The discovery reveals how dinosaurs used this multipurpose opening — scientifically known as a cloacal vent — for pooping, peeing, breeding and egg laying. .

The dinosaur's derrière is so well preserved, researchers could see the remnants of two small bulges by its "back door," which might have housed musky scent glands that the reptile possibly used during courtship — an anatomical quirk also seen in living crocodilians, said scientists who studied the specimen.

It does look a bit like the back opening on a crocodile, he said, but it's different in some ways.

Even so, this dinosaur likely had copulatory sex, unlike some birds that bump butts when they do a "cloacal kiss" during reproduction, Vinther said.

To get a more complete picture of Psittacosaurus' cloacal vent, Kelly compared it with those of living land vertebrates.

The vent is the opening, and the cloaca, which comes from the Latin word for "sewer," is the muscular chamber behind it.

Based on its preserved anatomy, the opening could have been either horizontally oriented, like a bird's, or vertically oriented, like a crocodile cloaca, she said.

And, just like in most land vertebrates (except for mammals, which have more than one hole for defecation, urination and reproduction), this dinosaur used its hole for everything, which explains why researchers found a fossilized poop in its butt.

"It's like a Swiss Army knife of excretory openings," Vinther said.

The same Psittacosaurus cloacal vent was described in October 2020, when another team posted their research in the BioRxiv database, meaning it has yet to be peer-reviewed or published in a journal.

Vinther, who had shared his data with that team for another project, said the researchers used the cloacal vent data without his permission