Placental mammals — the evolutionary lineage that includes humans — co-existed with non-avian dinosaurs for a short time before the dinosaurs went extinct, according to new research.
Life reconstruction of the extinct bear-dog Amphicyon ingens. Image credit: Roman Uchytel.
About 66 million years ago, a massive asteroid crashed into Earth near the site of the small town of Chicxulub in what is now Mexico.
This impact unleashed an incredible amount of climate-changing gases into the atmosphere, triggering a chain of events that led to the extinction of non-avian dinosaurs and 75% of life on the planet.
Debate has long raged among paleontologists over whether placental mammals were present alongside the dinosaurs before the end-Cretaceous mass extinction, or whether they only evolved after the dinosaurs were done away with.
Fossils of placental mammals are only found in rocks younger than 66 million years old, suggesting that the group evolved after the mass extinction. However, molecular data has long suggested an older age for placental mammals.
In the new study, University of Bristol paleobiologist Emily Carlisle and colleagues used statistical analysis of the fossil record to determine that placental mammals originated before the mass extinction, meaning they co-existed with dinosaurs for a short time.
However, it was only after the asteroid impact that modern lineages of placental mammals began to evolve, suggesting that they were better able to diversify once the dinosaurs were gone.
The researchers collected extensive fossil data from placental mammal groups extending all the way back to the mass extinction 66 million years ago.
“We pulled together thousands of fossils of placental mammals and were able to see the patterns of origination and extinction of the different groups,” Dr. Carlisle said.
“Based on this, we could estimate when placental mammals evolved.”
“The model we used estimates origination ages based on when lineages first appear in the fossil record and the pattern of species diversity through time for the lineage,” said Dr. Daniele Silvestro, a paleobiologist at the University of Fribourg.
“It can also estimate extinction ages based on last appearances when the group is extinct.”
“By examining both origins and extinctions, we can more clearly see the impact of events such as the end-Cretaceous mass extinction or the Paleocene-Eocene Thermal Maximum (PETM),” said University of Bristol’s Professor Phil Donoghue.
“Primates, the group that includes the human lineage, as well as Lagomorpha (rabbits and hares) and Carnivora (dogs and cats) were shown to have evolved just before the end-Cretaceous mass extinction, which means our ancestors were mingling with dinosaurs.”
“After they survived the asteroid impact, placental mammals rapidly diversified, perhaps spurred on by the loss of competition from the dinosaurs.”
The study appears today in the journal Current Biology.
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Emily Carlisle et al. 2023. A timescale for placental mammal diversification based on Bayesian modelling of the fossil record. Current Biology, in press;
