A Mesozoic myth: Dinosaurs didn’t rule the Earth like we think

The various ways that dinosaurs interacted with the environments they called home is a relatively new area of study, one that was reliant on new fossil finds in the latter part of the 20th century, as well as technological advances such as CT scanning that’s allowed experts to analyze tiny fossil details that were previously entirely beyond the reach of the imagination. But there is one aspect of dinosaur ecology that paleontologists have been pondering ever since they recognized that an especially long “Age of Reptiles” preceded the “Age of Mammals.” 

The earliest mammals originated during the Mesozoic, right alongside the evolutionary explosion of reptiles. These ancient beasts seemed to be insectivores and universally small, with big eyes that suggested they were active at night. Paleontologists pointed out that not until after the Cretaceous did mammals get larger than the size of a house cat. Dinosaurs seemed to keep mammals in submission, our ancestors and relatives squeaking out a living under the feet of the “terrible lizards,” until a great extinction cleared the way for our forebears.

While 20th-century experts and science communicators depicted dinosaurs as sluggish, dim-witted, and even freakish, our early mammal relatives were thought of as evolutionary underdogs. Mammalian chauvinism led paleontologists to cast small, beetle-crunching mammals of the Mesozoic era as superior creatures that were being held down by the tyranny of cold-blooded reptiles, an antagonistic relationship that even spun off into a proposed reason for dinosaurian extinction. 

The Triassic, Jurassic, and Cretaceous worlds were rife with dinosaur eggs that were thought to be left unguarded by careless parent reptiles, perfect meals for mammals that were likely too small to be noticed. Perhaps early mammals staged a kind of prehistoric coup, one oft-repeated hypothesis suggested, growing in number as they ate nest after nest of dinosaur eggs until the great reptiles could no longer reproduce fast enough to sustain their numbers. Dinosaurs were treated as both fascinating but repulsive, bizarre creatures that got in the way of mammalian ascent and paid the ultimate price for it.

Even after the death-by-omelet hypothesis faded out of favor, as it could potentially explain only why dinosaurs disappeared and not any of the other forms of life that vanished at the end of the Cretaceous, the image of Mesozoic mammals as meek, nocturnal, and downtrodden hung on. The idea undoubtedly spoke to our own modern-day feelings about dinosaurs, creatures that were fascinating but that we also spoke of as “dominating” and “ruling” the Earth. The narrative for our ancestors was one of subjugation by the reptiles, with an emphasis on large body size as superior seeming to add credence to the notion that mammals couldn’t really thrive until the big dinosaurs were gone. 

A few rare fossils even seemed to speak to this idea, like dinosaur claw marks preserved around fossil mammal burrows found in Utah — evidence of a raptor-like dinosaur scrabbling into the ground to try and nab whatever mammals might be hiding inside.

But just as paleontologists had underestimated dinosaurs for decades, so, too, had they misunderstood Mesozoic mammals. The supposed antagonism between dinosaurs and mammals has been overplayed, and in some ways misunderstood because of a focus on competition for ecological prominence. Mammals of the Triassic, Jurassic, and Cretaceous were not all shrew-like creatures that ate bugs and feared reptilian talons. Mammals can truly be said to have thrived during the Mesozoic, and new finds are continuing to alter the picture.

Paleontologists often debate whether “mammal” should refer to what are often termed crown mammals — defined by the last common ancestor of monotremes, marsupials, and placental mammals — or include all the forms more closely related to mammals than reptiles, which are broadly called mammaliaforms (of which crown mammals are a subset). In a general sense, though, fuzzy creatures that we would likely call a mammal if we saw one alive were scurrying around by the Triassic, 225 million years ago, at the latest. 

The beasts flourished among the forests, floodplains, and deserts of the Mesozoic world, spinning off a variety of different forms. There were ancient mammal equivalents of squirrels, shrews, otters, aardvarks, flying squirrels, and more. And some of them were demonstrably not afraid of dinosaurs. A fossil of the badger-size mammal Repenomamus described in 2005 was found with baby dinosaur bones in its stomach, and another Repenomamus described in 2023 was biting a small horned dinosaur called Psittacosaurus in the ribs when both were suddenly overcome by a lahar debris flow of volcanic material and preserved. A focus on body size had misled paleontologists and caused dinosaur experts to overlook how varied Mesozoic mammals had become.

The emerging picture is that competition between different forms of early mammals restricted the evolution of our ancestors, not the dinosaurs. During most of the Mesozoic, it was relatively archaic forms of mammals — the mammaliaforms — that evolved into an array of different shapes and niches. The early beasts effectively took up a great deal of ecological space and their lineages held onto those spaces even as crown mammals emerged. It was only during the Cretaceous, as the older mammaliaforms began to disappear, that the ancestors of today’s marsupial and placental mammals began to evolve in new ways and establish the basis for their great flourishing in the Cenozoic era, or from the asteroid impact sixty-six million years ago to the present day. The story played out in parallel with the comings and goings of different dinosaur groups, having more to do with interactions between mammal lineages than supposed conflict with the dinosaurs. 

And even though all Mesozoic mammals found so far haven’t been any bigger than an adult house cat, the fossil record may still hold some surprises. In 2019, paleontologists named an enormous protomammal from 205-million-year-old rocks in Poland, therefore living alongside the dinosaurs. It belonged to a tusked, piglike group called dicynodonts, but it grew to be more than 5 tons (4,500 kg) in weight and lived alongside early dinosaurs and other reptiles. The fact that such a large protomammal not only survived so long into the “Age of Reptiles,” but had gone unrecognized until well into the 21st century, hints there may be other pockets in Mesozoic time when mammals and their relatives grew larger than paleontologists have expected. After all, the vast majority of mammal species alive today are small — the size of rodents such as mice and squirrels. The same was true in the Mesozoic. The diverse forms mammals evolved into, and the various niches they opened, are signs of success and not suppression by reptiles.

The legend, therefore, isn’t true. Dinosaurs did not rule anything. The reptiles were part of an ever-changing world, influenced by both other living things and abiotic factors such as climate and rainfall patterns, while dinosaurs themselves changed the ecosystems around them. What dinosaurs ate and where they walked changed the world, just as the reptiles were shaped by the vegetation around them and the prehistoric climates. Their paleoecology is so often what we imagine when we try to picture how dinosaurs fit into and moved through their ancient environment, a shifting mural of prehistoric life that experts are only just beginning to fully perceive.