Herbivorous dinosaurs developed replacement teeth to eat tough food, research shows

At the end of the Cretaceous period, duck-billed hadrosaurs were the most advanced herbivores on Earth. New research has shown just how voracious these dinosaurs were: their teeth wore down in less than two months on average, as they consumed enormous amounts of plants. Some of the most successful herbivores on Earth may have had hundreds of thousands of teeth over their lifetime.

The ornithopods are a group of dinosaurs that includes Iguanodon, Hypsilophodon, and their relatives, including the rare rhabdodontids. Ornithopods first appeared in the Middle Jurassic, but were most abundant in the Cretaceous, when they became the dominant herbivores in much of the world.

This evolution took them from small generalists to large and specialized “herbivorous machines” that can rival today’s cows and sheep. The research, led by Dr. Attila Ősi of Eötvös Loránd University in Hungary, shows that the dinosaurs achieved this after developing a large number of replacement teeth that allowed them to eat even the toughest plants in large quantities.

“The teeth and jaws of ornithopods have changed dramatically over the course of their evolution,” says Attila. “Earlier members of the group, such as Iguanodon, needed more than 200 days to develop their teeth and at least as long to wear them down through chewing. But by the end of the Cretaceous, hadrosaurs were wearing down their teeth in just 50 days.”

“We believe this is because the later ornithopods must have fed on tough plants that quickly wore down their teeth. Because they wore down at an enormous rate, these dinosaurs had to build up tooth banks in their skulls to avoid starvation.”

The results of the study were published in the journal Nature communication.

The best herbivores among the dinosaurs are

While herbivory is one of the most common lifestyles among animals, eating plants is surprisingly difficult. Unlike meat, which is easily broken down in the intestines, plants are generally made up of tough fibers and complex carbohydrates that are difficult to digest.

Teeth are at the forefront of this nutritional battle, breaking down plants and cutting them into smaller pieces so that gut bacteria can break them down more efficiently. But as co-author Professor Paul Barrett explains, this takes its toll on teeth.

“Over the course of a herbivore’s life, their teeth gradually wear down,” says Paul. “This puts an upper limit on the lifespan of some mammals, such as elephants or cows. Once the teeth are gone, the animal can no longer eat and dies.”

“This is not a problem for reptiles. They are able to constantly form new teeth, and as soon as the previous one is worn down, a replacement tooth comes to the surface from below. Therefore, dinosaur teeth are widespread fossils and therefore a valuable way to study the evolution of these animals.”

The team was particularly interested in studying the teeth and jaws of ornithopods, which eventually became the most advanced herbivores to ever live on our planet. By studying well-preserved skulls, they were able to understand how dinosaur skulls evolved into increasingly complex shapes that were better suited to eating plants.

“We can see that their adaptations to herbivory become more complex as they evolved,” explains Paul. “At the beginning they had single rows of relatively simple teeth with little wear, probably because these dinosaurs focused on fruits and softer plants.”

“When hadrosaurs evolved, they already had far more teeth, which developed a large blade-like edge on one side and a series of grooves behind it. This structure is unique to these dinosaurs and ensured that the upper and lower teeth remained sharp when rubbing against each other.”

Later ornithopods also moved their jaws in new ways: they could move them back and forth and sideways, allowing them to grind plants even more deeply. Their bodies also became much larger, allowing them to accommodate larger guts that could release the nutrients in the plants more effectively.

Different dinosaurs took different approaches to herbivory. But the team found that some groups of ornithopods, such as the tenontosaurids and their more advanced iguanodont relatives, all followed a strikingly similar evolutionary path. They believe this is an example of convergent evolution.

“About 110 million years ago, these ornithopods rapidly evolved a number of similar traits,” explains Paul. “Their teeth increased in number, their jaws became more tightly interlocked, and they developed more replacement teeth, making them more effective herbivores.”

“We also see this in horned dinosaurs, which include species like the Triceratops. It is reasonable to assume that these changes occurred for similar reasons.”

Could flowers be responsible for this?

Although there is compelling evidence that the environment changed in the Early Cretaceous, it is difficult to determine exactly what happened. To uncover possible causes, the team examined worn areas of dinosaur teeth, called wear facets, for signs of microscopic changes.

“Before the Early Cretaceous, ornithopods’ teeth had many large pits,” says Attila. “This suggests that they ate large quantities of plant seeds and possibly also ingested a lot of dust and soil, as they foraged close to the ground.”

“Later forms have fewer pits but many more scratches. This suggests that they now ate harder plants or had a different diet.”

One possible explanation could be that dinosaurs did not actively change their diet, but that certain plants became more common. It is possible that the increase in flowers is responsible for this, but this does not quite fit with the available evidence.

“Although it’s suspicious that flowering plants started to diversify around this time, they were still quite uncommon back then,” says Paul. “In fact, horsetails, ferns and conifers were much more common until the Late Cretaceous for dinosaurs looking for something to eat.”

“Because it is very difficult to separate the fossil records of plants and dinosaurs, it is unlikely that we will ever have sufficiently detailed evidence to prove a connection, even though it is a very interesting idea.”

Once the work on ornithopods is complete, the team hopes to gradually expand their research to other herbivorous dinosaurs, such as ankylosaurs and horned dinosaurs, to better understand why these reptiles were so successful and how evolution shaped the diets of the different groups.

“We would like to take samples from other dinosaurs to see if the trend towards larger body size, greater tooth number and altered tooth wear that we found in ornithopods is more widespread,” says Attila. “If we can find out what changes herbivores went through at that time, we can understand the role of these dinosaurs in Mesozoic ecosystems much better.”

This story has been republished with permission from the Natural History Museum. Read the original story here.