Dr Sereno said it is "very rare" for a plant-eater to have such sharp-edged, enlarged canines. He argued they were most likely used for self-defence and competitive sparring, based on his examination of wear and chipped enamel on the teeth.
Bristles similar to those of Pegomastax were also recently found in China on another tiny fanged plant-eaters, or heterodontosaur, called Tianyulong.
Dr Sereno said when dwarf-sized heterodontosaurs like Pegomastax lived some 200 million years ago, they would have scampered around in search of suitable plants, looking something like a "nimble, two-legged porcupine".
"Pegomastax and kin were the most advanced plant-eaters of their day," he said.