The end-Cretaceous mass extinction 66 million years ago was characterized by a worldwide ecological catastrophe and rapid species turnover. Arboreal (tree-dwelling) species were especially at risk of extinction due to large-scale devastation of forested environments caused by wildfires from the Chicxulub asteroid impact. New research led by Cornell University and University of Cambridge scientists reveals that most of the surviving mammals did not rely on trees, though the few arboreal mammals — including ancestors of primates and marsupials — that lived on may have been versatile enough to adapt to the loss of trees.
Life reconstruction of an early primate species called Purgatorius mckeeveri (foreground) and a species of archaic ungulate mammal (forest floor). Image credit: Andrey Atuchin.
“One possible explanation for how primates survived the end-Cretaceous mass extinction, in spite of being arboreal, might be due to some behavioral flexibility, which may have been a critical factor that let them survive,” said Jonathan Hughes, a doctoral student in the Department of Ecology & Evolutionary Biology at Cornell University.
The earliest mammals appeared roughly 300 million years ago and may have diversified in tandem with an expansion of flowering plants about 20 million years prior to the end-Cretaceous mass extinction event.
“When the Chicxulub asteroid struck, many of these mammal lineages died off,” Hughes said.
“At the same time, the mammals that did survive diversified into all the new ecological niches that opened up when dinosaurs and other species became extinct.”
In the study, Hughes and colleagues used published phylogenies (branching, tree-like diagrams that show evolutionary relatedness among groups of organisms) for mammals.
The researchers then classified each living mammal on those phylogenies into three categories — arboreal, semi-arboreal and non-arboreal — based on their preferred habitats.
They also designed computer models that reconstructed the evolutionary history of mammals.
They compared information known from living mammals against available fossils to help provide additional context for their results.
Generally, the models showed that surviving species were predominantly non-arboreal through the end-Cretaceous mass extinction event, with two possible exceptions: ancestors of primates and marsupials.
Primate ancestors and their closest relatives were found to be arboreal right before the mass extinction event in every model.
Marsupial ancestors were found to be arboreal in half of the model reconstructions.
They authors also examined how mammals as a group may have been changing over time.
“We were able to see that leading up to the end-Cretaceous mass extinction event, around that time frame, there was a big spike in transitions from arboreal and semi-arboreal to non-arboreal, so it’s not just that we are seeing mostly non-arboreal species, but things were rapidly transitioning away from arboreality,” Hughes said.
The results was published in the journal Ecology and Evolution.
Jonathan J. Hughes et al. Ecological selectivity and the evolution of mammalian substrate preference across the K-Pg boundary. Ecology and Evolution, published online October 11, 2021; doi: 10.1002/ece3.8114