Perhaps they were more efficient than dinosaurs? They knew their targets?
Dinosaurs so thoroughly dominated the ancient world that they suppressed the evolutionary possibilities open for mammals. This notion has been around for decades now, and it seems to be backed up by the fact that no known mammal became larger than badger-sized during the time dinosaurs were the most prominent animals on land. But, thanks to decades of new fossil discoveries, paleontologists have realized an important twist in the story—it wasn’t dinosaurs that suppressed the evolution of our ancient mammalian ancestors, but other forms of ancient mammal.
Paleontologists who study fossil mammals have suspected the story of “dinosaur dominance” was too simple, but, until now, the critical evidence was missing. Clues that the picture was more complicated now come from a new Current Biology paper by University of Oxford paleontologist Elsa Panciroli and colleagues that followed the evolutionary spread of various mammals during the Age of Dinosaurs—from more ancient types called mammaliaforms to the group of mammals found all around us today, the therians. Because the ancient mammaliaforms diversified and spread into so many forms first, the researchers found, our own mammalian ancestors and relatives didn’t have a chance to truly dig their claws into the world’s ecosystems until after the asteroid strike that sparked a mass extinction 66 million years ago.Riley Black, “Other Mammals, Not Dinosaurs, Kept Our Ancestors Down” at Smithsonian Magazine
This would make a great documentary. Unfortunately, the paper at Current Biology is closed access.