Evolutionary flexibility: Unique traits help mammals survive mass extinctions

A recent study into the mammalian family tree across multiple mass extinctions reveals unexpected traits among the survivors, challenging the long-held belief that generalist or “boring” mammals primarily endured such catastrophic events. 

Survival of the unspecialized 

The researchers discovered that the mammals which persisted through mass extinctions possessed unique and advanced traits for their respective eras. This contradicts the notion of the “survival of the unspecialized.”

“The idea of the ‘survival of the unspecialized’ goes back to the 1800s, and the conventional wisdom is that generalized animals are the least likely to go extinct. But we found that the ones that survived more often only seemed generalized in hindsight, when compared with their later descendents,” explained study senior author Ken Angielczyk of the Field Museum.

“They were actually pretty advanced animals for their time, with new traits that might have helped them survive and provided evolutionary flexibility.”

Focus of the study

This revelation was supported by a massive family tree of synapsids, constructed by study co-lead author Spencer Hellert of Columbia College Chicago. Synapsids are a group of animals whose only surviving members are mammals.

“What’s been thought previously is that every time a new group of mammals evolves, you start out with a small generalist animal, since when disaster strikes, those are the guys that keep on trucking – they can hide anywhere, they can eat whatever is around,” said study co-lead author Spencer Hellert. “The kind of mammal that survives a mass extinction won’t be a specialist like a panda bear that can only eat bamboo.”

Evolutionary radiations 

Hellert’s extensive family tree, one of the largest of its kind, incorporated data from all previously established family trees for synapsids, providing a comprehensive and rigorous approach to summarizing information from various sources. 

The research team examined the evolutionary radiations in synapsids over time, focusing on the species’ diets and body sizes, and discovered that the pattern of survival was not solely reliant on generalist characteristics. At certain points, larger synapsids and not just generalist insect-eaters were among the survivors.

How the research was initiated 

Study co-lead author David Grossnickle, assistant professor at the Oregon Institute of Technology, published a study in 2019 that highlighted how small, insect-eating mammals are often the lineages that survive challenging times. He approached Hellert and Angielczyk to see if this trend may also apply to earlier mammals and their ancestors.

“We were pretty surprised – it’s pretty well-established that those mammal radiations go from these small insectivores into the bigger taxa repeatedly, so I was kind of expecting to see that as we went back into synapsid history. And when we went back, that pattern starts to disappear,” said Grossnickle.

Novel characteristics 

Upon closer analysis, the researchers found that some survivor species, initially perceived as unspecialized, actually bore novel characteristics. 

For instance, certain mammals from the dinosaur era possessed advanced teeth designed not just for cutting into prey but also for grinding, akin to a mortar and pestle. This “fancier” tooth structure might have offered a significant advantage during times of food scarcity, allowing these mammals to consume a broader range of foods.

Specialized animals 

However, the study’s findings should not be interpreted as suggesting that specialized animals, such as pandas with their bamboo-exclusive diet, are less prone to extinction than generalist species like raccoons. 

Instead, the research highlights that the species making it through mass extinctions possessed traits that were far from generic.

“Animals with novel traits like new tooth features, or jaws that work a little better at breaking down different food items, don’t really take over ecologically until the incumbent, older lineages go extinct,” said Grossnickle. 

“You often need an extinction event like the one that killed the dinosaurs to knock out some of those older groups, and then it allows those fancier animals to persist and diversify.”

Study implications 

According to the researchers, the results of their study have broader implications for scientists’ understanding of how evolution works. 

“We don’t really know if there is a consistent set of features typically possessed by the ancestors of evolutionary diversifications,” said Angielczyk. 

“The fact that we see this complexity in the diversifications of mammals and their ancient relatives means we need to examine other groups to see if the situation in mammals is an exception, or business as usual.”

The study is published in the journal Nature Ecology and Evolution.

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