Extinction changes the rules of evolution of body size

When they looked beyond the marine genera that became extinct to consider those who were the first of their kind, the authors found an even more dramatic shift in body size patterns before and after extinctions. During the background periods, the newly evolved genres tend to be slightly bigger than the ones that came before. During recovery from a mass extinction, the pattern reverses and it becomes more common for the creators of most classes to be tiny compared to the remaining species that survived the cataclysm.

The genera of gastropods, including sea snails, are among the few exceptions to the smaller reconstruction model. The genera of gastropods born during the recovery intervals tended to be larger than the survivors of the previous disaster. Almost everywhere, the authors write, “selectivity over body size is more pronounced, in any direction, during mass extinction events and their recovery intervals than during periods of background.”

Think of it as the biosphere’s version of choosing starters and bench warmers based on height and weight more than skill after losing a big game. There just might be some logic to this game plan in the evolution arc. “Our next challenge is to identify the reasons why so many creators after mass extinction are small,” said lead author Jonathan payne, Professor Dorrell William Kirby at Stanford Earth.

Scientists do not yet know whether these reasons may be related to global environmental conditions, such as low oxygen levels or rising temperatures, or to factors related to interactions between organisms and their local environment, such as scarcity of oxygen. food or the shortage of predators. According to Payne, “Identifying the causes of these patterns can help us not only understand how our current world came into being, but also project the long-term evolutionary response to the current extinction crisis. “

Fossil data

This is the latest in a series of articles by Payne’s research group which exploit statistical analyzes and computer simulations to uncover the evolutionary dynamics of body size data from marine fossil records. In 2015, the team recruited high school interns and undergraduates to help calculate the size and body volume of thousands of marine genera from photographs and illustrations. The resulting dataset included most genera of fossil invertebrate animals known to science and was at least 10 times larger than any previous compilation of fossil animal body sizes.

The group has since expanded the dataset and tested it for the models. Among other results, they found that a larger body size became one of the main determinants of the risk of extinction for marine animals for the first time in the history of life on Earth.

For the new study, Monarrez, Payne and co-author Christmas Heim at Tufts University used body size data from marine fossil records to estimate the likelihood of extinction and origin based on body size over most of the past 485 million years . By matching their body size data with audience occurrence records Paleobiology database, they were able to analyze 284,308 occurrences of marine animal fossils belonging to 10,203 genera. “This dataset has allowed us to document, in different groups of animals, how evolutionary patterns change when mass extinction occurs,” Payne said.

Future recovery

Other paleontologists have observed that smaller animals become more common in the fossil record as a result of massive extinctions – often calling it the “Lilliput effect,” after the Tiny People’s Realm in the novel. from the 18th century by Jonathan Swift. Gulliver’s travels.

The results of the new study suggest that animal physiology offers a plausible explanation for this model. The authors found the classic pattern of shrinkage in most classes of marine animals with low levels of activity and slower metabolism. Species in these groups that first evolved right after mass extinction tended to have smaller bodies than those that appeared during background intervals. In contrast, when new species evolved into more active groups of marine animals with faster metabolism, they tended to have larger bodies as a result of extinction and smaller bodies in normal times.

The results highlight the mass extinction as a two-act drama. “The extinction part is changing the world by not only removing a lot of organisms or a lot of species, but by removing them in various selective patterns. Then the recovery is not just equal for all those who survive. A new set of bias goes into the recovery model, “Payne said.” It is only by combining these two that you can truly understand the world that we get to five or 10 million years after an extinction event. “