'Oldest' hard-shelled fossil

They have discovered the fossilised remains of a marine animal - perhaps a sponge or coral - which they say lived nearly 550 million years ago.

The creature's hard, shelly parts are far more complex than anything else found from this time.

It gives researchers an insight into a period of Earth history just before lifeforms on our planet are thought to have gone through many rapid changes.

"This was a crucial time in evolution in terms of the fossil record," said Dr Rachel Wood, from Schlumberger Cambridge Research, UK. "It is when the light turned on, as it were.

"It's when multi-celled animals started to get hard skeletons. This [organism] has a much more complex biomineralised shell than we've seen before. It's also very big - it can reach up to a metre."

Giant reef

Dr Wood and colleagues report their findings in the journal Science. They found their fossil organism in what remains of a giant reef in southern Namibia.

The creature - classified as Namapoikia reitogensis - lived in the deep fissures or cracks in the reef. It is comprised of a series of tubules, each just a few millimetres in diameter.

These hard parts are fully biomineralised; they are not leathery like the skeletons of other creatures from this time.

If it was a coral, each tube would have probably housed a polyp with tentacles. If it was more like a sponge, its entire surface would have been covered in a thin veneer of tissue; small canals would have acted as a filtration unit to pull food out of the water.

The features displayed in the fossil have until now only been seen in specimens that are 15 million years younger.

The primitive nature of N. reitogensis means it is impossible to say whether it is really a Poripheran (sponge) or Cnidarian (coral).

"It's a characteristic of a lot of the Precambrian organisms that they share features of many groups," Dr Wood told BBC News Online. "They are what are known as stem groups. In other words, they haven't diverged into all the groups we know today."