Most dinosaurs were vegetarian, research suggests
Most dinosaurs were vegetarian rather than meat-eating beasts, research suggests.
21 Dec 2010
While Tyrannosaurus Rex sums up the image of a dinosaur wreaking terror by ripping flesh with powerful jaws, many of its closest relatives were more content nibbling leaves.
A new study of the diet of 90 species of theropod dinosaurs challenged the conventional view that nearly all theropods hunted prey, especially those closest to the ancestors of birds.
Rather it showed that among the most bird-like dinosaurs known as coelurosaurs plant eating was a common way of life.
Their diet may have also helped them survive and exploit new environments becoming the most successful group of dinosaurs throughout the Cretaceous Period, 145-65 million years ago.
Dr Lindsay Zanno of the Chicago Field Museum said: "Most theropods are clearly adapted to a predatory lifestyle, but somewhere on the line to birds, predatory dinosaurs went soft."
Theropods are a group of bipedal dinosaurs colloquially known as "predatory" dinosaurs and include the iconic hunters Tyrannosaurus and Velociraptor.
Among theropod dinosaurs, all modern birds and several groups of their closest extinct relatives belong to a subgroup known as Coelurosauria.
Most were feathered and most intelligent dinosaurs and those with the smallest body sizes also belong to this group.
However researchers have been only left with fossilized bones and teeth to work with and so had to deduce their diets.
For example the bone-crunching teeth and jaws of Tyrannosaurus rex were the tools of a megapredator or that the tooth batteries of Triceratops were used for shearing plant material.
However many coelurosaurian dinosaurs have more ambiguous adaptations such as peg-like teeth at the front of the mouth or no teeth at all so determining their diet has been a challenge.
Dr Zanno added: "These oddball dinosaurs have been the subject of much speculation but until now, we have not had a reliable way to choose between competing theories as to what they ate."
But a small number of fossilized dinosaur dung, stomach contents, tooth marks, the presence of stones within the stomach that serve as a gastric mill for digesting vegetation have been found along with a number of species.
And two dinosaur species preserved locked in the throes of combat have been found to cast light on the mystery of what dinosaurs ate.
The researchers found almost two dozen anatomical features statistically linked to direct evidence of herbivory including a toothless beak.
"Once we linked certain adaptations with direct evidence of diet, we looked to see which other theropod species had the same traits. Then we could say who was likely a plant eater and who was not."
Applying their data on diet, the researchers found that 44 theropod species distributed across six major lineages were eating plants and that the ancestor to most feathered dinosaurs and modern birds had probably already lost its appetite for flesh alone.
Because plant eating was found to be so widespread in Coelurosauria, the hypercarnivorous habits of T. rex and other meat eating coelurosaurs like Velociraptor should be viewed "more as the exception than the rule."
Besides identifying diet, the researchers analyzed whether different groups of coelurosaurs followed the same evolutionary pathways toward an herbivorous diet.
They found that over time, species lost their flesh-rending teeth, developing strange tooth types such as peg, wedge, and leaf-shaped teeth, and ultimately, some lost most or all of their teeth altogether and replaced them with a bird-like beak.
The beaks then continued to evolve into a myriad of forms and help support a high degree of dietary diversity in modern birds.
One theory why they were so successful was that the break up of continents and origin of new habitat opened up new dietary niches for coelurosaurs to explore.
Dr Zanno said: "The ability to eat plant materials may have played a pivotal role in allowing coelurosaurian dinosaurs to achieve such remarkable species diversity.
"But more study is needed to understand what role dietary shifts may play in evolutionary processes."
Because ceolurosaurian dinosaurs include the closest extinct relatives of birds, understanding their biology is also extremely important to understanding how, why, and under what conditions birds evolved and first took flight.
"We don't know what drove the ancestors to birds to take flight," she says, "seeking food in the trees is just one of many possibilities."
Using statistical analysis to find correlations between physical traits and diet could offer a new window as to how evolution works.
"Being able to establish diet in extinct animals with confidence will allow us to start tackling even broader questions, such as whether animals tend to increase in body and diversity when they evolve herbivory."
The findings are published in the journal Proceedings of the National Academy of Sciences.