3-D Reconstruction of Fossil Reveals Secret Sex Life of Dinosaurs

The newly discovered orifice is the oldest known fossilized cloacal vent in existence

A view of the full fossilized dinosaur above another image of a closer view of the fossilized cloaca
This fossil is the oldest known preserved dinosaur cloacal vent. Jakob Vinther, University of Bristol and Bob Nicholls/Paleocreations.com 2020

Paleontologists reconstructed a dinosaur cloacal vent for the first time, giving insight into how dinosaurs may have used this opening to defecate, urinate, breed and lay eggs—just like birds and reptiles do today. The fossil, currently on display in the Senkenberg Natural History Museum in Frankfurt, Germany, belonged to the Psittacosaurus, a dog-sized dinosaur with bird-like hips and a beak that walked the Earth during the Cretaceous period between 100 to 122 million years ago.

The Frankfurt specimen was first unearthed in China decades ago, but the well-preserved cloaca wasn't noticed until paleontologist Jakob Vinther, of the University of Bristol England observed the flattened cloaca when studying the preserved dinosaur's skin for evidence of camouflage, reports Chris Stokel-Walker for New Scientist. The researchers describe their findings this week in the journal Current Biology.

The cloaca was so well preserved that Vinther realized it could be reconstructed using 3-D computer modelling, reports Katherine J. Wu for the New York Times. Scientists studied preserved rear ends of birds, reptiles, and other vertebrates to figure out what the cloacal vent may have looked like. To do this, Vinther recruited help from paleoartist Robert Nicholls and Diane Kelly, a biologist from the University of Massachusetts in Amherst, who is an expert in the evolution of genitalia, reports the New York Times.

The Psittacosaurus's reconstructed cloaca shows the opening is lined with dark-colored lips pinched at only one end and included scent glands on each side that may have released pheromones to attract mates, reports the New York Times. The opening could have also been either oriented horizontally like a bird or vertically like a crocodile, Diane Kelly, a biologist of the University of Massachusetts Amherst, tells Laura Geggel of Live Science.

"It is very unique. Most cloacas form a kind of slit. Sometimes it's a vertical split, sometimes it's a smiley face, sometimes it's a sour face. This thing has a V-shaped structure with a pair of nice flaring lips, and there's not a living group of animals that have morphology like that," Vinther tells Kate Hunt for CNN. "It is somewhat similar to crocodiles but still unique."

A cloaca is present in modern-day animals, including sharks, ducks and amphibians as well. It is not present in placental mammals or bony fishes.

The highly pigmented cloaca also suggests that it may have been used as a visual mating display, similar to the way baboons use their behinds to woo a partner, reports CNN. Study authors suggest the dark pigmentation could have also protected the dinosaur from microbial infections.

Researchers still do not know the sex of this particular dinosaur. Animals with cloacal vents usually tuck their genitals inside the body, but only the cloaca was preserved in the Frankfurt specimen.

The specimen did, however, have fossilized fecal matter poised for an exit, “right near where it’s supposed to” be, Vinther told the New York Times.