sudden disaster led to the extinction of a large number of terrestrial creatures.

However, the dinosaurs luckily escaped this disaster and successfully "ascended to power", dominating the world in the subsequent Jurassic and Cretaceous periods.

So, what caused the mass extinction of terrestrial life at the end of the Triassic period? Why were dinosaurs able to survive the natural disaster of biological extinction and develop rapidly after the disaster?

On July 2, Researcher Sha Jingeng and Dr. Fang Yanan from the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, in collaboration with American scholars, published a latest research result online in the internationally renowned journal Science Advances, showing that volcanic winter was the main cause of the mass extinction of terrestrial life at the end of the Triassic; and for the first time revealed that feathers with heat-insulating functions and long-standing adaptation to the cold polar climate were the reasons why dinosaurs escaped the volcanic winter at the end of the Triassic and quickly "ascended to power."

The intriguing evolution of Triassic organisms

The Triassic Period is the first period of the Mesozoic Era, which began 252 million years ago and ended 201 million years ago, lasting about 51 million years.

In the Permian period before the Triassic period, the Earth had just staged a tragic elegy for life. The mass extinction event at the end of the Permian period is known as the largest extinction event in the history of the Earth. The tree of life never recovered. In the Early Triassic period, it took millions of years for the few remaining organisms to restore the Earth's environment to a state suitable for most organisms to live in.

However, the tenacity of life is beyond imagination. After the recovery and evolution after the mass extinction at the end of the Permian period, the Triassic organisms showed a prosperous scene, with reptiles and gymnosperms growing unprecedentedly. It is particularly noteworthy that compared with the strange appearance of Paleozoic animals, the Triassic reptiles are more like modern organisms.

For example, theodonts, dinosaurs, and mammal-like reptiles rose rapidly in the Triassic. By the late Triassic, dinosaurs had become a diverse group and occupied an important position in the ecosystem. The Triassic is therefore called the "dawn of the dinosaur age." However, the dinosaurs at that time were still relatively weak and far from being the kings.

Although the animals and plants of the Triassic period made tremendous efforts to restore the Earth's ecology and promote the evolution of life, nature still ruthlessly trampled them underfoot: at the end of the Triassic period about 200 million years ago, the Earth's Pangaea disintegrated, and drastic changes in the environment caused a new round of mass extinction.

Although the impact of the end-Triassic mass extinction event was the weakest of the five mass extinction events, a large number of marine and terrestrial organisms still became extinct, such as bivalves, brachiopods, ammonites, corals, radiolarians, ostracods, foraminifera, etc. in the ocean, and most non-dinosaurian archaea, therapsid reptiles and some large amphibians on land also became extinct.

But this extinction event also gave the Triassic a special significance, that is, the Triassic was the only era in the history of the world that experienced a mass extinction again after emerging from a disaster and advancing the evolution of life by leaps and bounds. It had a profound impact on the development of Mesozoic organisms, especially the heyday of the dinosaurs.

Who was responsible for the Triassic extinction?

There have always been many explanations for the cause of the mass extinction event at the end of the Triassic period. In recent years, researchers have increasingly focused their attention on volcanic eruptions.

Scientists who support the volcanic eruption hypothesis believe that in the early Mesozoic era, the disintegration of the supercontinent Pangaea triggered intense volcanic activity, which led to an increase in the concentration of carbon dioxide in the atmosphere and caused a greenhouse effect.

Previous studies have shown that the atmospheric carbon dioxide concentration at that time reached 1000-6000ppm, there were no glaciers at the earth's poles, and forest coverage reached all the way to the North and South Poles of the Pangea continent.

However, Researcher Sha Jingeng and Dr. Fang Yanan from the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Professor Paul Olsen from Columbia University in the United States and others have different views on the changes in the earth's environment after volcanic eruptions.

They conducted high-resolution measurements and research on the terrestrial Late Triassic to Early Jurassic strata of the Haojiagou section in the Junggar Basin in Xinjiang, my country.

