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Nov 20, 2023

A Hiccup in the Annals of Animal Evolution: That Thing Was a Plant

When did animals arise? During the Cambrian Period, over half a billion years

When did animals arise? During the Cambrian Period, over half a billion years ago. Except for the bryozoans, after the earliest one turned out to be algae

Stone the primeval crows. Protomelission wasn't an early bryozoan animal that emerged over half a billion years ago. It was a plant, and that new identification may change everything we thought about animal evolution, scientists reported in a paper Wednesday.

When animals began to evolve is a huge argument, and the paper published in the journal Nature – by Jie Yang of the Yunnan University with Martin Smith of Durham University and colleagues – doesn't resolve it. It does, however, argue that a fossil identified in 2021 as a bryozoan that emerged in the early Cambrian Period over half a billion years ago was actually a dasyclad alga.

What's in that for you? It changes our understanding of the emergence of animals. They may not all have arisen in the Cambrian after all.

Ironically, bryozoans are also known as "moss animals." They are ectoprocts, which means anus outside. Uh, that means the anus isn't among the crown of tentacles at the mouth, as opposed to endoprocts. Well, you get the idea.

If Yang's team is right, then no unequivocal bryozoans have been found from the Cambrian, which indicates that this vast family arose later, the authors write.

The new paper is based on Protomelission-like fossils from southern China. A 2021 paper by Zhiliang Zhang and colleagues (also published in Nature) was also based on fossils from southern China, as well as from Australia.

The 2021 paper explained that bryozoans have been found from the Ordovician period that follows the Cambrian, so it's reasonable to assume that bryozoans arose in the Cambrian. And the researchers claimed to have finally found one – Protomelission.

But Yang, Smith et al. now argue that Protomelission was alga, not animal. Anyway, Smith sees no reason why bryozoans couldn't have emerged during the Ordovician, he says, and explains why he suspects that this is exactly what they did.

Body beautiful

Bryozoans today are a vast group of aquatic animals that come in a great range of forms, many startlingly beautiful. Some are freshwater, some marine, and they're distant cousins of more familiar animals like mollusks and flatworms.

Bryozoans are often mistaken for coral because both are colonial water animals. But bryozoans aren't related to coral, which is a cnidarian – a cousin of the jellyfish.

As for dasycladales algae, they're unicellular behemoths. A single individual – if left alone to live its life in shallow warm seawater – can grow to 20 centimeters (8 inches) long. One potential cause for confusion is that dasycladales, bryozoans and algae all secrete external skeletons made of calcium carbonate.

In any case, in contrast to the 2021 paper, Smith says he sees no reason why the bryozoans couldn't have emerged in the Ordovician, which is when their fossils actually appear. "I see no reason to doubt the Ordovician origin of bryozoans – but I’m not convinced that this points to a Cambrian origin," Smith says.

"Mineralization [creating that external skeleton] is a key part of the bryozoan body plan, so any theory that requires long gaps in the fossil record makes me uncomfortable, particularly when people have been looking so hard for Cambrian candidates. And even a late Cambrian origin would put the dawn of the phylum long after the heat of the Cambrian explosion, which was largely over by 515 million years ago – the age of Protomelission."

In short, many animal lines did emerge during the Cambrian explosion, but bryozoans may not have been among them.

Smith notes that the 2021 paper was based on "phosphatized microfossils – 3D remains left after dissolving rock in acetic acid, basically vinegar. This is like finding the bones of a squirrel – it gives some information about the original morphology, but not the complete picture."

Indeed, a squirrel skeleton looks like a rat's while a squirrel looks like a rat with a bushy tail. One is adored while the other is reviled.

Back to the bryozoans. The new paper is based on "macrofossils," actual complete organisms, albeit squashed flat.

And that's better?

Well, as Smith explains, imagine that the squirrel had been run over. The team took the information from the phosphatized microfossils and the "road-killed" flattened Protomelission macrofossil and pieced together the creature's 3D structure (from the Zhang paper) with its silhouette (from the squished evidence).

"Zhang et al.'s fossils look like a catalogue of empty matchboxes, and these authors could only infer the presence of a stalked, tentacled, feeding apparatus emerging from each module, or ‘matchbox,’" Smith says. "Our fossils showed what was really inside the modules: no tentacles but instead a tapering flange that couldn't possibly have been a bryozoan feeding apparatus – but which would have been ideal for photosynthesis. And it matches the spine/leaf-like projections of dasyclad algae."

He concludes: "These flanges are what move us from 'might or might not be a bryozoan' to 'definitely is not a bryozoan.'"

A caveat. Since the two papers were based on different modes of preservation, the researchers can't be 100-percent certain that both specimens are actually Protomelissons. But for sure what they've worked on weren't bryozoans. Maybe the creatures that the 2021 team explored were bryozoans after all; Smith says he highly doubts it, but fair's fair.

The Ediacaran exception

Even if Protomelisson had been a bryozoan, it wouldn't have been the earliest animal. That honor may have belonged to the ediacarans, a bunch of fantastical life forms gracing the oceans a hundred million years before all the above – but what ediacarans were is also debated. Animal? Plant? Something else entirely?

"Ediacarans have been interpreted as all sorts of different things through the ages," Smith explains. "The consensus now is that almost all Ediacarans are stem-group animals; that is, more closely related to animals than anything else, but not properly animals in the same manner that dinosaurs are precursors to birds but don't count as birds proper."

Smith adds that there are some candidates among the ediacarans for super-early sponges and maybe even proto-cnidarians but no confirmed proper animals: no bilaterians, characterized by bilateral symmetry – a left half and a right half. Bilaterians did clearly emerge during the Cambrian – but guess who didn't? The bryozoans.

And what are we to learn? Some animals don't have solid parts that could leave fossils, so we can't consider their earliest history. But theoretically, if all animal phyla did arise in the early Cambrian, this strongly suggests that during the Cambrian explosion, evolution switched into a higher gear – resulting in a creative frenzy never seen since, Smith says.

"Evolution by natural selection is biology's grand unifying theory, but if it did something different in the Cambrian, it raises the question of how well we understand the engine of biodiversity," he says. "If bryozoans – and perhaps some less fossilizable phyla – arose later, then the trajectory of life was not set in stone in the early Cambrian."

By the way, the authors also compared the Protomelission-like fossils to those of a geographically widespread group of small, spine-shaped fossils known as cambroclaves. They concluded that some cambroclaves may also be dasyclad algae. Oops.