Image: Group of heterodontosaurs enjoying a vegetarian meal.
Nobumichi Tamura
This artist's illustration shows a group of heterodontosaurs enjoying a vegetarian meal. New analysis of a juvenile heterodontosaur skull reveals the species had canine teeth as well as plant-chomping molars, suggesting they were omnivores.
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updated 10/24/2008 1:12:37 PM ET 2008-10-24T17:12:37

The world's second smallest known dinosaur skull belonged to a toothy heterodontosaur whose appetite probably far exceeded the size of its head, suggests a new study that proposes the dino consumed both plants and meat.

Although scientists suspect the dinosaur was mostly vegetarian, its theorized "occasional omnivory" could mean it represents a transitional phase between carnivorous dinosaurs and herbivores.

A tiny skull, which co-author Laura Porro stumbled upon in the archives of Cape Town's Iziko South African Museum, narrowly missed becoming the world's smallest known dinosaur head. It measures less than 2 inches in length and belonged to a juvenile weighing less than two sticks of butter.

"The skull of a baby dinosaur called Mussasaurus, or mouse lizard, from Argentina is smaller, at only 3 centimeters, (and is) probably the world's smallest complete dinosaur skull," Porro told Discovery News.

Heterodontosaurus lived during the Early Jurassic period around 190 million years ago. Adults grew to about the size of a modern-day turkey, reaching just over 3 feet in length and weighing five to six pounds.

Porro, a University of Chicago paleontologist, and her colleagues studied the juvenile's skull and determined the individual was probably buried alive in a sandstorm, a mode of death that left its remains in "relatively good condition."

The researchers focused on the dinosaur's "bizarre suite of teeth."

"Heterodontosaurus is unusual among dinosaurs — and all reptiles — in that its teeth dramatically change shape along the jaw," Porro said.

The front of the jaw is lined with large, fang-like canine teeth exist. These are normally associated with carnivores. But this species also has heavily worn, molar-like grinding teeth at the back. These are more characteristic of plant eaters.

"Also, unlike other early plant-eating dinosaurs, Heterodontosaurus has large, powerful arms and sharp, re-curved claws on its hands, again similar to those of meat-eating dinosaurs," she said, adding that "perhaps Heterodontosaurus represents a transition stage."

Of particular interest concerning the newly discovered skull is that the juvenile already had a fully developed set of canines. Other researchers thought these teeth might have emerged later, to be used by fighting adults as for today's wart hogs, but the fact that youngsters had them strengthens the theory that they were used for tearing into flesh for consumption.

Image: Skull
Journal of Vertebrate Paleontology/Natural History Museum
This image shows a 2-inch heterodontosaur skull that weighs less than two sticks of butter. It is the second-smallest intact dinosaur ever found
The researchers, whose findings are published in the fall issue of the Journal of Vertebrate Paleontology, suspect the dinosaur occasionally ate insects, reptiles and small mammals.

CT scans and X-rays found the species did not have replacement teeth, like those of most other dinosaurs that replace their teeth throughout their lives. Instead, it had just one adult set, as mammals do today.

While Porro said "true mammals and dinosaurs appear at roughly the same time (during the late Triassic) their ancestors diverged much earlier -- over 320 million years ago." It's therefore an open question as to why Heterodontosaurus possessed a more mammalian tooth strategy.

Paul Sereno, one of the world's leading paleontologists and an expert on Heterodontosaurus, told Discovery News that he believes "these are some of the most interesting and early of dinosaur finds."

"Rare among dinosaurs, these little guys have canines alongside cheek teeth adapted for slicing plants," he said. "The closest modern analog we can think of is the small muntjac deer from China."

This deer is an herbivore, but males grow fang-like canines similar to those possessed by Heterodontosaurus.

Sereno concluded that the study helps to "shed light on the anatomy of this interesting group of tiny dinosaurs, which have been known previously mainly from a single skeleton."

© 2012 Discovery Channel

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