By Lin Shujuan (China Daily)

Newly discovered fossils of a primitive turtle species, the oldest of its kind found in the world, gives credence to the belief that turtles originated from an aquatic environment and its shell formed before its dorsal carapace, according to today`s Nature magzine.

A group of Chinese and Canadian paleontologists were responsible for the discovery in May last year in Guizhou province.

The fossil dates back to the late Triassic period, 220 million years ago. The species had teeth, and more importantly, had no complete carapace, Li Chun, a scientist at the Beijing Institute of Vertebrate Paleontology and Paleoanthropology and head of the research team, said.

This handout image shows an artist`s impression of an ancestral turtle from the Triassic Period found in South China`s Guizhou Province in May 2007. A stunningly intact 220-million-year-old fossil found in southwestern China appears to have settled a long-simmering debate over reptile evolution. [Agencies]

Li is the author of a report on the discovery appearing in Nature magazine.

"The fossils show a fully developed plastron like today`s turtles, but also a plate in the middle of its back, and broadened ribs, suggesting the evolution of a dorsal shell," he said.

Previous records of primitive turtles - about 14 to 18 million years younger - show no obvious physical differences from today`s turtles.

If the species represent an early stage in the evolution of a turtle`s shell, which Li and his fellow research members believe, that means the plastron of turtles evolved before their carapace.

Li said the species also challenge another popular theory that turtles originated from a terrestrial or half-aquatic animal.

The researchers excavated three fossils in marine deposits. Their analysis showed the species shared the same biological features as modern soft-shelled turtles, which can adapt well to an aquatic environment.

Robert Reisz, a professor of biology with the University of Toronto at Mississauga, Canada, said the discovery "is very important" as it offers an alternative perspective "about the origins of turtles and the evolution of their striking body configuration".

However, the discovery has not solved a long-going controversy on the subject.

Reisz said the possibility exists that the turtles` partially developed carapace could be an adopted feature rather than a primitive one as a result of habitat change from land to water.

"There is still controversy about which particular group of fossil reptiles gave rise to turtles," he said.