Scientists Have Finally Found a Dinosaur's Butthole, and It's Pooping

A dinosaur that died some 120 million years ago has left behind such an exquisitely preserved cloacal opening—an orifice used for defecation, urination, and copulation—that scientists have been able to describe this multi-purpose organ in detail for the first time.

The “unusually fine preservation and fortuitous orientation” of this early Cretaceous genital-bum combo, which belonged to a dog-sized dinosaur called Psittacosaurus, enabled paleontologists to reconstruct its cloaca in three dimensions, according to a study published on Tuesday in Current Biology. 

Led by Jakob Vinther, a paleontologist at the University of Bristol, the researchers added that ”no other non-avian dinosaur fossil preserves the cloaca” to their knowledge. The work could answer questions about the sexual lives of dinosaurs, the mechanics of their waste removal systems, and the similarities between cloacas in extinct and modern animals.  

“Dinosaur skin is rare and often patchy and when we look at those fossils, the cloacal region seems to be always missing,” Vinther said in an email. “If you think of when animals decay, they often rupture through this opening, being the weakest part of the body surface.”

The Psittacosaurus specimen, however, was buried in a lake and entombed in a deoxygenated environment of mud and volcanic ash, which curbed its decay and prevented scavengers from tearing it to shreds. The animal also just happened to be oriented at an angle that showed off its cloaca, as well as what appears to be a fossilized piece of poop in the orifice.

“It is a combination of how it is exposed where you see it from below at an angle and then also the overall exquisite preservation that we have,” Vinther said.

The rare specimen reveals tantalizing insights about this under-studied part of dinosaur anatomy, including hints that the animal may have used visual or scent signals to communicate with other animals, perhaps in mating displays.

From our mammalian perspective as humans, it might seem very odd to have just one hole, or “vent” as the orifice is typically called, performing such different functions. But we are actually the outliers: Most vertebrate animals have evolved cloaca to tend to their sexual and waste removal needs, as opposed to the multiple vents mammals use to accomplish the same tasks.

Birds, reptiles, amphibians, and fish are among the animals that possess cloaca, providing scientists with a comprehensive dataset for studying how these vents operate in modern biological contexts. But because cloacas do not fossilize as easily as bone or scales, paleontologists are left with little to work with when trying to understand the finer mechanics of how dinosaurs pooped, peed, and bred.

The Psittacosaurus specimen, which belongs to the Senckenberg Museum of Natural History in Frankfurt, Germany, offers a stunning exception to this rule. Vinther first glimpsed the dinosaur remains in 2009, while at the museum to study another fossil set. 

“We took a look at it in the exhibit and I noticed the color patterns preserved,” Vinther said. ‘In 2016 we described those color patterns and then I also noticed the cloaca, which is what we describe now.”

The orifice is so well preserved that there even appears to be “an underlying cream-coloured amorphous mass inferred to be a coprolite”—the term for fossilized feces—“immediately inside the cloacal opening,” the study reports. In other words, the dinosaur may have had a round of poop in the chamber when it died, which was fossilized along with its cloaca.

It’s not clear whether Psittacosaurus had a slit-shaped vent, which is seen in crocodylians, or a rounded vent, which is seen in birds. But there is clear pigmentation around the specimen’s cloacal opening, as well as some characteristic swellings next to the vent, which could open a window into the potential behavior of Psittacosaurus. 

Vinther and his colleagues speculate that the dinosaur may have flashed visual signals to other animals with its pigmented colors. The swellings may have contained scent glands, similar to those seen in modern crocodilians, enabling the dinosaur to communicate messages, such as its receptivity to mates. 

“We see that the dinosaur made the cloacal opening visually attractive,” Vinther said, adding that “this may be a rare glimpse back to a time when dinosaurs roamed the Earth and strutted their rear-end multipurpose opening at each other in order to get lucky.”