Life looked different after an asteroid crashed into the planet around 66 million years ago. The dinosaurs died out, the arboreal mammals declined, and the terrestrial mammals thrived. The traditional story that’s told is that the asteroid decimated the dinosaurs and that the decimation of the dinosaurs allowed mammals to trade the treetops for the ground.
But what if that traditional story is wrong? What if the mammalian transformation from arboreal to terrestrial was already underway at the time of the asteroid?
According to a new paper in Palaeontology, things might have happened that way, as the move from arboreal to terrestrial was already being made by many mammals before the asteroid arrived.
Read More: Did Prehistoric Mammals Live With Dinosaurs, and What Were They Like?
Mammals Adapted to the Ground
When the asteroid arrived at the end of the Cretaceous period, the dinosaurs disappeared. But they weren’t the only creatures that were affected.
“Tree dwelling mammals struggled after the asteroid impact,” said Christine Janis, a study author and a professor of paleontology at the School of Earth Sciences at the University of Bristol, according to a press release.
Though it’s clear that arboreal mammals declined while terrestrial mammals thrived in the aftermath of the impact, it isn’t clear whether these mammals were also declining and thriving before the asteroid, too. Setting out in search of answers, Janis and her team turned to fossil fragments from the Late Cretaceous, before the disappearance of the dinosaurs.
Studying the structure of these fragments, Janis and her team found that some species of mammals were already starting to switch from arboreal habitats to terrestrial ones, swapping out one home for another, millions of years in advance of the asteroid.
“It was already known that plant life changed toward the end of the Cretaceous, with flowering plants, known as angiosperms, creating more diverse habitats on the ground,” Janis said in the release. “What had not been documented was whether mammals were becoming more terrestrial, in line with the habitat changes.”
Read More: Did Humans and Dinosaurs Ever Live Together?
Finding Traces of Mammal Movement
Looking specifically to fragments of limb bones, Janis and her team found that many therian mammals, or mammals from the marsupial and placental branches of the mammalian family tree, were starting to transition out of a lifestyle in the trees to a lifestyle on the ground in the final years of the Cretaceous, when dinosaurs were still alive and well.
While tracking these changes in the lifestyles of the therians would typically involve the investigation of full skeletons, Janis and her team were able to stick to the analysis of fossil fragments. That’s because the traces of the therians’ motion are contained in the articular surfaces, or the tips, of these fossils, where they indicate whether a specific species was more suited to arboreal or terrestrial movement.
“We’ve known for a long time that mammalian long bone articular surfaces can carry good information about their mode of locomotion, but I think this is the first study to use such small bone elements to study change within a community, rather than just individual species,” Janis said in the release.
Ultimately, these findings indicate that the fate of the dinosaurs wasn’t the only factor that influenced the lives and lifestyles of mammals around the end of the Cretaceous. “The vegetational habitat was more important for the course of Cretaceous mammalian evolution than any influence from dinosaurs,” Janis said in the press release.
Read More: Dinosaurs May Be the Reason Why Humans Have Low Lifespans
Article Sources
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Palaeontology. Down To Earth: Therian Mammals Became More Terrestrial Towards the End of the Cretaceous
Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.
