Whew. It turns out the earliest known ancestor of humans is not a sea creature a millimeter in size that lacked an anus. The opposite was thought to be true after the release of a 2017 study in which scientists named Saccorhytus coronaries as a 540 million-year-old member of a category of animals called deuterostomes, which gave rise to vertebrates. But a new study says Saccorhytus was placed in the wrong group of animals. "We had specimens that were better preserved, so we knew immediately that the authors had got it flat-out wrong," study author Philip Donoghue tells the Guardian.
He and other researchers from the UK and China collected hundreds of new specimens and as LiveScience explains, they "used a type of particle accelerator called a synchrotron to produce detailed X-ray images of the fossil that revealed microscopic details about its body plan." They made what they say is a telling find: the holes around its mouth aren't "purported pharyngeal openings," or primitive gills as originally thought. Rather, they are "taphonomic artifacts": the base of spines that had been broken off, reports the BBC. "Saccorhytus would have lived in the oceans—in the sediment with its spines holding it in place," study author Emily Carlisle explains.
Adds Donoghue, "[it] looks like a tiny wrinkly ball with a bunch of spines and a mouth with rings of teeth around it. I like to describe it as an angry minion." Per the study published in Nature, the researchers assert that Saccorhytus belongs to the group Ecdysozoa, which includes arthropods (insects, spiders, and crustaceans). As for the missing anus (Saccorhytus' single hole would have served as both mouth and anus), Carlisle notes that most ecdysozoans do have an anus. "It could be that it lost it during its own evolution—perhaps it didn't need one because it could just sit in one spot with one opening for everything." (Read more discoveries stories.)