The first described cloacal vent region in a non-avian dinosaur belongs to a fossil Psittacosaurus from China. Its preserved skin, scales and pigmentation reveal what the outside looked like—but not the animal’s sex, how it mated or what internal reproductive structures it had.
What was discovered?
In a paper published in Current Biology on 22 February 2021, Jakob Vinther, Robert Nicholls and Diane A. Kelly described the cloacal region of Psittacosaurus specimen SMF R 4970. The fossil comes from the Early Cretaceous Jehol deposits in Liaoning, China. The authors presented it as the first described cloacal vent region from a non-avian dinosaur. Read the paper in Current Biology.
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“Dinosaur butt” is a catchy shorthand; the anatomical term is a cloaca, or cloacal vent. In many vertebrates, a cloaca is a shared external opening for digestive, urinary and reproductive functions. The fossil preserves the external region, not its internal structures.
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Where is the cloaca in the fossil?
It is the opening at the rear underside of the animal, beneath the tail. In the preserved specimen, researchers identified it by the distinctive skin and scale pattern around the vent, rather than by an internal organ. The paper reports that the region can be retrodeformed to show its three-dimensional form.
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The University of Bristol’s release about the discovery includes an image and additional explanation of the preserved area. See the University of Bristol release.
What details did the fossil preserve?
The vent’s external anatomy differs from the nearby skin: it has a dorsal lobe and lateral margins, with distinctive surrounding scales and pigmentation. The Bristol release says the outer margins were heavily pigmented with melanin. These are observations of the fossil’s preserved exterior; they do not reveal the animal’s internal reproductive anatomy.
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What can’t this fossil tell us?
The cloacal region does not establish whether the dinosaur was male or female, how it mated, or whether it had a penis. The fossil does not preserve the internal features needed to answer those questions. As Diane A. Kelly put it in the Bristol release: “Those distinguishing features are tucked inside the cloaca, and unfortunately, they’re not preserved in this fossil.”
Could the cloaca have been used for signalling?
The authors suggested the external anatomy may have played a visual or olfactory signalling role. The Bristol release also reports their speculation that the lobes might have held scent glands. These are possible functions, not demonstrated behaviour: the specimen preserves pigmentation and shape, not signalling in life or scent glands.
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What did later work add?
A 2022 study of Psittacosaurus integument re-examined the same specimen using laser-stimulated fluorescence and described a longitudinal cloacal opening. It discussed possible crocodilian-like internal anatomy, but that interpretation remains an inference from the preserved exterior, not direct evidence of the cloacal interior. Read the 2022 study in Communications Biology.
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