“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,” said Prof Philip Donoghue of the University of Bristol’s School of Earth Sciences, who co-led the study.
“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.”
In addition to all the statements made by scientists, “It’s a bit confusing – [most] ecdysozoans have an anus, so why didn’t this one?” also said researcher Emily Carlisle to BBC Radio 4’s Inside Science.
What is Saccorhytus?
Saccorhytus is an ancient creature. It is believed that it lived approximately 540 million years ago in the Fortunian stage of the Cambrian Period. If confirmed as a deuterostome, the saccorhytus would represent the oldest known species of this superphylum. Saccorhytus’ lack of anus is also an intriguing feature.
Although there is a question mark on minds about how the digestive system works, Saccorhytus is very important for evolutionary biology.
“This is a really unexpected result because the arthropod group have a through-gut, extending from mouth to anus. Saccorhytus’s membership of the group indicates that it has regressed in evolutionary terms, dispensing with the anus its ancestors would have inherited,” says Shuhai Xiao from Virgina Tech, USA, who co-led the study. “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.”