Ancient mammal relatives cast light on recovery after mass extinction

A study of how the ancient relatives of modern mammals recovered after mass extinction, published in Proceedings of the Royal Society B today, raises fresh questions about the capacity for life to recover from cataclysmic events.

A growing number of species in the modern world face extinction due to global climate change, habitat destruction and over-exploitation.

An international group of researchers have been studying the fossil record in a bid to understand how mass extinctions came about, and how species and ecosystems recovered in the aftermath.

The study suggests that the survivors of mass extinctions are often presented with new ecological opportunities. The loss of many species in their communities allows them to evolve new lifestyles and new anatomical features as they fill empty niches. However, it turns out that not all survivors respond in the same way, and some may not be able to fully exploit the new opportunities arising after a mass extinction. 

At the end of the Permian Period - about 252 million years ago - as many as 90 per cent of marine organisms and 70 per cent of terrestrial species were wiped out in the largest mass extinction in Earth’s history. Lead author Dr Marcello Ruta, University of Lincoln, and his teamexamined how a group of ancient relatives of mammals called anomodonts responded in the aftermath. Their findings show that anomodonts remained anatomically conservative even as the number of species recovered. 

Dr Ruta said, “Groups of organisms that survive such a mass extinction are said to have passed through an evolutionary bottleneck similar to the genetic bottleneck that may occur in a population if many of its members die off.  A genetic bottleneck sometimes allows the population to move to a new evolutionary trajectory, but other times it constrains the future evolution of the population. Near the end of the Permian, a large number of anomodont species existed that displayed a wide range of body sizes and ecological adaptations, including terrestrial plant eaters, amphibious hippo-like species, specialized burrowers and even tree-dwelling forms.”

The variety of anatomical features found in anomodonts declined steadily over their history. Even in the aftermath of the mass extinction, when there should have been a lot of empty ecological space, anomodonts did not evolve any fundamentally new features. This suggests that the evolutionary bottleneck they passed through during the extinction constrained their evolution during the recovery.

Dr Ruta added, “The results underscore that recoveries from mass extinctions can be unpredictable, a finding that has important implications for the species extinctions being caused by human activity in the world today. We cannot just assume that life will return to the way it was before the disturbances.”