Researchers at Durham University, UK, and Northwest University, Xi'an, China, examined 15 exceptionally preserved fossils of the annelid worm Iotuba chengjiangensis dating from the early Cambrian period 515 million years ago.

The fossilized remains included evidence of the worms' guts and kidneys and revealed they had an unexpectedly complex structure similar to that of other annelid worms.

The researchers say this means that annelids—or segmented worms—diversified into different lineages some 200 million years earlier than previously thought and were part of the evolutionary leap known as the Cambrian explosion.

The Cambrian explosion saw a huge rise in organisms between 540 and 530 million years ago—as shown by fossil records—and saw the appearance of many of the major groups that make up animal life on Earth.

The findings are published in the journal Proceedings of the Royal Society B.

Study co-author Dr. Martin R. Smith, in the Department of Earth Sciences, Durham University, said, "We know that the main animal lines we see today emerged during the Cambrian explosion, but we always thought annelid worms were late to the party, and their major subgroups didn't begin to diversify until nearly 200 million years later.