Researchers from China and the UK have solved the mystery surrounding Saccorhytus coronaries – a millimeter-long spiny creature that was found in 2017 and believed to have inhabited oceans 500 million years ago. The veil of mystery over the critter persisted for years, as the scientists could not locate the creature’s anus. Turns out – they should not have even tried.
The critter was originally determined to be a member of a deuterostomes group of animals - the ancestors to the modern vertebrates (including humans). However, scientists' latest research suggests that it actually belongs to another group called ecdysozoans – the predecessors of the modern spiders and various insects. As it turned out, the whole mishap happened over the creature's anus – or rather lack thereof.
Ecdysozoans normally have an anus and hence the critter was initially ruled not to belong to them. But, upon closer discovery using x-rays, scientists saw it resembles ecdysozoans internally more than the ancestors of the vertebrates.
Like them, Saccorhytus has a large mouth and also features tiny holes around it that served as bases for spines that kept the critter anchored in one place. Those holes also served as a source of confusion as they were originally believed to be gills – another mark of the proto-vertebrates.
As to Saccorhytus' absence of anus, scientists are yet to find out what happened to it. One theory suggests that there was an even earlier representative of ecdysozoans that has not been discovered, but did not have an anus.
Another theory goes that Saccorhytus did not actually need one.
"It could be that it lost it during its own evolution - perhaps it didn't need one because it could just sit in one spot with one opening for everything," Emily Carlisle, one of the researchers, said.