Fossil experts say the creatures are both prosauropods, creatures with small heads and long necks. They could walk on two or four legs and they ate only plants. Their remains were found in Madagascar.

John Flynn from the Field Museum in Chicago: "This is an extremely important discovery"

The animals probably roamed the Earth in the Middle to Late Triassic - 225 to 230 million years ago - which makes them marginally older than Herrerasaurus and Eoraptor, the current record-holders for dinosaur seniority.

The remains of eight other prehistoric creatures were uncovered with the prosauropods. The discovery is exciting because the fossils date from a period in Earth's history scientists know very little about.

John Flynn from Chicago's Field Museum where the fossils are now on display

The Middle to late Triassic was a time when a wide variety of reptiles, amphibians, and other vertebrates populated the land. By its close, early dinosaurs and mammals had appeared, but a sparse fossil record remains.

Now palaeontologists have discovered an area in Madagascar stretching over more than 1,000 kilometres that they hope will give up some of that period's secrets. They were led to the location by a young, local man called Mena who had found what he thought were old bones.

"The fossils are exquisitely preserved. They show a level of detail far superior to everything else from that time," says John Flynn, of the Field Museum in Chicago, and one of a large team of scientists currently working on the project.

Modern-day reptiles

Of the eight animals discovered in addition to the prosauropods, three are members of the branch of animals that includes modern-day reptiles and five are members of the branch that includes mammals. Most of the eight animals have never been seen before.

The team were unable to use radiocarbon dating to age the fossils because of the nature of the rock in Madagascar. Instead, they had to use other clues to date their finds, for example, by analysing the distribution and anatomy of other fossils in the rock strata.

The Madagascar fossil record is suspiciously lacking in fossils of aetosaurs - small, armored reptilian herbivores that were abundant about 228 million years ago. The team have therefore concluded that their find is more ancient - probably closer to 230 million years old - making the two new prosauropods the oldest dinosaurs ever discovered.

A Chicago Field Museum employee holds the upper and lower jawbones of the "oldest dinosaur"

The five pre-mammalian animals discovered could shed light on the origins of the first true mammals. Researchers still do not know how this line of large, cold-blooded "mammal-like reptiles" evolved into small warm-blooded mammals after the extinction of the dinosaurs. "The new fossils will help us tease that evolutionary transformation apart," said Mr Flynn.

The paleontologists also expect that their find will provide clues as to how the break-up of the supercontinent Pangaea, which began in the Triassic, affected the course of evolution.

The fossils now reside at the Field Museum of Chicago, but once the study of them is complete many will be returned to Madagascar.