It has recently been supposed that humans could trace their ancestry back to a strange microscopic creature with a mouth and no anus. Thanks to analysis of 500 million year old fossils at the Swiss Light Source SLS, we can be relieved to find out this is not true: Saccorhytus is not a deuterostome like us, but an ecdysozoan. The findings, published today in Nature, make important amendments to the early phylogenetic tree and our understanding of how life developed.
In 535 million year old rocks in China is a mysterious microfossil whose evolutionary affinity is hotly debated. Saccorhytus was originally described in 2017 as an ancestral deuterostome, a member of the group from which our own deep ancestors emerged. It is microscopic in size – about a millimetre in diameter – and resembles a spikey, wrinkly sack, with a mouth surrounded by spines and holes that were interpreted as pores for gills – a primitive feature of the group. This made for a very unexpected origin of deuterostomes: within sand-grain sized organisms that may have lived among the sand or floating in the sea. However, the evidence supporting this view was always very weak – were those holes around the mouth really gill pores?
The researchers tried to address this question by collecting new specimens of Saccorhytus, dissolving tonnes of rock with strong vinegar and picking through the resulting grains of sand for these rare fossils. The fossils are no longer rare – the teams recovered hundreds of specimens, many much better preserved than any seen before, providing new insights into the anatomy and evolutionary affinity of Saccorhytus.
“Some of the fossils are so perfectly preserved that they look almost alive,” says Yunhuan Liu, professor in Palaeobiology at Chang’an University, Xi’an, China. “Saccorhytus was a curious beast, with a mouth but no anus, and rings of complex spines around its mouth.”
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Image: The reconstructions show the fossil of Saccorhytus from the front
Credit: Graphic: Dinghua Yang