The research team recalibrated the ancient latitude of the Junggar Basin and believed that it was located in the Arctic region of Pangea (about 71 degrees north latitude) during the Late Triassic to Early Jurassic. During the field survey, they discovered for the first time beautifully preserved dinosaur footprint fossils in the late Triassic to early Jurassic lake-facies mudstones of the Junggar Basin.

Sha Jingeng told reporters that in the Late Triassic, herbivorous dinosaurs were mainly distributed in mid- and high-latitude regions, while low-latitude regions were dominated by non-dinosaurs and non-archosaurian archosaurs. After the mass extinction event at the end of the Triassic, dinosaurs quickly spread around the world, while other archosaurs that originally dominated low-latitude regions became extinct.

The traditional view is that the mass extinction of terrestrial life at the end of the Triassic was caused by global warming due to the release of large amounts of carbon dioxide by volcanic eruptions, but the response of terrestrial life after the extinction event seems to refute this view.

The research team speculated that the supervolcanic eruption first brought about a volcanic winter, which refers to a sudden drop in the temperature of the earth's surface caused by the large amount of volcanic ash and aerosols released by volcanic eruptions, which blocked solar radiation.

Although volcanic winter lasts for a short time, only a few years to decades, and then the greenhouse effect begins, it does not prevent volcanic winter from becoming the primary factor in "killing" most terrestrial organisms.

"If the temperature rises, land creatures will migrate from low latitudes to high latitudes. But in fact, land quadrupeds including dinosaurs migrated from high latitudes to low latitudes, and the pseudo-suchians originally in low-latitude areas became extinct. Therefore, we infer that it was the 'cold' of the volcanic winter that caused the mass extinction of land creatures, rather than the 'heat' as traditional views believe." Sha Jingeng said.

Why did the dinosaurs survive the disaster?

During the mass extinction event at the end of the Triassic period, dinosaurs were like the chosen ones of God. They not only established their dominance over the world, but also revitalized the entire ecosystem, as if all the suffering on Earth was a prelude to their dominance.

Coincidentally, the research team discovered dinosaur footprints and ice raft deposits at the same time in the Late Triassic to Early Jurassic strata in the Junggar Basin.

Ice raft deposits are sand or small gravel (0.1-15 mm in diameter) floating in mudstone. Fang Yanan has two speculations about ice raft deposits: one is that when the lake water freezes in winter, the sand and gravel under the water also freeze. When spring comes, some ice blocks, like bamboo rafts, transport the frozen gravel to the center of the lake, where it melts and settles in the mud; the other possibility is that strong winds in winter blow the gravel on land to the ice surface, and when the lake ice melts in spring, the gravel falls to the bottom of the lake.

"This shows that even in the greenhouse Earth period when there was no glacier at the poles, there was seasonal freezing (below zero degrees)," Fang Yanan said.

The presence of dinosaur footprints and ice raft deposits in polar regions suggests that dinosaurs were well adapted to the seasonally cold polar climate, which is how they survived the volcanic winter.

At the same time, these dinosaurs also have "external appearance" blessings.

"Evidence of feather fossils has been found in some branches of non-avian theropod dinosaurs and two basal herbivorous ornithischian dinosaurs. Based on the phylogenetic scaffolding method, we infer that dinosaurs were born with feathers, and these feathers were most likely used for insulation." Fang Yanan said that although the large adult sauropod dinosaurs discovered so far have no feathers, it is possible that they were born with feathers and lost them as they grew up, just like adult African elephants have no hair, but baby elephants do have hair.

The primitive feathers with heat-insulating function ensured that herbivorous dinosaurs could withstand the cold winters in mid- and high-latitudes, and thus enjoy the rich and stable plant resources in mid- and high-latitudes.

Most other archosaurs without feathers for insulation became extinct in the volcanic winter, with only a few smaller species escaping the disaster by hiding in caves.

In short, the end-Triassic extinction event led to the extinction of a large number of plants and animals, but it inadvertently opened the door for the newly emerging reptiles, dinosaurs, to dominate the earth. Eventually, dinosaurs were crowned kings and became the top creatures in the food chain, and they sat in this position for 140 million years.

Source: Science and Technology Daily

Editor: Wang Yu

Review: Julie

Final judge: Wang Yu