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 in Scientific Reports, paleontologists from The Field Museum and their collaborators demonstrate that some ancient mammal relatives, known as therapsids, were suited to the 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 13 or 14 years based on the record of growth preserved in their bones," said Field Museum paleontologist Ken Angielczyk, one of the paper's authors. "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 still 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 meters long and have weighed hundreds of pounds–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 environments 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, paleontological research helps us understand the world around us today," said Angielczyk. "By studying how animals like Lystrosaurus adapted in the face of disaster, we can better predict how looming environmental changes may affect modern species."
Summary of Major Findings
1. Study of bone microstructure and body size distributions in the forerunners of mammals (therapsids) reveals distinct life history changes during the Permo-Triassic Mass Extinction (252 Mya).
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 great 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.
- 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 a tiny mouse to a massive rhino. South Africa contains the best fossil record of early therapsids in the world.
Our paper does NOT say
- We do not demonstrate behavioral or physical evidence of early reproduction. Rather, our main empirical dataset comes from body size distributions and bone histology, 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 histologic findings.
To Dr. Jennifer Botha-Brink: National Research Foundation (South Africa): Early Triassic Recovery Project NRF-65244
To Dr. Adam Huttenlocker: National Science Foundation Doctoral Dissertation Improvement Grant 1209018; National Science Foundation Postdoctoral Fellowship in Biology NSF DBI-1309040