520 Million Year Old Animal Fossils Might Not Be Animals After All

A species that lived around 520 million years ago and was thought to be the oldest known bryozoan is instead a type of colony-forming algae, according to a new study.

Bryozoans are filter-feeding, tentacle-bearing animals that live in complex, apartment-like colonies attached to rocks, shells, or other surfaces on seabeds or lake bottoms. The problem is that other animals and algae inhabit the same type of modular construction. While Entrance door of the protomélissioni was first described in 1993, scientists did not classify it as a bryozoan until 2021.

Now analyzes of even better-preserved fossils than previously described show that the species may not have been a bryozoan after all, says Martin Smith, a paleobiologist at Durham University in England.

Where previous fossils only preserved the skeletal framework of colonies, the new fossils, discovered in southern China, also include soft parts of the organism, Smith says. And instead of the tentacles believed to have been found in a perfectly preserved bryozoan, the fossils have simple leaf-shaped flanges typical of certain types of algaehe and his colleagues report on March 8 to Nature.

A photo of a fossil shell with a dark brown band attached to part of it.
New fossils of Entrance door of the protomélissioni (dark brown band shown attached to a fossil shell) suggest that the species is not the oldest known type of bryozoan after all.Zhang Xiguang

If confirmed, the new discovery means that the oldest unequivocally known bryozoan fossils are only around 480 million years old. This, according to Smith, makes bryozoans the only major group of animals not to have first appeared during the Cambrian Period, a surge in biological diversification that some scientists have called “the big bang of life” and which ended about 488 million years ago (SN: 04/24/19).

As a result, the Cambrian was not, as previously thought, a unique interval of innovation in evolutionary history during which all the blueprints of animal life were mapped out, the researchers conclude.

“The question is, has evolution lost its ability to create new body plans?” said Smith. The team’s new discovery suggests not, he says.

Not everyone agrees that the new fossils are not bryozoans. The leaf-like flanges described by Smith and his colleagues could just as well be interpreted as body parts of individual animals in the bryozoan colony, says Paul Taylor, an invertebrate paleontologist at the Natural History Museum in London who has no not participated in the study.

Because the tentacles that bryozoans use to snatch prey from the water are soft tissue and don’t typically store well, their absence from new fossils isn’t at all surprising, Taylor notes.

For Taylor, the new findings are not enough to dismiss P. gatehousei as a Cambrian bryozoan, but they highlight the inherent uncertainty in identifying fossils with single body plans. More fossils preserving additional features, such as those that preserve the organism’s early growth stages, are needed to settle the question, he says.

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