- Victoria Gill
- BBC Science Correspondent
Scientists say they have solved an evolutionary mystery involving a 500-million-year-old microscopic spiny creature with a mouth but no anus.
When discovered in 2017, this tiny, sac-shaped fossil of a marine animal was said to be the oldest known ancestor of humans.
Saccorhyntys coronarius, as this ancestral being is known, was tentatively classified within the group of deuterostomes, which are characterized by his anus forms before his mouth during his embryonic development.
These are the primitive ancestors of vertebrates, including humans.
Now, a new study suggests that Saccorhytus should be classified in an entirely different group of animals.
A team of researchers in China and the UK carried out a very detailed X-ray analysis of the creature and concluded that belongs to a group called ecdysozoawhich are ancestors of spiders and insects.
One of the origins of this evolutionary confusion was the animal’s lack of an anus.
Emily Carlisle, a researcher who has studied Saccorhytus in detail, told BBC Radio 4’s Inside Science about this. “It’s a little disconcerting, (the majority) of ecdysozoans have anus, so why not this one?
An “intriguing option” to explain this, he said, would be that an ancestor even before this entire group had no anus, and that Saccorhytus evolved after that.
“It’s possible that it lost it during its own evolution, maybe it didn’t need it or it could just sit in one place with a single opening for everything,” he said.
However, the main reason for the “repositioning” of Saccorhytus in the Cambrian tree of life—the division of the geological time scale belonging to the Paleozoic Era—is that, on initial examination, the holes surrounding its mouth were interpreted as pores for gills, a primitive feature of deuterostomes.
When the scientists looked in more detail, using powerful X-rays to closely examine the 1mm creature, they realized that it was actually the base of the spines that had broken off.
Scientists who study these fossils try to place each animal on a tree of life, much like a family tree, something that allows them to build the big picture to understand. where do they come from and how does it evolveRon.
“Saccorhytus would have lived in the oceans, in the sediment with its spines holding it in place,” explained Carlisle, who works at the University of Bristol in the UK.
“We think he just sat there (where he was found), in a very strange environment with many animals They would look like some living creatures out there, but a lot of totally weird ones.”
The rocks containing these Cambrian fossils are still being studied.
“There’s still a lot we can learn about his environment,” Carlisle added.
“The more I study paleontology, the more I realize how much is missing. In terms of this creature and the world it lived in, we’re really just scratching the surface.”
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