Breeding Young to Survive: Growth Patterns in Therapsids during Earth’s Biggest Extinction Event

When you think of mass extinction events, you often picture the events themselves: either a single, massive event that wreaks havoc on the environment and its organisms, or a longer-term series of events that changes the structure of ecosystems and the diversity of organisms that inhabit those habitats. Either way, catastrophe.

Meteorcrater
Arizona’s Meteor Crater, Image CC BY 3.0, via Wiki Commons.

There are the famous “Big 5” extinction events, and we often think of these mass extinctions in terms of the number of organisms that went extinct, and less often from the success stories of organisms that survived, and how they bounced back.

What allows an organism to survive in situations where there may not be suitable habitat or enough food? What if these organisms are failing to achieve maturity? How does a species survive if its individuals are not living long enough to breed? Doesn’t that spell extinction for that species?

In most cases, yes. But in a new Open Access study published last week in Nature Scientific Reports, by Jennifer Botha-Brink, Daryl Codron, Adam Huttenlocker, Ken Angielczyk, and Marcello Ruta, shows that some organisms were able to successfully able to evolve around this caveat.

Their study focused on the most severe mass extinction event in Earth’s history, the Permo-Triassic Extinction Event (PTME). Around 96% of marine species and 70% of terrestrial vertebrate species perished due to several possible causes, including possible meteor impacts, volcanism, or increase of greenhouse gases. The continents in the Triassic became more arid, due to less rainfall and increased temperatures. The effects from one condition to the next created a cascade of problems that impacted diversity that persisted for around 5 million years.

So even if you successfully survived the PTME, you still had to survive the 5 or so million years of the ecological equivalent of a Mad Max film.

Botha-Brink et al (2016) looked at the growth patterns in therapsids from Karoo Basin deposits of South Africa, and focused on those that survived the PTME and persisted into the Triassic. The Karoo Supergroup is a good choice because its deposits span the Carboniferous clear into the Jurassic, which provides an advantage when looking at survivals and trends across the PTME. One organism got special attention, Lystrosaurus, because of its abundance in the fossil record from the Karoo, as well as its presence in other ecosystems globally during the post-extinction recovery phase.

srep24053-f1
Examples of bone microstructure from Lystrosaurus specimens from the Permian (a–c) and Triassic (d–f). Arrows indicate distinct growth marks. From Botha-Brink et al. (2016)

Using bone microstructure as a proxy for ontogenetic growth and development of Lystrosaurus and other therapsids across the Permo-Triassic boundary, the team measured distinct growth marks. Growth marks preceding slower-forming tissues are an indicator of a slower growth process, and thus a longer time for an animal to reach full-size maturity. The team observed that Permian taxa had many of these growth mark patterns present in their bones, whereas Triassic specimens showed little-to-no growth mark patterns that would suggest they were reaching skeletal maturity. Likewise, slower-forming types of bone are absent from all specimens of Lystrosaurus that the team examined.

Botha-Brink et al. (2016) hypothesized from these observations that early Triassic therapsids were not reaching full maturity before they died. Likewise, an examination of body size distributions in Lystrosaurus before and after the PTME showed that individuals after the extinction event trended smaller than  the larger individuals from the Permian.

So, based on these observations, the team concluded that therapsids in the Early Triassic were reproducing at a younger age, before full size and maturity was achieved. But since we cannot know for sure when first reproduction was actually occurring in these organisms, either before or after the PTME, the team conducted a series of population dynamics simulations and concluded that, indeed, Early Triassic species would have had to modify their breeding habits in order to survive, notably reproducing while still juveniles or young adults. This was achieved either through larger clutches/litters or more breeding events.

This study does note, however, that it focuses solely on specimens from the Karoo Group in South Africa, but it does have larger implications. Will similar trends be observed in other sites across the globe? This study is great in outlining how bone histology can shed light on life history events, and presents a theory on just how these early Mesozoic organisms, like Lystrosaurus, were able to persist in such an extreme, turbulent time in Earth’s history.

Read the original paper here:
Botha-Brink, J. et al. (2016) Breeding Young as a Survival Strategy during Earth’s Greatest Mass Extinction. Science Reports 6, 24053; doi: 10.1038/srep24053

Featured image: Lystrosaurus murrayi by Dmitry Bogdanov, courtesy Wikimedia Commons

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Published by Sarah Z. Gibson

Dr. Sarah Z. Gibson is a paleontologist and science communicator based in Minnesota. Her research focuses on the evolutionary history of ray-finned fishes from the Early Mesozoic. https://orcid.org/0000-0002-6784-3980

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