There are no ifs and buts about it: A roughly 530-million-year-old tiny creature that lacks an anus is not, as previously thought, the oldest member of a broad group of animals that includes everything from starfish to humans. .
Despite his absent anus, Saccorhytus coronarius It had no shortage of holes in its wrinkled, potato-shaped body, including a ring of small openings around its gaping mouth. Previously, those holes had been identified as an early version of gill slitstypically used for breathing (Serial Number: 3/2/17). Gill slits are commonly found in deuterostomes, so their presence apparently nailed the creature’s place on the animal family tree.
But a new 3-D reconstruction of the half-millimeter-long species based on fossil images shows that those holes are remains of broken spinesresearchers report Aug. 17 in Nature. The identification of the spines helped shift the creature into a group with arthropods and nematodes, called the Ecdysozoa.
After millions of years, the fossils can look very different from the original specimens, which makes it difficult to identify biological characteristics (Serial number: 3/8/22). The majority coronary system the fossils have collapsed “like a very sad balloon collapsing in on itself,” says paleontologist Philip Donoghue of the University of Bristol in England. The 3-D reconstruction brings coronary system to life, even if it looks like an angry minion, he says.
Donoghue and his colleagues X-rayed many coronary system fossils that represent the different stages of decomposition of organisms. The images revealed that an inner layer of skin once pushed through the pores and spread outward, forming spines. During fossilization, that inner layer was lost and the holes were left behind.
While the spines practically lock coronary system in his new group, one puzzle remains: the missing anus. It’s not inherently strange: absence has evolved independently in many species, such as jellyfish, which vomit up their food waste. But both deuterostomes and ecdysozoans usually have anuses, so coronary system an awkward fit in any group.
Still, “if you don’t have an anus,” Donoghue jokes, “you won’t feel very comfortable anywhere.”