A small, sleek cousin of the lumbering armoured dinosaurs Stegosaurus and Triceratops has been discovered, shedding new light on the origins of an ancient group of plant-eaters.

While Stegosaurusis celebrated for the defensive plates that ran the length of its spine and Triceratops for its bony protective ruff and horns, the new species was a slim, light and fleet-footed creature, about the size of a modern fox.

Despite these differences, Eocursor parvus, which means “little dawn-runner”, belongs to the same group as its heavier and slower relatives, the ornithischians. These were among the most successful herbivorous dinosaurs, which thrived in huge herds in the Cretaceous period between 145 million and 65 million years ago.

Eocursoris among the earlier members of the group, and the most complete ornithischian skeleton yet found from the Triassic period, between 250 million and 200 million years ago.

“We know that ornithischians were a very successful and important group of plant-eating dinosaurs that first appeared 220 million years ago,” said Richard Butler, a palaeontologist at the Natural History Museum in London, who led the research team.

“However, the few ornithischian fossils from the Triassic are incomplete and controversial, so we know virtually nothing about the group’s early evolution. Eocursor is enormously important because it helps to fill this gap in the fossil record.”

The miniature dinosaur fossil was discovered in 1993 in Orange Free State (now Free State), South Africa, but the specimen has only recently been studied exhaustively by scientists. It has been identified as a new species from details of its skull, backbone, arms, pelvis and legs.

“Eocursor is a very small and primitive dinosaur that would have eaten plants with its leaf-shaped teeth and had an unusually large, grasping hand,” Dr Butler said. “The lower leg bones are very long, suggesting it would have been able to run fast on its hind legs to escape from predators.”

Details of the discovery are published in the journal Proceedings of the Royal Society. Dr Butler and colleagues from the Iziko South African Museum in Cape Town and the University of Cambridge compared the specimen with those of other ornithischian dinosaurs. “It seems that primitive ornithischians were scarce, and the group succeeded by taking advantage of the extinctions of other plant-eating reptiles . . . about 200 million years ago,” Dr Butler said.

Times Recommends

• Time-worn Freddie gives up Test matches
• Brighton company 'involved in cyber attack'
• The art of survival: it’s all in the mind