The cloaca is the black, ridged area on the underside of the tail, at the centre of the image
3D Camouflage in an Ornithischian Dinosaur, Current Biology (2016) Psittacosaurus sp. SMF R 4970, Whole Specimen
A fossil dinosaur originally discovered in northwestern China is so exquisitely preserved that the shape of its cloaca – the opening used for excretion and mating – is visible for the first time.
The evidence has actually been in plain sight. The psittacosaurus – a kind of early ceratopsian related to Triceratops that lived around 120 million years ago – has been on public display at the Senckenberg Museum of Natural History in Frankfurt, Germany, for over a decade and several scientific papers have already been written about its primitive feathers and colouring.
Only now, though, has a team led by Phil Bell at the University of New England in Australia formally described the cloaca. Bell declined to discuss the finding until the paper is published in a peer-reviewed journal.
Birds and reptiles have a cloaca – a single orifice used for excretion, urination, mating and laying eggs – so it has always been assumed that dinosaurs had them too. The cloaca of the psittacosaurus confirms this expectation.
Only the external part of the cloaca has been preserved. The vent is around 2 centimetres long, is flush with the surrounding area rather than protruding as some cloacas do, and is surrounded by darkly pigmented tissue (see picture, above).
The internal anatomy has not been preserved, so the fossil doesn’t definitively resolve questions about how dinosaurs mated. However, the cloaca has a longitudinal opening like those of crocodiles, which do have penises. By contrast, most birds – the living descendants of dinosaurs – do not.
“It is a triumph of discovery to have such a delicate region so perfectly preserved in a fossil so old,” says John Long of Flinders University in Australia, who wasn’t involved in the research. “We have various other different parts preserved but not a cloaca.”
Unfortunately, it doesn’t reveal much. It isn’t possible to tell the sex of this particular animal, but the cloaca’s resemblance to those of crocodiles suggests that this type of dinosaur had a penis.
“The crocodilian-like vent of psittacosaurus implies that, unlike lizards and later-diverging birds, psittacosaurus probably had a muscular, unpaired, and ventrally-positioned copulatory organ,” the researchers write in their paper.
Most birds mate cloaca to cloaca – called cloacal kissing – so many biologists assume dinosaurs mated this way too. However, some birds such as ducks and ostriches have long, flexible penises that emerge from their cloacas during mating. In ducks, the erection process, called eversion, takes just a third of a second.
It is thought that the ancestors of modern birds had penises, so it is plausible that the dinosaurs from which they evolved had them, too. It would have been very difficult for large dinosaurs to mate without very long penises, says Long.
Reference: bioRxiv, DOI: 10.1101/2020.10.11.335398
More on these topics: