Contrary to previous belief, mammals that survived past mass extinctions were not always omnivores; many mammals had unique traits that aided their survival, prompting a reassessment of evolutionary patterns.
Life reconstruction of Thrinaxodon, an ancient mammalian relative from the Triassic period of Earth's history. Thrinaxodon was similar in size and shape to modern minks and was close in size to the inferred ancestor of ancient mammal relatives known as cynodonts, and it may have shared that ancestor's preference for animal foods. Source: AprilNeander
66 million years ago, an asteroid hit the Earth, triggering a devastating mass extinction. The dinosaurs (except for a few birds) and many mammals became extinct. But some small mammals survived, laying the foundation for all mammals today.
For decades, scientists have believed that mammals and their close relatives survived challenging times, such as mass extinctions, because they were generalists, capable of eating almost anything and adapting to any situation in their lives. A new study of a mammal family tree that has survived multiple mass extinctions shows that the species that survived were not as widespread as scientists thought: Instead, possessing new and different traits may be the key to success in the aftermath of the catastrophe.
"The idea of 'natural selection and survival of the fittest' dates back to the 19th century, and conventional wisdom held that animals that were generalized were least likely to become extinct. But we found that those animals that survived were often only more generalized in hindsight compared to their later descendants." Field Museum MacArthur Paleontology "They were actually quite advanced animals for their time, and their new traits may have helped them survive and provided evolutionary flexibility," said Ken Angielczyk, curator of zoology and senior author of the study published Oct. 5 in the journal Nature Ecology & Evolution.
The skulls of two ancient mammal relatives, or synosaurs, show the approximate body shapes of the ancestors of the two major synosaurs that diverged from each other. Wallasaurus (pictured below), from the Permian period of Earth's history, was about the same size as the ancestor of all great apes. Morganosaurus (pictured above) from the Jurassic period of Earth's history was about the same size as most mammals during the dinosaur age and the ancestors of modern mammals. Both Wallasaurus and Morgangutenosaurus fed on other animals. Photo credit: Photo by KenAngielczyk
"The previous idea was that whenever a new group of mammals evolved, there would be a small generalist animal at the beginning because when disaster struck, these animals were able to move on -- they could hide anywhere and eat anything around them," said Spencer Hellert, an assistant professor at Columbia College Chicago, a research associate at the Field Museum and co-first author of the study. "Mammals that survived mass extinctions wouldn't be bamboo-eating specialists like pandas."
David Grossnickle, an assistant professor at the Oregon Institute of Technology and co-first author of the study, published a study in 2019 highlighting that small insectivorous mammals were often the lineages that survived challenging times, including the extinction event of the dinosaurs, and were the precursors to major species diversification. He approached Hellert and Angelk to see if this trend held true for early mammals and their ancestors.
Hellert created a vast family of taxa, of which mammals were the last surviving members. This family tree is one of the largest fossil trees ever constructed, taking into account all previous family trees that scientists have produced for this taxon. This method is a more formal, rigorous, and repeatable method that can summarize information from a large number of trees, rather than randomly picking a few trees and sticking them together.
"We couldn't test this idea without a large family tree, along with general information about the animals' diet and body size," Angelchik said. "We then looked at what happened to the five major evolutionary radiations of great apes over time. This process repeated itself when new catastrophes led to the extinction of most of these species."
The researchers, including co-authors Graeme Lloyd and Christian Kammerer, found that the story of great ape evolution is not one of "small and specialized survival." In some cases, it was the larger synodactyls that survived, and the winners weren't just insect-eating generalists.
"We were quite surprised -- the radiation of mammals from small insectivores to large groups has been well established, so I was kind of expecting to see that as we went back into the history of great apes. As we go back in time, that pattern starts to disappear."