No ifs, ands or butts about it: A teeny roughly 530-million-year-old critter that lacks an anus is not, as previously thought, the oldest member of a wide-ranging animal group that includes everything from starfish to humans.
Despite its absent anus, Saccorhytus coronarius had no shortage of holes on its wrinkly 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 slits, typically used for respiration (SN: 2/3/17). Gill slits are commonly found in deuterostomes, so their presence seemingly nailed the critter’s spot on the animal family tree.
But a new 3-D reconstruction of the half-millimeter-long species based on fossil imaging shows those holes are instead remnants of broken spines, researchers report August 17 in Nature. The identification of the spines helped shift the creature into a group with arthropods and nematodes, called Ecdysozoa.
After millions of years, fossils can look very different from the original specimens, which makes it challenging to identify biological features (SN: 3/8/22). Most S. coronarius fossils have been flattened “like a very sad balloon that’s collapsed in on itself,” says paleontologist Philip Donoghue of the University of Bristol in England. The 3-D reconstruction brings S. coronarius to life — even if it does look something like an angry minion, he says.
Donoghue and his colleagues took X-rays of many S. coronarius fossils representing different stages of the organisms’ decay. The images revealed that an inner skin layer once pushed through pores and extended outward, forming spines. During fossilization, that inner layer was lost, and the holes were left behind.
While the spines pretty much lock S. coronarius into its new group, a puzzle remains: the absent anus. It’s not inherently weird — the absence has evolved independently in many species such as jellyfish, which vomit their food waste. But both deuterostomes and ecdysozoans usually have anuses, making S. coronarius an uncomfortable fit in either group.
Still, “if you haven’t got an anus,” Donoghue jokes, “you’re not going to be very comfortable anywhere.”