A microscopic fossil beast without an anus is not the first human ancestor

Aug. 17 () –

An international team has discovered that a mysterious microscopic creature discovered in China from which humans were once believed to have descended It is part of a different family tree.

The ‘Saccorhytus’ has the appearance of a spiny and wrinkled sacwith a large mouth surrounded by spines and holes that were interpreted as pores for gills, an early feature of the deuterostom group, from which our own deep ancestors arose.

However, extensive analysis of 500-million-year-old fossils from China has shown that the holes around the mouth are bases of spines that were dislodged during fossil preservation, finally revealing the evolutionary affinity of the Saccorhytus microfossil. ‘. The results are published in Nature.

“Some of the fossils are so perfectly preserved that they seem almost alive,” said Yunhuan Liu, professor of paleobiology at Chang’an University in China. Saccorhytus was a curious beast, with a mouth but no anus, and rings of complex spines around the mouth.”

The findings introduce important modifications in the primitive phylogenetic tree and in the understanding of how life developed.

The true story of Saccorhytus’s ancestry lies in the internal and external microscopic features of this tiny fossil. By taking hundreds of X-ray images at slightly different angles, with the help of powerful computers, a detailed 3D digital model of the fossil could be reconstructed.

Researcher Emily Carlisle, from the School of Earth Sciences at the University of Bristol (UK), explains that “fossils can be quite difficult to interpret and Saccorhytus is no exception. We had to use a synchrotron, a type of particle accelerator, as a basis for our analysis of the fossils.”

“The synchrotron provides very intense X-rays that can be used to take detailed images of the fossils,” he continues. “We took hundreds of X-ray images at slightly different angles and used a supercomputer to create a 3D digital model of the fossils, that reveals the minute features of its internal and external structures.

Digital models showed that the pores around the mouth were closed by another layer of the body extending through, creating spines around the mouth. “We believe that these would have helped ‘Saccorhytus’ to capture and process their prey”, suggests Huaqiao Zhang, from the Nanjing Institute of Geology and Paleontology (China).

The researchers believe that Saccorhytus is actually an ecdysoszoan: a group containing arthropods and nematodes. “We considered many alternative groups with which the ‘Saccorhytus’ could be related, like corals, anemones and jellyfish, which also have a mouth but no anus“explains Professor Philip Donoghue, from the University of Bristol’s School of Earth Sciences, who co-led the study.

“To solve the problem, our computational analysis compared the anatomy of Saccorhytus with all other groups of living animals, concluding a relationship with arthropods and their relatives, the group to which insects, crabs and roundworms belong” , he points out.

Saccorhytus’s lack of an anus is an intriguing feature of this microscopic and ancient organism. Although the obvious question is the alternative route of digestive waste, this feature is important for a fundamental reason in evolutionary biology.

How the anus arose — and sometimes its subsequent disappearance — contributes to our understanding of how animal body plans evolved. Moving ‘Saccorhytus’ from the deuterosome to the ecdysozoon means removing the anus that disappears from the history of the deuterosome and adding it to that of the ecdysozoon.

“This is a really unexpected result because the arthropod group has a through intestine, which extends from the mouth to the anus. Saccorhytus’s membership in the group indicates that it has regressed in evolutionary terms, regardless of the anus that their ancestors would have inherited“, says Shuhai Xiao, from Virginia Tech (United States), who co-led the study.

“We still don’t know the precise position of ‘Saccorhytus’ within the tree of life,” he acknowledges. but it may reflect the ancestral condition from which all members of this highly diverse group evolved.”