Our anus-less ancestor
There's a big question behind this tiny fossil.
Meet your ancestor – or at least, a reconstruction of it. This millimetre-long Cambrian fossil was found in limestone deposits in Shaanxi, central China. Unveiled in Nature today, the new species, named Saccorhytus coronarius, is 540 million years old, making it the oldest known example of a deuterostome – a major group of the animal kingdom which includes vertebrates (like us).
In deuterostome embryos, the anus generally develops first, then the mouth. In another major group, the protostomes (such as insects, worms and molluscs), the mouth develops first. When the evolutionary lines of deuterostomes and protostomes split, it reflected a basic design modification in the embryo. And, for some reason, this arse-about design change laid the groundwork for more sophisticated developments in deuterostomes, such as our big brain.
Yet while Saccorhytus had a big mouth, it showed no evidence of an anus. The authors speculate that in the adult, the mouth may have served for both feeding and excretion. They suspect the small conical structures on its body were the precursors of gills.