Its bone structure and light form suggest it moved swiftly
Scientists have described a new primitive dinosaur species, Eocursor parvus, which lived in the Late Triassic - about 210 million years ago.
Unearthed in South Africa's Free State, the creature appears to have been a small, agile plant-eater.
The team tells a Royal Society journal that Eocursor sheds light on the early evolution of the Ornithischia.
This important group included the well known herbivororous dinosaurs Triceratops and Stegosaurus.
The fossil specimen was first identified in 1993 but only recently appraised.
It is by far the most complete example of a Triassic ornithischian known, comprising skull and skeletal material, including bones of the backbone, arms, pelvis and legs.
In its day, Eocursor would have been little bigger than a fox. Its bone structure and light form suggest it moved swiftly.
The scientists say the creature provides the earliest evidence for the origins of many skeletal characteristics seen in the ornithischian group, including the backward-pointing pelvis.
The specimen provides new information on an uncertain time
A comparison has been done across a wide range of specimens and this indicates that Late Triassic ornithischians were really quite rare. The group then diversified in the subsequent early Jurassic, filling empty herbivorous niches following mass extinctions of other creatures.
"We know ornithischians were a very successful and important group of plant-eating dinosaurs that first appeared 220 million years ago, in the late part of the Triassic Period," explained Dr Richard Butler, a palaeontologist at the Natural History Museum, London, UK.
"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. The lower leg bones are very long, suggesting it would have been able to run fast on its hindlegs to escape from predators."
The name Eocursor comes from the Greek eos, meaning "dawn" or "early", the Latin cursor meaning "runner" and parvus meaning "little"
"The earliest dinosaurs we know are about 228 million years old, so this one is only just a bit younger than that," commented Dr Paul Barrett, a NHM researcher unconnected with the new study.
"The fossil record for early meat-eating dinosaurs is slightly better; and for some of the other plant-eaters, we also have not-too-bad a record. But for the ornithischians, we have almost nothing; so in that sense, this is a major find," he told BBC News.
The assessment is reported in Proceedings of the Royal Society B: Biological Sciences.
The research team included co-workers at the Iziko South African Museum in Cape Town and the University of Cambridge, UK.