How Dinosaur Ancestors Survived Earth's Biggest Mass Extinction

Researchers have uncovered the remains of a never-before-seen ancient creature in a discovery that sheds light on how a group of animals that includes dinosaurs and crocodiles survived the Earth's largest-ever mass extinction and thrived after the event.

Chinese scientists discovered bones of the newly identified species in the region of Xinjiang, located in the northwest of the country, amid a rich deposit of plant and animal fossils from the Permian period (around 299-252 million years ago) and Triassic period (around 252-201 million years ago), a study published in the journal The Science of Nature shows.

Estimates of the age of the specimen, which has been named Vigilosaurus gaochangensis, indicate that it is around 252 million years old, placing it right at the boundary of the two periods—a tumultuous time marked by a mass-extinction event that wiped out the majority of life on Earth.

The Permian-Triassic extinction event, as it is known, is thought to have resulted from an intense period of volcanic activity that spewed out vast quantities of greenhouse gases, ash and other debris, which led to significant global warming as well as other environmental effects such as ocean acidification.

The researchers discovered a fossilized hind limb of V. gaochangensis and subsequently determined that the remains represented a new species. The lizard-like creature would have measured less than 20 inches long and was likely quite agile.

The species is one of the earliest relatives of the archosaurs—a group of animals that includes the extinct non-avian dinosaurs and pterosaurs, as well as birds and crocodiles.

The researchers found that V. gaochangensis has several bodily features indicating that it likely walked upright on all fours rather than in a sprawling stance seen in today's lizards, that walk with their limbs extended to the sides and bellies almost touching the ground.

The findings suggest that the ancestors of archosaurs acquired this key evolutionary innovation before the Permian-Triassic extinction, which enabled later members of the group, such as dinosaurs, to subsequently dominate the Earth for millions of years. Previously, researchers had suggested that this ability evolved after the event as a response to drastic climatic changes.

Archosaurs began to flourish and diversify in the Triassic and became dominant over the course of the wider Mesozoic Era (around 252 to 66 million years ago) as life on Earth recovered from the cataclysmic event. But there is evidence of their earliest relatives from before the Permian-Triassic extinction event.

The fossil record of these early relatives though is scarce and there is a significant gap in the scientific knowledge about how and when archosaurs transformed from a sprawling to an upright posture, in which the limbs are tucked under the body.

Some previously known archosaur ancestor tracks had indicated that these animals evolved the ability to walk upright on all fours prior to the extinction event, but to date there has been little evidence from actual fossil finds to support this idea.

The latest findings may go some way to filling this knowledge gap by providing unique fossil evidence of an early archosaur relative that had likely evolved the ability to walk upright on all fours before the mass extinction, the researchers said.

The posture shift from sprawling to upright is considered to be a major evolutionary transition for archosaurs and can be seen in birds, dinosaurs, crocodiles, pterosaurs and their relatives.

"The evolution of walking ability was not prompted by climate change. But having such a structure made it easier for archosaurs to adapt to the warming conditions in the early Triassic," co-author of the study, Chen Jianye, a researcher at the Institute of Vertebrate Palaeontology and Palaeoanthropology at the Chinese Academy of Sciences, told the South China Morning Post.

Walking in a more upright stance has a number of potential evolutionary advantages. It allowed many archosaur species to move faster and sustain their vigorous movements for longer, meaning they were better able to adapt to a changing world.

"The new posture opened up a lot of opportunities for occupying new ecological niches, providing new possibilities in animal evolution," Jianye said.

Upright posture in those archosaurs that developed the ability is often associated with endothermy—a trait describing so-called warm-blooded animals that can maintain a constant body temperature independent of the environment—a study published in the journal Paleobiology showed.

Endothermy was another key evolutionary advantage that arose in some archosaurs. But while endothermy offers significant benefits, such as constant alertness, they come at a cost—these animals have to eat much more to sustain themselves. The ability to walk upright would have enabled endothermic archosaurs to become more effective predators to compensate for this disadvantage.

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