Ancient mammal relatives cast light on recovery after mass extinction

A study of how the ancient relatives of modern mammals recovered after mass extinction 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.

The fossil record has been used by scientists in a bid to understand how mass extinctions came about, and how species and ecosystems recovered in the aftermath.

Research 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.

Dr Marcello Ruta (University of Lincoln, UK), Dr Kenneth Angielczyk (Field Museum of Natural History, Chicago), Professor Jörg Fröbisch (Museum für Naturkunde, Berlin) and Professor Michael Benton (University of Bristol) examined how a group of ancient relatives of mammals called anomodonts responded in the aftermath of the largest mass extinction in Earth’s history.

Their findings, published this week in Proceedings of the Royal Society B and titled “Decoupling of morphological disparity and taxic diversity during the adaptive radiation of anomodont therapsids”, show that anomodonts remained anatomically conservative even as the number of species recovered.

The mass extinction at the end of the Permian Period – about 252 million years ago – had profound effects on organisms on land and in the sea, with as many as 90 per cent of marine organisms and 70 per cent of terrestrial species wiped out.

Lead author Dr Marcello Ruta, of the University of Lincoln, UK, 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.

This is the first study of its kind to address simultaneously changes in species number and anatomical diversity in anomodonts, and to quantify their response to the most catastrophic extinction on record.

Anomodonts are abundant, diverse, and well-studied, which makes them ideal models for evolutionary analyses.

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.”

Funding was provided by the Natural Environment Research Council, the Deutsche Forschungsgemeinschaft, the Alexander von Humbolt Foundation and the German Federal Ministry for Education and Research.

Marine worms reveal the deepest evolutionary patterns

The study of ancient worms could offer a more solid understanding of evolutionary patterns and processes, according to new research.

Scientists from the universities of Bath and Lincoln have revealed new findings on the evolutionary relationships and structure of priapulids – a group of carnivorous mud-dwelling worms living in shallow marine waters.

The research, carried out by evolutionary biologists Dr Matthew Wills, Dr Sylvain Gerber, Mr Martin Hughes (all University of Bath) and Dr Marcello Ruta (University of Lincoln), features in October’s Journal of Evolutionary Biology.

Dr Wills first pioneered a study on existing and extinct priapulids in 1998. Fourteen years on, the team looked at a new and expanded data set of anatomical features to see how knowledge of these worms has been affected by new fossil finds.

He explained: “The fossils from the Cambrian period can cause a real headache for evolutionary biologists. Instinct tells us to expect simple organisms evolving over time to become increasingly more complex. However during the Cambrian period there was an apparent explosion of different major groups of animals, all appearing simultaneously in the fossil record. We looked at priapulid worms, which were among the first ever predators. What’s remarkable is that they had already evolved into a diverse array of forms – comparable to the morphological variety of their living cousins – when we first encounter them in the Cambrian fossil record. It’s precisely this apparent explosion of anatomical diversity that vexed Darwin and famously attracted the attention of Harvard biologist Stephen Jay Gould.”

Dr Ruta, from the School of Life Sciences at the University of Lincoln, continued: “Our work has shown that despite many new fossil finds, including many from China in the last decade, the picture remains largely unchanged. This is really important because the fossil record is notoriously incomplete. It is often difficult to know whether a pattern is just an artifact of this incompleteness, or biologically meaningful. Our study resolutely confirms the latter. Priapulids are fascinating animals with much potential in evolutionary studies. They have a long history, with the earliest known species being 505 million years old, and with some of their extinct relatives being even older. They were important components of ancient bottom-dwelling marine invertebrate communities, and their predatory habits are well documented in the fossil record. However, for all their abundance and diversity, priapulids are a remarkable and often cited example of a morphologically conservative group, their overall shape and proportions having changed relatively little during their history. This research will help us to understand evolutionary patterns in ‘deep time’. This is looking at the tempo (evolutionary rates) and mode (the study of the way, manner or pattern of evolution) to uncover the ancient events when organisms first began to diversify and break from one another. For example, what makes a mammal a mammal and so on.”

The research gives prominence to the importance of an adequate and unbiased inclusion of data, where possible, from both fossil and living species in assembling evolutionary family trees. Fossils inform our understanding of evolutionary patterns and processes, and show unique morphological traits that are no longer observed in living species.

Dr Ruta added: “Detailed scrutiny of other groups of organisms is needed, in order to decipher the rate at which structural, functional and ecological changes occur and how acquisition of new traits impact on group diversification. Ultimately, combined results from these investigations will offer a solid framework for understanding the very roots of Life’s grandeur and the astounding variety of species alive today.”

The full article can be accessed online at http://onlinelibrary.wiley.com/doi/10.1111/jeb.2012.25.issue-10/issuetoc