Scientists who identified the newly identified species, Nyasasaurus parringtoni, believe it may have been the first dinosaur ever to have lived.
It would probably have stood upright and lived by Lake Nyasa in Tanzania, feeding on vegetation. Although it was the size of a dog, it had a five foot long tail.
Its fossilised upper arm and six vertebrae date from a few million years after the "Permo-Triassic mass extinction", the greatest of all time, and provide important insight into the origin and early evolution of dinosaurs.
All life on Earth today is descended from the four per cent of species that survived the extinction 250 million years ago, which scientists believe could have been caused by an asteroid impact or volcanic eruptions.
The researchers said: "We can say, with confidence, the lineage leading to dinosaurs evolved by the Anisian, the early part of the Middle Triassic epoch, approximately 243 million years ago."
The creature roamed Africa about 10 million years before more familiar dinosaurs like the small, swift footed Eoraptor and Herrerasaurus that date back to about 230 million years ago, a period known geologically as the Triassic.
The discovery, described in the Royal Society journal Biology Letters, means the dinosaur lineage originated 10 to 15 million years before previous fossils showed, appearing in the Middle Triassic rather than the Late Triassic period.
It is a period of evolution about which there is little evidence and which has generated sharp debate among scientists.
Dr Sterling Nesbitt, a paleontologist at the University of Washington, said: "If the newly named Nyasasaurus parringtoni is not the earliest dinosaur, then it is the closest relative found so far.
"For 150 years, people have been suggesting there should be Middle Triassic dinosaurs, but all the evidence is ambiguous.
"Some scientists used fossilised footprints, but we now know other animals from that time have a very similar foot.
"Other scientists pointed to a single dinosaur like characteristic in a single bone, but that can be misleading because some characteristics evolved in a number of reptile groups and are not a result of a shared ancestry."
Working with the seven pieces of its remains that were collected in the 1930s and are housed at the Natural History Museum in London, the researchers calculated the animal measured seven to 10 feet in length.
It was as tall as three feet at the hip and may have weighed between 45 and 135 pounds, or 20 to 60 kilos.
The species was named after Prof Rex Parrington, the University of Cambridge palaeontologist who unearthed the bones.
Four vertebrae from a second specimen of Nyasasaurus which were also used in the research are kept in the South African Museum in Cape Town.
It is not certain that the dinosaurs originated in Tanzania because when Nyasasaurus lived the world's continents were joined in the land mass called Pangaea.
Tanzania would have been part of southern Pangaea, which included Africa, South America, Antarctica and Australia.
Dr Paul Barrett, a dinosaur researcher at the Natural History Museum, said: "The new findings place the early evolution of dinosaurs and dinosaur like reptiles firmly in the southern continents."
The bones of the new animal reveal a number of characteristics common to early dinosaurs and their close relatives.
For example, the bone tissues in the upper arm bone appear as if they are woven haphazardly and not laid down in an organised way indicating rapid growth, a common feature of dinosaurs.
Sarah Werning, a biologist at the University of California, Berkeley, who conducted the bone analysis, said: "We can tell from the bone tissues Nyasasaurus had a lot of bone cells and blood vessels.
"In living animals, we only see this many bone cells and blood vessels in animals that grow quickly, like some mammals or birds.
"The bone tissue of Nyasasaurus is exactly what we would expect for an animal at this position on the dinosaur family tree.
"It is a very good example of a transitional fossil – the bone tissue shows Nyasasaurus grew about as fast as other primitive dinosaurs, but not as fast as later ones."
Another example is the upper arm bone's distinctively enlarged crest, needed to anchor the upper arm muscles. The feature, known as an elongated deltopectoral crest, is also common to all early dinosaurs.
Dr Nesbitt said: "Nyasasaurus and its age have important implications regardless of whether this taxon is a dinosaur or the closest relatives of dinosaurs.
"It establishes dinosaurs likely evolved earlier than previously expected and refutes the idea dinosaur diversity burst onto the scene in the Late Triassic, a burst of diversification unseen in any other groups at that time."
The late English paleontologist Dr Alan Charig, included as a co-author on the paper, named the specimen but never documented or published in a way that was formally recognised.