Scientists Describe Ancient Bag-Like Sea Creatures From China
Scientists have described a new kind of sea creature in what's now central China. It lived 540 million years ago, and the tiny, baggy organism could occupy a peripheral spot on our own evolutionary tree.
When scientists like Simon Conway Morris discover a new animal, they get to name it. He and his colleagues in China don't seem to give compliments where they aren't deserved.
"We arrived at the word Saccorhytus, which basically means a wrinkled bag," says Conway Morris, a paleobiologist at the University of Cambridge in the U.K.
Actually, this thing is a lot uglier than a wrinkled bag. It's basically a giant gaping mouth with spikes and some extra holes — probably for oozing waste.
"Doesn't sound too appetizing, does it?" says Conway Morris.
Fortunately, it was no bigger than a grain of rice.
Morris and his colleagues in China and Germany announced the these ancient sand-dwellers (they found 45 of them) Monday in the journal Nature.
He says the animals likely lived about 540 million years ago, "slithering or clambering" in sediment in shallow seas.
"It has a very small body. It doesn't have a tail. It does not appear to have eyes," says Conway Morris. And it doesn't appear to have an anus, either, though its pleated, expandable mouth sure did look like one.
It sounds primitive, but compared to other life in that era, this little blob of an animal was on the cutting edge. This was a time in Earth's history when sea pens and mats of algae were about the most exciting things around.
Conway Morris and his colleagues think it's a really important find, that the animals could be a distant side-shoot of our own evolutionary tree — a humbling part of it that doesn't come up much.
"What we have here is an animal which we would suggest is in fact the earliest known deuterostome," he says. Deuterostomes are a huge group of organisms that, over the next millions of years would come to include starfish, sea squirts and anything with a spine, including humans. (Interestingly, "deuterostome" is Greek for "second mouth." That's either a polite euphemism, or a reference to the fact that now, everything in this group — us included — develops an anus first and then a mouth after that.)
Instead of that key orifice, Saccorhytus coronarius had eight holes down the sides of its round body, where waste might have just passed through willy nilly.
"And why's that important? Because basically one of the very few hallmarks of all deuterostomes is what we call gill slits," says Conway Morris.
Structures like those may have been precursors to gills, which were in turn an important stepping stone on the way to animals that were capable of walking and breathing on land.
This critter is a glimpse of what the common ancestor of many of today's animals might have looked like, says Peter Van Roy, a paleobiologist with Yale University and Ghent University in Belgium who wasn't involved in the paper.
"Things that are really that early — 540 million years old — are really important because they give major insights into the origins of all these major groups," says Van Roy.
No one is arguing that this is our common great great great grandparent, but it's an ancestor of a major animal group that includes humans. So humans probably came from something that looked like it.
"We come from these tiny, small blobs. That, I think, is an important thing for people to realize," says Van Roy.
Saccorhytus lived during a dramatic point in evolutionary history, the Cambrian explosion, when there was a big boom in diversity of creatures that resulted in all the major animal groups present today.
"Things start becoming big and start developing hard parts and structures to capture other animals and tear them apart — basically, a much more violent world," says Van Roy. Food chains were just starting to assemble. "It was the beginning for the world as we know it now, a world that's dominated by active animals — things walking around, swimming around, not just sitting there," he says.
There's a mystery about when, exactly, the explosion in animal diversity occurred. Molecular data, which allows researchers to estimate the age of the ancestor of two living species based on genetic differences between them, suggest it occurred as far back as 800 million years ago. But paleontologists don't see hard evidence of that spike in diversity until millions of years later, around 520 million years ago.
"So, there's this big gap in the record based on what would be predicted by molecular clocks and what we know from the fossil record," says Rahman.
This rare fossil find, he says, suggests that diversity could have gotten started earlier than fossils suggest.
"Perhaps one of the reasons we didn't see so many animals in the fossil record is because the animals were really small," he says. Small enough to fit between grains of sand.
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