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Precambrian Trails from Living Fossils

John on November 21, 2008 at 3:28 pm

Science Daily has the story of a group of researchers who’ve discovered a very unusual “living fossil.” Gromia sphaerica is a macroscopic single celled organism about the size of a grape. The researchers discovered it crawling across the floor of the ocean near the Bahamas. And that’s where the story gets really interesting:

The finding is significant, because similar fossil grooves and furrows found from the Precambrian era, as early as 1.8 billion years ago, have always been attributed to early evolving multicellular animals.

“If our giant protists were alive 600 million years ago and the track was fossilized, a paleontologist unearthing it today would without a shade of doubt attribute it to a kind of large, multicellular, bilaterally symmetrical animal,” says Matz, an assistant professor of integrative biology. “We now have to rethink the fossil record.”

[...]

The bilateral animals, or “Bilateria,” appeared in the fossil record in the early Cambrian about 542 million years ago, quickly diversifying into all of the major animal groups, or phyla, still alive today. This rapid diversification, known as the Cambrian explosion, puzzled Charles Darwin and remains one of the biggest questions in animal evolution to this day.

Very few fossils exist of organisms that could be the Precambrian ancestors of bilateral animals, and even those are highly controversial. Fossil traces are the most accepted evidence of the existence of these proto-animals.

Until now, the common explanation for the unusually rapid evolution of animal phyla (the so-called Cambrian explosion) has been to suggest that precambrian trails are evidence of ongoing animal evolution long prior to the Cambrian. For example, this PBS website for the show Evolution makes just such an argument:

Even organisms that hadn’t evolved hard parts, and thus didn’t leave fossils of their bodies, left fossils of the trails they made as they moved through the Precambrian mud. Life was flourishing long before the Cambrian “explosion”.

Note the scare quotes around “explosion.” With this latest discovery, the scare quotes may need to come off. Returning to the Science Daily article:

With their find, Matz and his colleagues argue that fossil traces cannot be used alone as evidence that multicellular animals were evolving during the Precambrian, slowly setting the stage for the Cambrian explosion.

“I personally think now that the whole Precambrian may have been exclusively the reign of protists,” says Matz. “Our observations open up this possible way of interpreting the Precambrian fossil record.”

Given that the recently discovered trails came from live animals thought to be “living fossils” very much like their Precambrian ancestors, perhaps some genetic investigation will provide new leads.

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