Bizarre ancient critter has spines but no anus 

Spiny discovery moves this minion lookalike off a distant limb on the human family tree

a 3-D reconstruction of Saccorhytus coronarius, which looks like a purple spiky cylinder with a large mouth

This is a 3-D reconstruction of the extinct animal Saccorhytus coronarius. The views of the front (left) and back (right) helped scientists reclassify this critter.

Y. Liu et al/Nature 2022

No ifs, ands or butts about it. A teeny critter that lived several hundred million years ago did not have an anus. It had been thought to be an early member of a group of animals that includes everything from starfish to humans. But no more. New 3-D imaging of holes on its body suggests it was more closely related to insects. 

Old fossils can look very different from the original animals. This makes it tough to identify features.  

Most fossils of this animal had been flattened. It made the Saccorhytus coronarius fossils look “like a very sad balloon that’s collapsed in on itself,” says Philip Donoghue. He works at the University of Bristol in England. As a paleontologist there, he studies fossils. His team’s new 3-D pictures bring S. coronarius to life. It appears a bit like a wrinkly potato, he says — or perhaps an angry minion. But a very small one. It’s just about a millimeter (four-hundredths of an inch) long. 

Despite lacking an anus, the fossil version of this animal’s body has no shortage of holes. Its mouth is surrounded by a ring of small openings. Scientists had thought they must be an early version of gill slits, typically used for breathing. Gill slits are commonly found in a group of animals called deuterostomes. Gill slits seemed to nail the ancient critter to that part of the animal family tree, which also includes humans. 

Some nifty detective work 

As part of their new work, the researchers took X-rays of many fossils that began decaying before turning into rock. That showed them what happens to an animal after its death. Those images revealed that an inner tissue layer once pushed through pores on S. coronarius. That tissue extended outward, forming spines. After the critter died, that inner layer was lost. This left only the holes. 

That shows those holes are actually broken spines. The discovery helped Donoghue’s team move this creature to another group. Called Ecdysozoa (EK-di-suh-ZOH-uh), it includes insects, worms and related animals. The spines pretty much lock S. coronarius into this new group.  

The researchers shared their findings August 17 in Nature

That still leaves the puzzle of why this critter had no anus. Some other animals lack one. Jellyfish are one example. To get around the problem, they vomit out their food wastes. But members of both deuterostomes and ecdysozoans usually do have anuses. That makes S. coronarius an uncomfortable fit in either group. 

Still, “if you haven’t got an anus,” Donoghue jokes, “you’re not going to be very comfortable anywhere.” 

Anna Gibbs was the spring 2022 science writing intern at Science News. She holds a B.A. in English from Harvard College.

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