We all know the story by now. What was overwhelmingly bad for the dinosaurs was ultimately good for mammals, the mass extinction of 66 million years ago clearing the deck for the Age of Beasts to finally come to fruition. Mammals no longer had to suffer under the feet of the “terrible lizards” and could finally make the world their own.

Of course the tale’s not quite that simple. In recent years, the idea that Mesozoic mammals were all shrew-like insectivores has been toppled by an array of squirrel, beaver, and even aardvark-like mammals that proliferated through the Age of Dinosaurs. Not to mention that the origins of many familiar mammal groups – like primates – seem to pre-date the terrible K/Pg disaster. The event was certainly important, but how?

To answer that question, paleontologists Gemma Louise Benevento, Roger Benson, and Matt Friedman looked at differences in mammal jaw shape through time. What they were assessing was a sense of disparity – the number of different shapes mammal jaws took through time. And given that these shapes are related to diet, the details of the jaw can track how mammals created new niches for themselves through evolutionary time.

After analyzing the jaws of 256 ancient beasts from the days of the earliest mammals in the Triassic through the rise of dominant beasts in the Eocene, long after the catastrophe of 66 million years ago, Benevento and colleagues found that the evolutionary breathing room really did make a difference to our greater family. Despite evolving various body types and ecological niches, mammal jaws through the Triassic, Jurassic, and Cretaceous were relatively similar when it came to how they were using their jaws.

The big change occurred after the end-Cretaceous mass extinction. Mammals of the following time period, the Paleocene, show a slightly wider range of jaw shapes and expressed a greater degree of different jaw functions than their Mesozoic predecessors. The trend kept going through the Eocene, which started about 56 million years ago, as mammal evolution flourished. Not only were there early members of familiar groups by this time – like early bats, primates, and elephants – but also now-extinct groups like the carnivorous hyaenodonts, rhino-like brontotheres, and more.

What emerges, then, is a mammalian mosaic. Previous studies documented how mammal body size changed dramatically right after the end-Cretaceous extinction. Mammals evolved into a wider range of body sizes than ever before. But jaw shape and function is a different story.

Viewed as a whole, Benevento and coauthors write, mammal jaw shape didn’t really pick up much disparity until about 10 million years after the end-Cretaceous crisis. But when the researchers looked at therian mammals alone – that is, the group that contains placental and marsupial mammals – they found that this subgroup quickly responded to the ecological shakeup by evolving more jaw functions. The extinction effectively allowed them to take over ecological space previously occupied by more archaic forms of mammals, or, as the researchers write, “We tentatively argue therefore that therians were the primary beneficiaries of the K/Pg extinction event.”

There's still more to uncover. Much of what we know about the beginning of the Age of Mammals comes from western North America. Fossils from elsewhere in the world will be critical to examining and testing ideas about what happened. For now, though, the story goes beyond the dinosaurs. It wasn’t just that dinosaurs were keeping mammals suppressed. At a smaller scale, archaic mammals were affecting the evolution of the therians. It took the extinction of these older mammal forms, as well as the dinosaurs, to give the therians a change to flourish. If that asteroid impact never happened, it wouldn’t have just translated into extended dinosaurian dominion. The shape of mammalian evolution would have been dramatically different.