Chew on This: Powerful Jaws Fueled a Jurassic Herbivore Boom

Remember Ducky from The Land Before Time? The adorable little dinosaur was one of the duckbills—known to paleontologists as hadrosaurs—that roamed far and wide during the Cretaceous chapter of the great dinosaur story. Duckbill bones are so numerous in some places that these herbivorous dinos are sometimes called the “cows of the Cretaceous.” But what allowed these plentiful, shovel-mouthed dinosaurs to become so successful?

For a time, the success of the duckbills thought to be powered by the rise of a new kind of green food: flowering plants. The timing seemed to be right. The earliest definitive flowering plants, or angiosperms, date to about 125 million years ago. Before that, the ancestors and relatives of hadrosaurs—a broader group called ornithopods—were not particularly numerous in terms of species or populations. By about 100 million years ago, however, hadrosaur evolution had exploded, spilling dozens of new species across North America and Eurasia. Perhaps these dinosaurs were taking advantage of a new food source, evolving in tandem with ancient angiosperms that had begun to spread across the northern hemisphere.

“At one point, a number of researchers related the rapid diversification of hadrosaurs and ceratopsians to the initial evolutionary radiation of angiosperms with many ‘weedy’ forms” such as the spindly Archaefructus, says Hans-Dieter Sues, a paleontologist at the Smithsonian’s National Museum of Natural History. This idea eventually fell out of favor, however. In a new study in the journal Scientific Reports, University of Bristol paleontologist Edward Strickson and colleagues further bury the hypothesis, arguing that the evolutionary bursts that led hadrosaurs to become so ubiquitous had nothing to do with flowers. Instead, these big-mouths owe their success to their remarkable jaws and teeth, which allowed them to take advantage of a greater array of plant foods.

When they mapped out the differences in ornithopod jaw anatomy, as well as the dinosaurs’ family tree, Strickson and his coauthors didn’t find a close association between the Mesozoic vegetarians and flowering plants. The first big burst in ornithopod evolution was in the Jurassic, tens of millions of years before angiosperms even came on the scene, later followed by three tightly-grouped bumps in diversity around 90 million years ago. And regarding their chompers, the researchers found that hadrosaurs generally had very similar jaws across species. Whatever they hit on early, it worked for them. In other words, there was no sign that the teeth and jaws of hadrosaurs specifically evolved to crunch up flowering plants.

The key to the Day of the Duckbill, then, seems to be their specialized jaws. Most dinosaurs couldn’t chew. Their jaws were little different than biological sets of scissors or shears, which worked just fine when it came to swallowing things whole. But the ancestors of hadrosaurs evolved densely-packed rows of leaf-shaped teeth, not to mention jaw bones that could flex and swing to create a unique kind of chewing motion. This gave hadrosaurs the ability to grind their food, a major feat that—as fossilized poop and other lines of evidence have shown—allowed them to chew through conifers and other tough vegetation. Maybe instead of “big mouth,” Ducky should have been nicknamed “great mouth.”

But don’t discount flower power just yet. Looking at the rise of the hadrosaurs and the horned ceratopsians, Sues says that “there must have been some floral change or changes during the mid-Cretaceous to account for the sudden appearance of two very diverse groups of large-bodied, herd-forming dinosaurian herbivores.” And in the Southern Hemisphere, where these dinosaurs were largely absent, giant, long-necked herbivores called titanosaurs proliferated to fill that niche around the same time. “I still have a lingering suspicion that all these evolutionary radiations are at least in some way connected to the diversification of angiosperms,” Sues says.

 It’s a hunch paleontologists will likely be chewing over for a long while.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.