Some animals not only survived a mass extinction following a dramatic and sudden climate shift 252 million years ago, but thrived against the odds, and scientists now know why.
The answer, it seems, was to live fast and die young.
When a series of Siberian volcanoes propelled billions of tonnes of carbon into the atmosphere it radically changed the Earth's climate.
Most animals could not cope with the new conditions and died out, but not the ancient mammal relatives known as therapsids, which not only lived on, but thrived.
To find out why, a group of paleontologists from the University of Utah studied growth patterns in therapsids from the South African Karoo Basin, an area rich in fossils from the Permian to the Early Jurassic – 300-180 million years ago.
“Before the Permo-Triassic extinction, the famous therapsid Lystrosaurus had a life span of about 13 or 14 years based on the record of growth preserved in their bones,” said paleontologist Ken Angielczyk, one of the paper's authors.
“Yet, nearly all of the Lystrosaurus specimens we find from after the extinction are only two to three years old.
“This implies that they must have been breeding when they were still relatively young themselves.”
The therapsids also appear to have had shorter life expectancies and to have become smaller.
Before the mass extinction, Lystrosaurus would have been about the size of a pygmy hippo. Post-extinction, its size dropped to that of a large dog.
But in its smaller, shorter lifespan state Lystrosaurus not only survived, but dominated ecosystems across the globe for millions of years during the post-extinction recovery period.
It makes up some 70-90% of the vertebrate fossils found in Early Triassic rocks in the Karoo.
“Therapsid fossils like Lystrosaurus are important because they teach us about the resilience of our own extinct relatives in the face of extinction, and provide clues to which traits conferred success on lineages during this tubulent time,” says another author Adam Huttenlocker.
“Lystrosaurus was particularly prolific, making it possible to build a large dataset and to sacrifice some specimens for histology to study the growth patterns recorded in its bones.”
Scientists have noticed this change to younger breeding in other species in our own time.
In the past century, the Atlantic cod has undergone a similar effect due to industrial fishing that has removed most large individuals from the population.
This has not only shifted the average size of cod significantly downward, but meant the remaining individuals are forced to breed as early in their lives as possible.
Similar shifts have also been demonstrated in African monitor lizards exploited by humans.
“Although it's hard to see the effects in our daily lives, there is substantial evidence that we are in the middle of a sixth mass extinction right now,” says Huttenlocker.
Bill Condie is a science journalist based in Adelaide, Australia.
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