The Evolution of Herbivory in Ornithopod Dinosaurs

Under the leadership of Attila Ősi, an international research team concluded that the transformation of the teeth and body size of these herbivorous ancient animals might have related to the appearance of flowering plants. The research, conducted using the most modern techniques, was reported on the Nature Communications website.

One of the most important terrestrial herbivorous vertebrates of the Mesozoic era (the middle period of Earth's history) were the Ornithopoda dinosaurs. These were bipedal, agile forms (such as the Iguanodon from England or the Mochlodon from the Bakony Mountains in Hungary), which were still 1-2 meters long and weighed 50-100 kg during the Jurassic period, but by the Cretaceous period, they had reached a body length of 10-12 meters. Over their more than 100 million-year-long history, their teeth and body size changed, but we know almost nothing about what caused these changes.

Led by Attila Ősi, a professor at the Department of Paleontology at Eötvös Loránd University (ELTE), researchers from several domestic institutions (Széchenyi István University, University of Szeged, Hungarian Natural History Museum) and foreign institutions (Natural History Museum, London, University of Bucharest) sought to answer what specific evolutionary changes occurred in the teeth of these animals, how global this phenomenon was, and what could have driven the recognized anatomical changes.

The researchers studied dinosaur fossils from several European and North American collections using the latest techniques: CT scans were made of some jaws, volume calculations were performed on 3D digital models of the teeth, the wear surfaces of worn teeth were analyzed in 2D and 3D, and the growth rate of teeth was examined through histological sections.

Wear surfaces examined on a herbivorous dinosaur tooth

The results showed that this successful group of herbivorous dinosaurs began

to drastically increase the volume of teeth in their jaws

towards the end of the Jurassic period, thus providing larger and larger surfaces for chewing. Chewing – the collision of upper and lower teeth and the resulting breakdown of food – wears down the teeth. In early forms, 60-70 percent of teeth were worn out within six months, while the animal was already growing the next tooth (constantly replacing teeth).

By the middle of the Cretaceous period (about 100 million years ago), even this was not enough: the teeth either became extremely large (in some species, the crown height reached 5-7 cm) or not just one, but even 4-5 teeth were waiting in line under the tooth in use to develop, emerge, and wear down within a month or two.

"The level of tooth wear increased to such an extent in the late Cretaceous hadrosaurs (duck-billed dinosaurs) that, for example, the North American Edmontosaurus, which reached a body length of 9 meters, wore down nearly a thumb-sized amount of tooth in a single day (!). In these forms, not just one but two or even three teeth were in use at the same time in a single socket. There were at least 42 sockets in a single jaw, and there were four jaws," says Attila Ősi, leader of the OTKA research program supported by NKFIH.

Qualitative and quantitative changes in tooth wear in herbivorous dinosaurs

During the tooth wear studies, it was revealed that while in early forms, the tooth surfaces were uneven, presumably due to the consumption of fleshier leaves, buds, seeds, and fruits of trees, in later forms, they became increasingly smoother, covered mainly by long, even grooves, suggesting the consumption of more uniform, probably low-growing, fibrous plants that were in large quantities but nutrient-poor.

Thus, involving numerous dinosaur fossils, the research now provides numerical evidence that in the history of the most prominent Mesozoic herbivores,

there was a significant change in their feeding mode,

which can be dated to the middle of the Cretaceous period. This is when the largest-bodied herbivores appeared, when the forms with the largest teeth developed, and when the first herbivorous dinosaurs emerged, which sped up tooth replacement with more than one replacement tooth.

The big question is whether this change observed in herbivorous dinosaurs can be associated with the transformation of terrestrial vegetation. The latter is nothing less than the appearance and dominance of angiosperms, or flowering plants, by the end of the first half of the Cretaceous period. "This event roughly coincides in time with the changes documented in the teeth of herbivorous dinosaurs, but for now, there is still not enough data to clearly demonstrate a direct connection between the two," notes Paul Barrett, co-author of the article and professor at the Natural History Museum in London.

The cover photo shows the lower jaw of a herbivorous dinosaur with teeth.