A spiky creature reminiscent of a mini-monster with a large mouth and no anus does not belong to the earliest human ancestors, contrary to what was thought. This is what an international team of researchers writes in the specialist journal Nature. Accordingly, due to a misinterpretation of certain body openings, fossils of Saccorhytus coronarius were incorrectly assigned to the animal group of the Neumünder (Deuterostomia), to which we humans also belong.
Elaborate X-rays of hundreds of fossils have now shown that the animal should instead be assigned to the so-called molting animals (Ecdysozoa), which include insects, spiders and tardigrades. A 3D model published by the research team shows an egg-shaped creature with a central maw that takes up a large part of the body surface. Its body is studded with small and larger spikes. According to the study, the little animal probably lived on the seabed and could probably neither crawl nor crawl. Feeding and defecation were both through the mouth.
The first discovery of S. coronarius comes from the Chinese province of Shaanxi. The layer of rock from which the fossil comes is dated at a good 530 million years. The fossil is about a millimeter in diameter. It has holes around its mouth. These were initially interpreted as so-called gill pores, a primeval feature of new mouths. But this interpretation of the openings was wrong, as the researchers led by Zhang Huaqiao from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences now show.
They collected hundreds of other fossils of the same species, some of which were much better preserved than previous finds. The researchers took X-rays of these from different angles and had a computer build a 3D model with them. The research team was able to show that the alleged gill pores were in fact the bases of spines. “We believe these Saccorhytus helped capture and process its prey,” says co-author Huaqiao Zhang.
The fact that S. coronarius has no anus is a special feature. The researchers assume that the progenitors of Saccorhytus coronarius had an anus, but this regressed. How the anus arose in animals – and in some cases disappeared again – is important for understanding the evolution of animals and their bodies.