Extinction and origin patterns change after mass extinctions

Scientists at Stanford University have discovered a surprising pattern in how life reappears after a cataclysm. Research published on October 6 in Proceedings of the Royal Society B shows that the usual rules of evolution of body size change not only during mass extinction, but also during subsequent recovery.

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

Whether and how evolutionary dynamics change 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 will come back,” said senior author Pedro Monarrez, postdoctoral researcher at Stanford’s School of Earth. , Energy & Environmental Sciences (Stanford Earth). “When we look at nearly 485 million years of extinctions and recoveries in the world’s oceans, there appears to be a pattern in what comes up as a function of body size in certain groups.”

Rebuild smaller?

The study builds on recent research from Stanford that looked at body size and extinction risk in marine animals in groups known as genera, a taxonomic level above species. This study found that smaller genera are on average as much or more likely than their larger parents to disappear.

The new study found that this pattern holds true for 10 classes of marine animals during the long periods between mass extinctions. But mass extinctions are upsetting the rules in unpredictable ways, with the risks of extinction becoming even greater for smaller genres in some classes, and bigger losing genres in others.

The results show that the smaller genera of a class known as crinoids – sometimes called sea lilies or silver fairies – were much more likely to be wiped out during mass extinctions. In contrast, no detectable size difference between victims and survivors appeared during the “background” intervals. Among trilobites, a diverse group distant from modern horseshoe crabs, the odds of extinction decreased very slightly with body size during background intervals, but increased about eight-fold with each doubling of body length during extinction. massive.

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 authors 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 papers by Payne’s research group that harness statistical analysis 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 larger body size became one of the biggest determinants of extinction risk for marine animals for the first time in the history of life on Earth.

For the new study, Monarrez, Payne, and co-author Noel Heim of Tufts University used body size data from the marine fossil record to estimate the likelihood of extinction and origin as a function of body size at during most of the 485 million years. By combining their body size data with occurrence records from the public 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. 18th century book 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, recovery is not just equal for all 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. ”