An international team of researchers has discovered that a mysterious microscopic creature that humans were thought to descend from was part of a different family tree.
Resembling an angry Minion, the Saccorhytus is a pointed, wrinkled sac, with a large mouth surrounded by spines and holes that have been interpreted as pores for the gills – an early feature of the Deuterostome group, from which our own deep ancestors descended.
However, extensive analysis of old fossils from 500 tens of millions of years from China showed that the holes around the mouth are bases of spines that detached during fossil preservation, finally revealing the evolutionary affinity of the Saccorhytus microfossil.
“Some of the fossils are so perfectly preserved that they seem almost alive”, explains Yunhuan Liu, professor of paleobiology at the Chang’an University, Xi’an, China. “Saccorhytus was a curious beast, with a mouth but no anus, and intricate rings of spines around its mouth.”
The findings, published today in Mother nature, make significant changes to the early phylogenetic tree and understanding of the development of life.
The true story of Saccorhytus ancestry lies in the microscopic characteristics internal and external of this tiny fossil. By taking hundreds of X-ray images from slightly different angles, using 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 explained: “Fossils can be quite difficult to interpret and Saccorhytus is no exception. We had to use a synchrotron, a kind of particle accelerator, as the basis for our fossil analysis. The synchrotron provides very intense X-rays that can be used to take detailed images of fossils. We took hundreds of X-ray images at slightly different angles and used a supercomputer to create a 3D digital model of the fossils, which reveals the tiny features of its internal and external constructions.”
Numerical models showed that the pores around the mouth were closed by another body layer extending through, creating spines around the mouth. “We think it would have helped Saccorhytus capture and process its prey,” suggests Huaqiao Zhang of the Nanjing Institute of Geology and Paleontology.500
Researchers believe that Saccorhytus is actually an ecdysoszoan: a group that contains arthropods and nematodes. “We considered many alternative groups that Saccorhytus might be related to, including corals, anemones and jellyfish which also have mouths but no anuses,” said Professor Philip Donoghue from the School of Earth Sciences. from the University of Bristol, who co-led the study. “To solve the problem, our computer analysis compared the anatomy of Saccorhytus with all other living groups of animals, concluding a relationship with arthropods and their mothers. and father, the group to which insects, crabs and roundworms belong.”
The absence of an anus in Saccorhytus is an intriguing feature of this microscopic and ancient organism. Although the question that comes to mind is the optional (out of the mouth, rather undesirable) digestive waste pathway, this feature is important for a fundamental reason in evolutionary biology. The way the anus appeared – and sometimes disappeared afterwards – contributes to the understanding of the evolution of animal body shapes. Moving Saccorhytus from the deuterosome to the ecdysozoan means removing an anus that disappears from the case history of the deuterosome and adding it to that of the ecdysozoan.
“This is a truly unexpected result because the arthropod group has a through-gut, extending from the mouth to the anus. Saccorhytus’ membership in the group indicates that he regressed in terms of evolution, dispensing with the anus that his ancestors would have inherited,” explains Shuhai. Xiao from Virgina Tech, USA, who co-led the study. “We still don’t know the precise place of Saccorhytus in the tree of life, but it may reflect the ancestral condition from which all members of this diverse group evolved.”
The international team included researchers from the University of Bristol, Chang’an University (Xi’an, China), Nanjing Institute of Geology and Paleontology (China), Chinese Academy of Sciences (Nanjing, China), Chinese Academy of Geological Sciences (Beijing, China), Shandong University (Qingdao, China), Swiss Light-weight Source, Virginia Tech (USA) and Initially Institute of Oceanography, Ministry of All-natural Useful resource (Qingdao, China).