Therapsids adapted to drastic climate change by having shorter life expectancies

Last updated Apr 6, 2016

Two hundred and fifty-two million years ago, a series of Siberian volcanoes erupted and sent the Earth into the greatest mass extinction of all time. Billions of tons of carbon were propelled into the atmosphere, radically altering the Earth's climate.

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

In a new study published today, 5 April 2016, 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."

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Summary of Major Findings

2. Our results show that post-extinction species took less time to reach adult size, had shortened life expectancies, high mortality rates, and were at heightened risk of extinction.

3. Simulations using ecological modelling show that breeding earlier, which would have led to shortened generation times, could have helped therapsids survive in the unpredictable, resource-limited post-extinction environment, and explains body size distributions observed in earliest Triassic species like Lystrosaurus.

4. The results help explain how the "disaster taxon" Lystrosaurus, not only survived, but spread to all areas of the globe and became the most abundant vertebrate after the Permo-Triassic Mass Extinction.

Background Facts

There have been five major mass extinctions in Earth's history.
The Permo-Triassic Mass Extinction (252 Mya) was the most catastrophic extinction in Phanerozoic history.
It killed 80-96% of all marine species and 70% of all terrestrial species.
Post-extinction ecosystems did not fully recover until some 5 million years after the event.
Therapsids include animals like Lystrosaurus, and another group called the cynodonts, which includes mammals and their immediate ancestors. Their body sizes ranged from that of a tiny mouse to a massive rhino. South Africa contains the best fossil record of early therapsids in the world.

The paper does NOT say

We do not demonstrate behavioural or physical evidence of early reproduction. Rather, our main empirical dataset comes from body size distributions and bone microstructure, which show direct evidence of shorter life expectancies in Triassic therapsids. Our inference of earlier breeding is then based on size distributions and expectations of survivorship models that we pursued based on our observations of the bone microstructure.

About the DST/NRF Centre of Excellence in Palaeosciences

The DST/NRF Centre of Excellence (CoE) for the Palaeosciences was awarded in 2013 to the University of the Witwatersrand and its collaborating institutions, namely the University of Cape Town, Iziko Museum in Cape Town, the National Museum in Bloemfontein, the Albany Museum of Rhodes University, and Ditsong Museum in Pretoria. It is hosted in the Evolutionary Studies Institute at Wits University.

Interviews:

Dr Jennifer Botha
National Museum, Bloemfontein
DST-NRF Centre of Excellence in Palaeosciences
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