Scientists characterize a dinosaur cloaca, or vent, for the first time

For the first time, scientists have described the cloaca, or vent, of a dinosaur -- the all-purpose opening used for reproduction and waste disposal.

Many mammals have distinct openings for defecating, urinating and reproduction, but most vertebrates boast a single hole. The latest discovery, published Tuesday in the journal Current Biology, suggests dinosaurs were no different.

Over the last decade, scientists have unearthed dinosaur fossils featuring feathers, as well as preserved skin pigments, but the latest discovery marks the first time researchers have characterized a cloaca.

"We simply didn't have any fossils preserving those parts," lead study author Jakob Vinther told UPI in an email.

"Preserving skin or feathers is quite rare, but often even when we find such fossils they are still incomplete and hence we didn't have the necessary areas preserved to see a thing such as the cloacal vent," said Vinther, a senior lecturer in macroevolution at Bristol University in Britain.

"During decay all the gases build up inside your digestive system and would explode out eventually and often that may be through the cloacal vent," Vinther said.

Vinther first noticed the outlines of the ancient cloaca several years ago while reexamining an extremely well-preserved specimen of a Psittacosaurus dinosaur housed at the Senckenberg Museum in Germany.

Because there was almost nothing for Vinther and his colleagues to use a reference point for their analysis of the cloaca, the work was slow-going. Even surveys of cloacal openings among modern vertebrates are limited.

Unfortunately, even well-preserved cloaca fossils, whether from dinosaurs or other vertebrates, provide few details about a specimen's working anatomical features, sexual or otherwise.

The new findings highlight the difficulties of determining the sex of ancient dinosaur specimens.

"Since we rarely ever find these parts preserved we haven't got much of anything to go from. Even with this cloaca we don't see a penis, if it had one," Vinther said.

"The penis would be tucked away inside and wouldn't reveal itself much from the way the cloaca appear. But if we had enough fossil specimens of the same species, we may be able to see some variation that could enable us to tell sexes apart," he said.

The dinosaur cloaca boasted features similar to those of crocodilian vents.

Preserved pigments around the outside of the cloaca suggest the dinosaur's vent was also used for sexual signaling, similar to baboons and some modern salamander species.

"It resembles crocodiles a bit by having a pair of lips on either side, but they don't converge in a backwards direction as they do in crocodile, so it is very much its own anatomy," Vinther said.

Though birds are prolific sexual signalers, their cloacas aren't all that conspicuous. As such, birds rarely rely on vents for display behavior.

"Birds engage in visual signalling like few other animal groups and we have been able to trace many aspects of bird behavior back to dinosaurs," Vinther said. "Perhaps these dinosaurs with their much more visible cloacas used them much more frequently for display, which our fossil is a unique glimpse back into."

Sexual anatomy is often a site of rapid diversification, and according to Vinther, the latest discovery suggests dinosaurs evolved a variety of different cloacal designs.

For now, however, scientists have only one example of dinosaur cloacal diversity.

"We can only cross our fingers that more fossils will eventually become available," Vinther said. "But it's a big ask."

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