快猫短视频

A MYSTERY is afoot. Something appears to have been crawling over the seabed more than a billion years ago. Yet according to the textbooks, animals didn鈥檛 exist until hundreds of millions of years later.

The puzzle originates from some fossilised trails found in Australian rock. The ridges are a millimetre wide and up to 2 milli-metres apart, and look like slime trails left by some worm-like creature inching across fine sea-floor mud.

When geologists found the trails 10 years ago, they assumed that the rocks must date from the Ediacara epoch 550 to 600 million years ago鈥攖he time from which the oldest generally accepted animal fossils come. But when Birger Rasmussen of the University of Western Australia dated the fossils using a new technique that was developed for non-volcanic rocks, he was surprised to find that they formed between 1.2 and 2 billion years ago.

Studies of the genetic differences between animals suggest that they began diverging between 700 and 1500 million years ago. But palaeontologists have always been sceptical about that because no animal fossils that old have been found, and because genetic clocks can underestimate the effects of rapid evolutionary spurts. And an earlier claim that rocks more than a billion years old found in India contain fossil trails has never been widely accepted (快猫短视频, 10 October 1998, p 6).

Rasmussen asked palaeontologist Stefan Bengtson of the Swedish Museum of Natural History in Stockholm to take a closer look at the trails. He interpreted them as slime trails left by a worm-shaped organism that glided along by stretching its body forwards and then contracting to pull up the rear. Modern ribbon worms move in the same way.

A worm-like animal would be the logical suspect if the traces dated from the Ediacara period, but because they existed at least 600 million years earlier Bengtson says 鈥渨e have been quite careful not to call these things animals鈥�. He says the mystery organism may not be the ancestor of any of today鈥檚 creatures, but could have been an evolutionary 鈥渄ead-end experiment鈥� that went extinct.

鈥淚t鈥檚 a very interesting structure,鈥� says Bruce Runnegar of the University of California at Los Angeles. He thinks the dating is accurate, but isn鈥檛 yet convinced the fossils are trails rather than some other biological structure. He plans to go to Australia to look at the rocks in the field.

Rasmussen and Bengtson have found more trails since writing up their findings. 鈥淭hey don鈥檛 change the story, but they make it considerably richer,鈥� Bengtson told 快猫短视频.