Since the 1980s, evolutionary biologists have debated whether mass extinctions and the recoveries that follow them intensify the selection criteria of normal times – or fundamentally shift the set of traits that mark groups of species for destruction. The new study finds evidence for the latter in a sweeping analysis of marine fossils from most of the past half-billion years.

Whether and how evolutionary dynamics shift in the wake of global annihilation has “profound implications not only for understanding the origins of the modern biosphere but also for predicting the consequences of the current biodiversity crisis,” the authors write.

“Ultimately, we want to be able to look at the fossil record and use it to predict what will go extinct, and more importantly, what comes back,” said lead author Pedro Monarrez, a postdoctoral scholar in Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth). “When we look closely at 485 million years of extinctions and recoveries in the world’s oceans, there does appear to be a pattern in what comes back based on body size in some groups.”

Build back smaller?

The study builds on recent Stanford research that looked at body size and extinction risk among marine animals in groupings known as genera, one taxonomic level above species. That study found smaller-bodied genera on average are equally or more likely to than their larger relatives to go extinct.

The new study found this pattern holds true across 10 classes of marine animals for the long stretches of time between mass extinctions. But mass extinctions shake up the rules in unpredictable ways, with extinction risks becoming even greater for smaller genera in some classes, and larger genera losing out in others.

The results show smaller genera in a class known as crinoids – sometimes called sea lilies or fairy money – were substantially more likely to be wiped out during mass extinction events. In contrast, no detectable size differences between victims and survivors turned up during “background” intervals. Among trilobites, a diverse group distantly related to modern horseshoe crabs, the chances of extinction decreased very slightly with body size during background intervals – but increased about eightfold with each doubling of body length during mass extinction.