Bigmouth strikes again University of Cambridge/PA Wire

The ancestor of all vertebrates, including fish, reptiles and humans was a big mouth but apparently had no anus.

The microscopic creature named Saccorhytus, after the sack-like features created by its elliptical body and large mouth, lived 540 million years ago. It was identified from microfossils found in China.

“To the naked eye, the fossils we studied look like tiny black grains, but under the microscope the level of detail is jaw-dropping,” says team member Simon Conway Morris, of the University of Cambridge, in the UK.


Researchers believe it was about a millimetre in size, lived between grains of sand on the sea bed and had a large mouth relative to the rest of its body.

They also think the creature was covered with a thin, relatively flexible skin, had some sort of muscle system which could have made contractile movements and allowed it to move by wriggling.

No anus

It probably ate by engulfing food particles, or even other creatures, but scientists were unable to find any evidence the animal had an anus.

“If that was the case, then any waste material would simply have been taken out back through the mouth, which from our perspective sounds rather unappealing,” says Conway Morris.

The creature is thought to be the most primitive example of a so-called “deuterostome” – a broad biological category that encompasses a number of sub-groups, including the vertebrates.

“We think that as an early deuterostome this may represent the primitive beginnings of a very diverse range of species, including ourselves,” says Conway Morris. “All deuterostomes had a common ancestor, and we think that is what we are looking at here.”

Precursors of fish gills

Saccorhytus gives us remarkable insights into the very first stages of the evolution of a group that led to the fish, and ultimately, to us, says team member Degan Shu, from Northwest University, in the US.

The creature also had small conical structures on its body which may have been the evolutionary precursor of the gills we now see in fish.

Most other early deuterostome groups are from about 510 to 520 million years ago, when they had already begun to diversify into vertebrates, sea squirts, echinoderms – animals such as starfish and sea urchins – and hemichordates, a group including things like acorn worms.

Read more: Comb jelly videos are rewriting the history of your anus

Journal reference: Nature, DOI: 10.1038/nature21072