Well, this is a surprise! The fossil record has revealed that placental mammals, the group that includes humans, dogs, and bats, originated in the Cretaceous period and did indeed co-exist with dinosaurs for a short time before the dinosaurs became extinct.

The asteroid that hit the Earth triggered the Cretaceous-Paleogene (K-Pg) mass extinction, which resulted in the death of all non-avian dinosaurs. Researchers have debated whether placental mammals were present alongside the dinosaurs before the mass extinction or only evolved after the dinosaurs’ extinction. Fossils of placental mammals are found only in rocks younger than 66 million years old, which is when the asteroid hit Earth, suggesting that the group evolved after the mass extinction. However, molecular data has long suggested an older age for placental mammals.

In a new research article published in the journal Current Biology, a team of palaeobiologists from the University of Bristol and the University of Fribourg used statistical analysis of the fossil record to determine that placental mammals first appeared before the mass extinction, meaning they co-existed with dinosaurs for a short period of 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.

To reach this conclusion, the researchers gathered extensive fossil data from placental mammal groups dating back to the mass extinction 66 million years ago.

Lead author Emily Carlisle of Bristol’s School of Earth Sciences said:

“We pulled together thousands of fossils of placental mammals and were able to see the patterns of origination and extinction of the different groups. Based on this, we could estimate when placental mammals evolved.”

Co-author Daniele Silvestro (University of Fribourg) explained:

“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. It can also estimate extinction ages based on last appearances when the group is extinct.”

Co-author Professor Phil Donoghue, also from Bristol, added:

“By examining both origins and extinctions, we can more clearly see the impact of events such as the K-Pg mass extinction or the Paleocene-Eocene Thermal Maximum (PETM).”

Primates, Lagomorpha (rabbits and hares), and Carnivora (dogs and cats) were found to have evolved just before the K-Pg mass extinction, meaning their ancestors coexisted with dinosaurs. Following the asteroid impact, placental mammals diversified quickly, possibly due to the reduced competition from dinosaurs.