Press release issued 14 August 2013
In the aftermath of the largest mass extinction in Earth history, anomodonts –ancient relatives of mammals – did not evolve any fundamentally new features, according to new research published today in Proceedings of the Royal Society B. This suggests that the evolutionary bottleneck they passed through during the extinction constrained their evolution during the recovery.
The study's findings are surprising as much research so far suggests that the survivors of mass extinctions are often presented with new ecological opportunities because the loss of many species in their communities allows them to evolve new lifestyles and new anatomical features as they fill the roles vacated by the victims. However, it turns out that not all survivors respond in the same way, and some may not be able to exploit fully the new opportunities arising after a mass extinction.
Dr Marcello Ruta of the University of Lincoln, with colleagues from the Field Museum of Natural History in Chicago, the Museum für Naturkunde in Berlin and the University of Bristol, studied how anomodonts responded in the aftermath of the end-Permian mass extinction, about 252 million years ago, when as many as 90 per cent of marine organisms and 70 per cent of terrestrial species became extinct.
Near the end of the Permian, a large number of anomodont species existed with 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 most successful group of anomodonts, with canine-like tusks in their upper jaws and turtle-like beaks, were the most important terrestrial herbivores of their time.
Dr Ruta said: "The number of anomodont species increased during the Permian, sharply dropped during the end-Permian extinction event, and then rebounded in the Middle Triassic Period (about 240 million years ago) before the final extinction of the group at the end of the Triassic.
"However, the variety of different anatomical features found in anomodonts – that is, their anatomical diversity – 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. Rather than allowing them to move to a new evolutionary trajectory, the genetic bottleneck they passed through constrained their future evolution."
Analyzing the response of animals and plants to this catastrophe helps scientists understand models of diversification and patterns of ecosystem reconstruction following large-scale biological crises.
Co-author Professor Michael Benton said: "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.
"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."
'Decoupling of morphological disparity and taxic diveristy during the radiation of anomodont therapsids' by Marcello Ruta, Kenneth D. Angielczyk, Jörg Fröbisch and Michael J. Benton in Proceedings of the Royal Society B.
Anomodonts are a diverse group of ancient mammal relatives (synapsids) known from the Permian and Triassic periods of Earth history, and anomodont fossils have been found on every continent. During this time they were the most diverse and abundant vertebrate herbivores (plant eaters) on land. They range from marmot size to hippo size, and the group includes many terrestrial members, some likely amphibious species, specialized burrowers, and a tree-dwelling species. The most successful group of anomodonts are called dicynodonts. Dicynodonts had stocky bodies with short limbs and tails. Most dicynodonts had a pair of canine-like tusks in the upper jaw, and a beak somewhat similar to that of turtles. Anomodonts survived the end-Permian extinction and re-diversified in the Triassic, before becoming extinct near the end of the Triassic.
Financial support for this research was provided by the Natural Environment Research Council, the Deutsche Forschungsgemeinschaft, and the Alexander von Humbolt Foundation and the German Federal Ministry for Education and Research.
Artist's reconstruction of the head of the anomodont Dicynodon lacerticeps
Image by Marlene Donnelly, Field Museum of Natural History, Chicago
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. Rather than allowing them to move to a new evolutionary trajectory, the genetic bottleneck they passed through constrained their future evolution.