Researchers have discovered a mysterious creature with a mouth but no anus is not an ancestor of humans after all.
When the saccorhytus was discovered in 2017 it was believed the spikey, wrinkly sack could be humans' oldest ancestor.
The spines and holes surrounding its mouth were interpreted as pores for gills, which is a primitive feature of the deuterostome group from which human ancestors emerged.
However, extensive analysis of 500 million-year-old fossils from China has shown that the holes around the mouth on the saccorhytus are bases of spines that broke away during the preservation of the fossils.
"Saccorhytus was a curious beast, with a mouth but no anus, and rings of complex spines around its mouth," said Yunhuan Liu, a palaeobiology professor at China's Chang'an University.
Researchers from Chang'an University and the University of Bristol believe the saccorhytus is an ecdysoszoan, which is a group that contains arthropods and nematodes.
"We considered lots of alternative groups that saccorhytus might be related to, including the corals, anemones and jellyfish which also have a mouth but no anus," University of Bristol professor Philip Donoghue said.
"To resolve the problem our computational analysis compared the anatomy of saccorhytus with all other living groups of animals, concluding a relationship with the arthropods and their kin, the group to which insects, crabs and roundworms belong."
Researcher Emily Carlisle from the University of Bristol's School of Earth Sciences said a detailed 3D digital model of the fossil was reconstructed from hundreds of X-ray images.
"Fossils can be quite difficult to interpret and saccorhytus is no exception," she said.
"We had to use a synchrotron, a type of particle accelerator, as the basis for our analysis of the fossils."
The actual creature was probably no more than a millimetre in size.
The digital models showed that pores around the mouth were closed by another body layer extending through, creating spines around the mouth.
Shuhai Xiao from Virginia Tech who co-led the study said it was an "unexpected result".
"Saccorhytus's membership of the group indicates that it has regressed in evolutionary terms, dispensing with the anus its ancestors would have inherited," he said.
"We still don't know the precise position of saccorhytus within the tree of life but it may reflect the ancestral condition from which all members of this diverse group evolved."
