Curtin University-led researchers used volcanic ash to determine when the oceans started teeming with complex life forms hundreds of millions of years ago as the planet thawed from the ice age.

Geologist Chris Kirkland said the fossils from the Coed Cochion Quarry, northwest of Cardiff, were some of the earliest evidence of large-scale multicellular organisms after four billion years of single-celled microbes.

He said the massive step in evolution was a transformative moment in Earth's biological history.

"Ediacaran fossils record the response of life to the thaw out from a global glaciation, which shows the deep connection between geological processes and biology," Professor Kirkland said on Tuesday.

He said the study would help the scientific world to better understand the "ancient ecosystems ... (and) unravel the mysteries of Earth's past and shape our comprehension of life's evolution."

Geologist Anthony Clarke said the volcanic ash that blanketed the marine animals acted as a time marker and the research team was able to date the fossils to 565 million years, accurate to 0.1 per cent.

"These creatures would in some ways resemble modern-day marine species such as jellyfish, yet in other ways be bizarre and unfamiliar," he said.

"Some appear fern-like, others like cabbages, whereas others resembled sea pens."

The fossils are named after the Ediacara Hills in South Australia's Flinders Ranges, where they were first discovered in the 1940s, leading to the first new geological period established in over a century.

"These Welsh fossils appear directly comparable to the famous fossils of Ediacara," Prof Kirkland said.

The study was published in the Journal of the Geological Society.

Australian Associated Press