Paleontologists have long debated why groups such as non-avian dinosaurs became extinct whereas mammals and other lineages survived the end-Cretaceous mass extinction approximately 66 million years ago. In new research, they reconstructed North American food chains and ecological habitats of land-living and freshwater animals before and after the extinction event.
Small primitive mammals live alongside a Triceratops; a softshell turtle climbs up a log, unaware that its freshwater surroundings will shelter it from the asteroid. Image credit: Henry Sharpe.
“It seems that the stable ecology of the last dinosaurs actually hindered their survival in the wake of the asteroid impact, which abruptly changed the ecological rules of the time,” said Dr. Alfio Alessandro Chiarenza, a researcher at the University of Vigo.
“Conversely, some birds, mammals, crocodilians, and turtles had previously been better adapted to unstable and rapid shifts in their environments, which might have made them better able to survive when things suddenly went bad when the asteroid hit.”
In the study, Dr. Alessandro Chiarenza and colleagues from Finland, Spain, the United Kigdom and the United States analyzed 1,600 fossil records from North America representing more than 470 genera of cartilaginous and bony fish, salamanders, frogs, albanerpetontids, lizards, snakes, champsosaurs, turtles, crocodylians, dinosaurs (including birds), and mammals.
They modeled the food chains and ecological habitats of these animals during the last several million years of the Cretaceous period, and the first few million years of the Paleogene period, after the Chicxulub asteroid hit.
Their results reveal that small Cretaceous mammals were diversifying their diets, adapting to their environments and becoming more important components of ecosystems as the Cretaceous period unfolded.
Meanwhile, the dinosaurs were entrenched in stable niches to which they were supremely well adapted.
“Mammals didn’t just take advantage of the dinosaurs dying,” the authors said.
“They were creating their own advantages through diversifying — by occupying new ecological niches, evolving more varied diets and behaviours and enduring small shifts in climate, by rapidly adapting.”
“These behaviors probably helped them to survive, as they were better able than the dinosaurs to cope with the radical and abrupt destruction caused by the asteroid.”
“Our study provides a compelling picture of the ecological structure, food webs, and niches of the last dinosaur-dominated ecosystems of the Cretaceous period and the first mammal-dominated ecosystems after the asteroid hit,” said Dr. Jorge García-Girón, a researcher at the University of Oulu and the University of León.
“This helps us to understand one of the age-old mysteries of paleontology: why all the non-bird dinosaurs died, but birds and mammals endured.”
“Dinosaurs were going strong, with stable ecosystems, right until the asteroid suddenly killed them off,” said University of Edinburgh’s Professor Steve Brusatte.
“Meanwhile, mammals were diversifying their diets, ecologies and behaviours while dinosaurs were still alive.”
“So it wasn’t simply that mammals took advantage of the dinosaurs dying, but they were making their own advantages, which ecologically preadapted them to survive the extinction and move into niches left vacant by the dead dinosaurs.”
A paper describing the findings was published in the journal Science Advances.
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Jorge García-Girón et al. 2022. Shifts in food webs and niche stability shaped survivorship and extinction at the end-Cretaceous. Science Advances 8 (49); doi: 10.1126/sciadv.add5040
