Written in Bone: Lystrosaurus

With its flat face and tusks protruding from its beaked mouth, no one would ever have called Lystrosaurus “beautiful”. But Lystrosaurus was the ultimate survivor–it made it through the greatest mass extinction in the Earth’s history, and dominated the land for millions of years afterwards.

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About 250 million years ago, near the end of the Permian period, the Earth looked different than it does now. None of the continents that we have today existed–they were all squeezed together into one enormous land mass that stretched from pole to pole, known as Pangaea (“all earth”). This single landmass was in turn surrounded by an immense ocean, known as Panthalassa. Cycads and ferns grew in profusion, but there were no grasses and no flowering plants. Insects and other arthropods crowded the land, where they were fed on by large salamander-like amphibians and lizardlike reptiles. Birds did not yet exist, and dinosaurs were a small and little-noticed group of turkey-sized predators. There were no mammals yet, but their early ancestors had appeared: reptiles known as “therapsids”, which were already showing some of the anatomical adaptations that would characterize the mammals.

One of these was Lystrosaurus, an inoffensive plant-eater about the size of a large dog. To us, it would have appeared like a strange combination of a lizard and a pig. Lystrosaurus had two tusks which it used to dig up plants, and a parrot-like beak. The face was flat, the body was short and stocky, and the chest was large, giving it a huge lung capacity which enabled it to survive on the dank stale air inside burrows. It has been speculated that it may have had a covering of bristle-like hairs, somewhat like an aardvark. Though Lystrosaurus had powerful front limbs and claws for digging, its legs were splayed out to the sides like a lizard, giving it a waddling gait. It had no armor or defensive weapons, and seems to have depended on its burrow for shelter from predators. It may have been nocturnal.

For some 20 million years, Lystrosaurus had been waddling around the landscape, munching on plants, little-noticed by anything else. But the pig-lizard was destined to play an important role in the history of life on Earth–it would be one of the few survivors of the deadliest mass extinction the planet has ever seen.

Over millions of years, as plate tectonics dragged the continents together to form Pangaea, the Earth’s climate had been changing. Shorelines disappeared as the continents joined, and shallow seas, one of the most populous habitats for life, disappeared with them. As the single land mass got bigger, its interior became warmer and drier, the humid forests which had characterized the earlier Carboniferous Period disappeared, and the interior was populated by animals who could adapt to the dry warm conditions. Including Lystrosaurus.

By itself, this global climate change would eventually have led to a wave of evolution, as earlier species died out and were replaced by new ones that had adapted to the altered weather patterns. But while all these changes were going on, something else was happening that would have lethal consequences.

In what is now modern-day Siberia, the same geological forces that had assembled the Pangaea super-continent were breaking the Earth’s crust open. Around 250 million years ago, a vast series of volcanic vents, stretching over a thousand miles, appeared. These rifts poured out wave after wave of molten basaltic lava, covering thousands of square miles with thick layers of volvanic rock–a geological formation known today as the Siberian Traps. The eruptions lasted for thousands of years. The volcanos also belched out huge amounts of ash, sulphur particles, and carbon dioxide. At first, the ash and particulate dust, circulated by winds, would have darkened the skies, shaded out a portion of sunlight, and caused global temperatures to cool significantly. But over time, the billions of tons of carbon dioxide that poured out each year would change the chemical composition of the atmosphere. The CO2 would cause a runaway greenhouse effect, trapping the sun’s heat and causing global temperatures to rise sharply.

The effect was devastating. Temperatures shot up. Carbon dioxide and sulphur in the ocean raised its acidity, while the warming temperatures decreased its ability to hold oxygen. On land, the vast interior of Pangaea was transformed into an endless desert stretching from horizon to horizon. It happened too quickly for evolution to cope, and the result was the Permian Mass Extinction. The oceans were hardest hit–about 95% of all sea life disappeared. Terrestrial life fared little better–about 75% of all land plants and animals went extinct. It is the closest that life on the planet has ever come to being completely wiped out.

But one of the survivors was the pig-lizard Lystrosaurus. It is not really known why this particular animal was able to survive when so many others died. It has been suggested that its simple diet of plant roots, combined with its burrowing habits and ability to extract oxygen from dust-laden and stale air, allowed it to eke out a meager living in the hellish aftermath of the climate disaster.

After a time, the Siberian eruptions ended, CO2 began to be slowly scrubbed from the air (by plants and by chemical processes), and a new period in the history of life, the Triassic, began. The Permian survivors began to recover, and as they adapted to the slowly-improving conditions, new species appeared. All life today is a descendant of the tiny handful of plants and animals that managed to make it through the end-Permian “Great Dying”.

One of the most successful of these survivors turned out to be Lystrosaurus. The pig-lizard waddled its way from one end of the supercontinent to the other, and as geological forces began to break Pangaea apart into the continents we know today, Lystrosaurus went along for the ride, evolving into several different species. For much of the early Triassic period, the Lystrosaurs were the most common large terrestrial animal, making up over 90% of all the skeletons found in some fossil bone beds.

It took about 30 million years, until the mid-Triassic, for life to once again reach the diversity it had before the Permian Extinction. By this time, the handful of Permian survivors had produced a number of new groups. The therapsids would go on to form modern mammals and, eventually, us. The dinosaurs began to diversify from their archosaurian ancestors: by the end of the Triassic they were the dominant form of life on Earth and would reign until a new mass extinction event wiped them out at the end of the Cretaceous. (Though before going extinct, the dinosaurs would produce our modern birds.)

The Lystrosaurs, however, could not compete with these new groups. After riding out the most devastating disaster in Earth’s history, the pig-lizards quietly died out during the early Triassic.

 

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