Paleontologists have examined new microfossils of a Cambrian microscopic animal called Saccorhytus coronarius, which was previously interpreted as a deuterostome, a large group of the animal kingdom which includes vertebrates (like humans).
An artist’s reconstruction of Saccorhytus coronarius. Image credit: Liu et al.
Saccorhytus coronarius is an ellipsoidal marine animal with a large mouth surrounded by spines and holes.
It was about a millimeter in size, and probably lived between grains of sand on the seabed.
The 535-million-year-old remains of Saccorhytus coronarius were first discovered in 2017 in the Kuanchuanpu Formation in Shaanxi Province, China.
The creature was interpreted as an anusless deuterostome. The interpretation was largely based on the presence of gills — a primitive feature of the deuterostome group.
However, the new fossils from the Kuanchuanpu Formation show that the holes around the animal’s mouth aren’t pores for gills but are bases of spines.
“Some of the fossils are so perfectly preserved that they look almost alive,” said Chang’an University’s Professor Yunhuan Liu.
“Saccorhytus coronarius was a curious beast, with a mouth but no anus, and rings of complex spines around its mouth.”
The true story of Saccorhytus coronarius’ ancestry lies in the microscopic internal and external features.
“Fossils can be quite difficult to interpret and Saccorhytus coronarius is no exception,” said University of Bristol’s Dr. Emily Carlisle.
“We had to use a synchrotron, a type of particle accelerator, as the 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.”
“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 structures.”
The digital models showed that pores around the mouth of Saccorhytus coronarius were closed by another body layer extending through, creating spines around the mouth.
“We believe these would have helped Saccorhytus coronarius capture and process its prey,” said Nanjing Institute of Geology and Palaeontology’s Dr. Huaqiao Zhang.
Saccorhytus coronarius. Scale bars – 200 μm in (a, d, f, h), 100 μm in (b, c, e, g, j) and 20 μm in (i). Image credit: Liu et al.
According to the team, Saccorhytus coronarius is in fact an ecdysozoan, a group that contains arthropods and nematodes.
“We considered lots of alternative groups that Saccorhytus coronarius might be related to, including the corals, anemones and jellyfish which also have a mouth but no anus,” said University of Bristol’s Professor Philip Donoghue.
“To resolve the problem our computational analysis compared the anatomy of Saccorhytus coronarius 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.”
Saccorhytus coronarius’ lack of anus is an intriguing feature of this microscopic, ancient organism.
Although the question that springs to mind is the alternative route of digestive waste, this feature is important for a fundamental reason of evolutionary biology.
How the anus arose — and sometimes subsequently disappeared — contributes to the understanding of how animal bodyplans evolved.
Moving Saccorhytus coronarius from deuterosome to ecdysozoan means striking a disappearing anus off the deuterosome case history, and adding it to the ecdysozoan one.
“This is a really unexpected result because the arthropod group have a through-gut, extending from mouth to anus,” said Virgina Tech’s Dr. Shuhai Xiao.
“Saccorhytus coronarius’ membership of the group indicates that it has regressed in evolutionary terms, dispensing with the anus its ancestors would have inherited.”
“We still don’t know the precise position of Saccorhytus coronarius within the tree of life but it may reflect the ancestral condition from which all members of this diverse group evolved.”
The team’s paper will be published in the journal Nature.
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Yunhuan Liu et al. 2022. Saccorhytus is an early ecdysozoan and not the earliest deuterostome. Nature, in press;
