New research using complex mathematical models has proposed that placental mammals co-existed with dinosaurs during the Late Cretaceous. Using sophisticated Bayesian statistical analysis an international team of researchers have estimated that placental mammals originated during the Late Cretaceous. However, it was only after the extinction of the non-avian dinosaurs that modern, recognisable lineages of placentals were able to diversify.

Debate has long raged amongst researchers over whether placental mammals were present alongside the dinosaurs before the mass extinction, or whether they only evolved after the non-avian dinosaur extinction. Fossils of placental mammals are only found in rocks younger than 66 million years old, after the end-Cretaceous extinction event. This suggests that the group evolved after the demise of the non-avian Dinosauria. However, molecular clock data indicates that placental mammals originated earlier.

When Did Placental Mammals Evolve?

Writing in the academic journal “Current Biology”, a team of researchers including palaeobiologists from the University of Bristol, the University of Fribourg (Switzerland) and scientists from Sweden used a complex statistical analysis to confirm placental mammals co-existed the dinosaurs.

Lead author of the study Emily Carlisle (School of Earth Sciences at the University of Bristol) commented:

“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.”

Placental Mammals Co-existed with Dinosaurs

The analysis indicates that primates (the ancestors of humans) probably evolved just before the K-Pg mass extinction event. In addition, the Lagomorpha (rabbits and hares) and the Carnivora were shown to have evolved when non-avian dinosaurs still roamed. The Carnivora is an extremely diverse Order of placental mammals. It includes cats, dogs, hyenas, civets, mongooses, bears, raccoons, pinnipeds (seals) and the mustelids (weasels, otters and their relatives).

To read an article from 2017 that examines evidence for an Early Cretaceous origin of placental mammals: Evidence of Placental Mammals – Dorset Fossils.

Co-author Professor Phil Donoghue (School of Earth Sciences, University of 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 Palaeocene-Eocene Thermal Maximum (PETM).”

Everything Dinosaur acknowledges the assistance of a press release from the University of Bristol in the compilation of this article.

The scientific paper: “A timescale for placental mammal diversification based on Bayesian modelling of the fossil record” by Emily Carlisle, Christine M. Janis, Davide Pisani, Philip C. J. Donoghue and Daniele Silvestro published in Current Biology.