Ancient mammal relatives called therapsids survived Earth’s greatest mass extinction 252 million years ago to become some of the most successful animals in the Early Triassic. Now, researchers studying the bones of these mammal forerunners have discovered their survival strategy: live fast, die young. The findings are published in Scientific Reports this week.
The planet lost over 80 percent of all marine species and 70 percent of all terrestrial species when a series of volcanoes erupted at the end of the Permian, ejecting so much carbon into the atmosphere, Earth’s climate was altered. Some animals, however, thrived in the aftermath of the Permo-Triassic Mass Extinction. The therapsid Lystrosaurus, for example, dominated Early Triassic ecosystems worldwide for millions of years.
A team led by Jennifer Botha-Brink of South Africa’s National Museum examined the bone tissue microstructure (a reflection of development rates) of 177 limb bones and three ribs from 34 therapsids unearthed from the South African Karoo Basin. These boundary-crossing and non-boundary-crossing animals spanned 20 million years of therapsid evolution. The team also studied body size distributions (based on skull length) of 246 Lystrosaurus specimens before and after the extinction event to model differences in survivorship rates: L. maccaigi and L. curvatus from before and L. murrayi and L. declivis after.
The researchers discovered that the life expectancy of Lystrosaurus was severely reduced after the extinction boundary. In addition to elevated mortality rates, the post-extinction species also reached adult sizes sooner and bred earlier.
Lystrosaurus specimen from the Albany Museum in Grahamstown, South Africa. Ken Angielczyk
"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," study co-author Kenneth Angielczyk from the Field Museum of Natural History said in a statement. "Yet, nearly all of the Lystrosaurus specimens we find from after the extinction are only 2 to 3 years old. This implies that they must have been breeding when they were still juveniles themselves."
Furthermore, Lystrosaurus also shrank in size. Before the extinction event, the extinct mammal relative was about a couple meters long and weighed as much as a pygmy hippo; after the extinction boundary, it was about the size of a big dog.
"We propose that these individuals were likely breeding young to compensate for dying at an early age," the authors wrote. Truncated development and shortened generation times could have helped Lystrosaurus survive harsh, unpredictable environmental conditions during the planet’s recovery period. According to the team’s simulations, extinction rates were predicted to be at least 40 percent in turbulent environments; but with changes to therapsid life histories, that figure dropped to just 3 percent.