Therapsids adapted to drastic climate change by having shorter life expectancies

Yet, some animals thrived in the aftermath and scientists now know why.

In a new study published in Scientific Reports, palaeontologists from the National Museum, Bloemfontein -- a partner of the DST/NRF Centre of Excellence in Palaeosciences, seated at the University of the Witwatersrand in Johannesburg -- and their collaborators demonstrate that ancient mammal relatives, known as therapsids, adapted to drastic climate change by having shorter life expectancies.

When combined with results from survivorship models, this observation leads the team to suggest that these animals bred at younger ages than their predecessors.

"Before the Permo-Triassic extinction, the therapsid Lystrosaurus had a life span of about 15 years based on the record of growth preserved in their bones," said National Museum palaeontologist Jennifer Botha-Brink, the lead author on the paper. "Yet, nearly all of the Lystrosaurus specimens we find from after the extinction are only 2¬-3 years old. This implies that they must have been breeding when they were juveniles themselves."

This adjustment in life history also meant a physical change for Lystrosaurus. Before the mass extinction, this creature would have been a couple of metres long and weighed hundreds of kilograms--about the size of a pygmy hippo. Post-extinction, its size dropped to that of a large dog, in large part due to its altered lifespan. Yet, these adaptations seemed to pay off for Lystrosaurus. Ecological simulations show that by breeding younger, Lystrosaurus could have increased its chance of survival by 40% in the unpredictable environment that existed in the aftermath of the extinction.

This change in breeding behavior is not isolated to ancient animals either. In the past century, the Atlantic cod has undergone a similar effect due to human interference. Industrial fishing has removed most large individuals from the population, shifting the average size of cod significantly downward. Likewise, the remaining individuals are forced to breed as early in their lives as possible. Similar shifts have also been demonstrated in African monitor lizards.

"With the world currently facing its sixth mass extinction, palaeontological research can help us understand how and why some animals, such as those like Lystrosaurus, thrived in the face of disaster," said Botha-Brink. Studying the reasons for differential survival in response to dramatic environmental perturbation amongst extinct species will allow us to better predict how today's climate change will affect modern species."