The asteroid that led to the extinction of the dinosaurs is thought to have paved the way for mammalian supremacy. However, a new study indicates that many species of mammals died off alongside the dinosaurs.

Metatherian mammals, the extinct relatives of modern marsupials, flourished in the shadow of dinosaurs during the late Cretaceous period. The metatherian mammals were large in number and had a wide range of diversity.

When the 10-km-wide asteroid struck what is now Mexico at the end of the Cretaceous period, the planet suffered incredible environmental damage. As the effects were felt across the globe, two-thirds of all metatherians living in North America perished. That includes more than 90 percent of the species living in the northern Great Plains of the modern day United States.

After the devastating asteroid strike, the metatherians never recovered their previous diversity. This could explain the scarcity of modern marsupials and why they are only found in unusual environments such as Australia.

With their populations decimated, metatherian mammals were rapidly replaced by the growing number of placental mammals. Placental mammals are any species that give birth to living, well-developed young, ranging from squirrels to humans. These mammals are ubiquitous in the world today.

“This is a new twist on a classic story,” said Dr. Thomas Williamson, lead author of the study. “It wasn’t only that dinosaurs died out, providing an opportunity for mammals to reign, but many types of mammals, such as most metatherians, died out too – this allowed advanced placental mammals to rise to dominance.”

Dr. Steve Brusatte, another author on the study, added “If a few lucky species didn’t make it through, then mammals may have gone the way of the dinosaurs and we wouldn’t be here.”

The original study was published in the journal ZooKeys.