Birds are living dinosaurs, the only evolutionary branch of the great reptiles to have survived the asteroid-driven mass extinction of 66 million years ago. But that doesn’t mean all avians survived unscathed through the fires and impact winter of the ancient catastrophe. For over 86 million years prior to that moment, birds with teeth thrived among the forests, floodplains and oceans of the Mesozoic world. Their story is seldom told, overshadowed both by the success of the beaked birds and the charisma of the non-avian dinosaurs. Only in the last two decades have paleontologists truly begun to understand how varied, widespread and successful toothed birds were, thriving for millions of years until the aftermath of the infamous impact.
“Most Mesozoic birds are outside the branch of the bird evolutionary tree that includes modern birds, so they have a suite of characters that are different from modern birds,” says Fort Hays State University paleontologist Laura Wilson. She notes the fossil species had feathers, hollow bones and other features in common with the pigeons and chickadees we see around us today. But understanding the success of toothed birds is not merely a matter of finding their living equivalents—it’s about understanding them on their own terms. Fossils of complete skeletons, bird parts preserved in amber and new techniques to compare the extinct birds with their living counterparts are allowing researchers to flesh out how these feathered, flying, toothy dinosaurs spread throughout the ancient world.
Paleontologists have known about toothed birds since the 1870s. The announcement of the toothed marine bird Hesperornis and the gull-like avian Ichthyornis, along with a beautifully preserved specimen of the earliest known bird Archaeopteryx, confirmed that prehistoric birds had teeth. Each of these fossils was critical to establishing that birds had evolved from some lineage of reptile, later recognized to be raptor-like dinosaurs.
Early birds such as Archaeopteryx don’t look all that different from the small, carnivorous dinosaurs they evolved from, with long, bony tails; claws; and, of course, teeth. Other features we associate with living birds—egg-laying, feathers, complex systems of air sacs—all evolved among non-avian dinosaurs first, too. And while Mesozoic bird fossils are rarely uncovered, owing to their small size and fragile bones, experts have discovered enough complete skeletons from deposits formed in the ash or fine-grained sediment at the bottoms of ancient lakes to get a feeling for how much the birds varied.
Most Mesozoic birds with teeth, for example, belonged to a group called the enantiornithes. Among their ranks were long-jawed birds with tiny teeth such as Longirostravis, crustacean-crunchers such as Eoalulavis and even potential sap-eaters like Enantiophoenix.
If you were to see an enantiornithine in life, you’d immediately recognize it as a bird. That doesn’t mean it was just the same as birds flying through modern skies. One critical difference, University of Hull biologist Jen Bright points out, is that the enantiornithines lacked cranial kinesis, or the ability of the beak to bend compared to the rest of the skull. If you’ve ever seen a bird yawn or a parrot flex its beak, you may have noticed the beak’s movement. The ability for the beak to move in respect to the rest of the skull likely helps modern birds open their beaks wider and close their jaws faster, and it might help provide more power to bird bites, but the enantiornithines lacked such advantages. And, of course, most enantiornithes had small, peg-like teeth concealed by lips.
The differences from modern birds created challenges for experts trying to understand what the Mesozoic birds fed on. “Morphology, plus geological context, is really all we have,” Bright says. Looking at the anatomy of a particular enantiornithine subgroup, for example, Bright and colleagues found that toothed birds such as Longipteryx and Rapaxavis shared some anatomical traits in common with modern birds that feed on small invertebrates and whatever other small morsels they can find. Many toothed birds fluttered among ancient forests, plucking insects out of the air and eating fruit from the trees of their time.
The enantiornithines were not the only toothed birds of the Mesozoic, however. Various other groups kept their teeth as early bird evolution took off. Named in 1872, Hesperornis was one of the earliest toothed birds known. The avian’s tiny teeth made it a darling among experts, a fossil with transitional features between reptiles and birds that was presented in lavish illustrations in a government-funded monograph. But some members of the United States Congress were unimpressed by the bird’s scientific importance, citing the thick volume as an example of irresponsible government spending. “Birds with teeth” briefly became an anti-spending rallying cry, even as researchers dug into the fascinating details of aquatic birds that paddled around seas where plesiosaurs and mosasaurs swam below.
“Hesperornis lived in this warm, shallow seaway that covered the middle of North America and has no modern analogue,” Wilson says. Even though it looked like a loon or a grebe, Hesperornis did not fly and preferred warmer waters than modern birds that swim with their feet, living among seas that were filled with large predatory marine reptiles. Somehow the bird carved out a niche for itself, chasing fish and squishy cephalopods along the ancient coasts of North America and parts of Asia.
Then an asteroid changed everything. Despite their Mesozoic successes, no toothed bird survived the aftermath of the impact at the end of the Cretaceous. The likes of Hesperornis, the enantiornithines and other grinning birds disappeared forever, beaked birds being the only surviving dinosaurs. Why toothed birds should go extinct while toothless birds survived doesn’t seem to make sense given their overall similarities, but, in those harsh years following the impact, what a bird ate likely made the difference between survival and extinction.
In 2016, paleontologists suggested that beaked birds survived the mass extinction because they already possessed beaks, gizzards and other adaptations to survive the post-impact years on seeds, nuts and other resilient foods that could have been preserved in the soil. Toothed birds, however, often chased live prey like insects and small reptiles, organisms that were decimated by the effects of the extinction and became extremely rare. Toothed birds and their bird-like raptor relatives may have perished because they had no more food as ecosystems collapsed.
“I absolutely think that eating an animal-based diet had something to do with the extinction of toothed birds at the [time of the impact],” Wilson says. If birds were even able to survive the initial heat pulse caused by falling impact debris, which many organisms did not, they would have met a world suddenly changed and almost devoid of the foods they previously relied upon. Even out at sea, where some Hesperornis might have been able to shelter from the heat, the bird had a high metabolism that required a great deal of food in ecosystems that were toppling as soot dust and light-reflecting compounds caused photosynthetic algae to die off, pulling the rug out from under ocean ecosystems.
When the last of the toothed birds were gone, they were gone forever—a lost part of the dinosaur story that changes a little more with each hollow bone recovered from the fossil record.