Although paleontologists know now much about dinosaurs and their appearance, they have not known anything about how their cloacal region — the all-purpose opening used for defecation, urination and breeding — appears. Paleontologists from the United Kingdom and the United States have now described the cloacal opening of Psittacosaurus, an ornithischian dinosaur that lived about 110 million years ago (Early Cretaceous epoch) in what is now China, and compared it to vents across modern land-dwelling vertebrate animals.
Dr. Jakob Vinther from the School of Earth Sciences at the University of Bristol and colleagues examined an exceptionally well-preserved specimen of Psittacosaurus sp. from the Early Cretaceous Jehol deposits of Liaoning, China.
The fossil is currently housed at the Senckenberg Museum in Frankfurt, Germany.
“I noticed the cloaca several years ago after we had reconstructed the color patterns of this dinosaur using a remarkable fossil on display at the Senckenberg Museum which clearly preserves its skin and color patterns,” Dr. Vinther said.
“It took a long while before we got around to finish it off because no one has ever cared about comparing the exterior of cloacal openings of living animals, so it was largely unchartered territory.”
“Indeed, they are pretty non-descript,” said co-author Dr. Diane Kelly, a researcher in the Department of Psychological and Brain Sciences at the University of Massachusetts Amherst.
“We found the vent does look different in many different groups of tetrapods, but in most cases it doesn’t tell you much about an animal’s sex.”
“Those distinguishing features are tucked inside the cloaca, and unfortunately, they’re not preserved in this fossil.”
The cloaca of Psittacosaurus is unique in its appearance but exhibits features reminiscent to living crocodylians such as alligators and crocodiles, which are the closest living relatives to dinosaurs and other birds.
Its outer margins are highly pigmented with melanin. This pigmentation provided the vent with a function in display and signaling, similar to living baboons and some breeding salamanders.
The large, pigmented lobes on either side of the opening could have harbored musky scent glands, as seen in living crocodylians.
Birds are one the few vertebrate groups that occasionally exhibit visual signaling with the cloaca, which the study authors now can extend back to the Mesozoic dinosaur ancestors.
“As a paleoartist, it has been absolutely amazing to have an opportunity to reconstruct one of the last remaining features we didn’t know anything about in dinosaurs,” said Robert Nicholls, of Paleocreations.
“Knowing that at least some dinosaurs were signaling to each other gives paleoartists exciting freedom to speculate on a whole variety of now plausible interactions during dinosaur courtship. It is a game changer!”
The team’s paper was published in the journal Current Biology.
Jakob Vinther et al. A cloacal opening in a non-avian dinosaur. Current Biology, published online January 19, 2021; doi: 10.1016/j.cub.2020.12.039