Did dinosaurs - such as this Diplodocus - pee?
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Photo by Ilja Nieuwland, distributed under a Creative Commons Attribution 3.0 Unported license.
Did dinosaurs - such as this Diplodocus - pee?

The Surprising Science of Dinosaur Pee

Dinosaurs pooped. The fossil record makes that abundantly clear. Scores of coprolites – fossilized feces – not only allow paleontologists to better understand the diet and ecology of dinosaurs, but let educators pass the mineralized excrement around to students and knowingly ask “Do you know what you’re holding right now?” in the hope of a shocked reaction. But did dinosaurs ever pee? That question hasn’t exactly been at the top of paleontology’s list in terms of dinosaur mysteries. Feeding behavior, colors, sounds, and other biological details carry a bit more prestige. Nevertheless, curious fossils found at two distant locations might record how Jurassic and Cretaceous dinosaurs relieved themselves on lakeshores and sand dunes.

The first possible dinosaur pee trace to be discovered was described only recently. At a 2002 meeting of the Society of Vertebrate Paleontology, Katherine McCarville and Gale Bishop reported on a strange “bathtub-shaped depression” among dozens of dinosaur tracks just south of La Junta, Colorado. The scour, set in the 150 million year old stone of a long-lost lakeshore, measures approximately ten feet long, five feet wide, and ten inches deep. The shape is similar to splats McCarville and Bishop created by streaming water onto sand.

There was no sign of rock overhangs or other structures that could have spilled water onto the ground at the fossil site. The only sources for an elevated stream of fluid, McCarville and Bishop pointed out, were sauropods like Apatosaurus and theropods such as Allosaurus that criss-crossed the shore. The shallow pit could have held the liquid waste of a dinosaur.

Exactly what species of dinosaur created the trace is impossible to say, but, McCarville and Bishop suggested, “The volume of fluid required to form a scour structure as large as the one in question suggests it may represent the expulsion of liquid urine from one of the sauropod dinosaurs crossing the tracksite.” All the more reason to be wary should you ever find yourself standing below a Diplodocus.

As yet, no one has formally published a study on Colorado’s Jurassic pee streak. The only published instance of dinosaurs marking the ground in such a fashion comes from a site far from Colorado, in Brazil’s Paraná basin.

In 2004 Marcelo Fernandes and coauthors described a pair of fossil splatters found among the Paraná basin’s fossil sand dunes that were trod by mid-sized theropod and ornithopod dinosaurs sometime around the beginning of the Cretaceous. The better-preserved of the two urine traces was a divot and accompanying ripples measuring six inches across at the widest point and almost three quarters of an inch deep.

The Paraná basin impressions weren’t mucky tracks, which would have shown push-up rims and other features related to a foot compressing sand, but traces made by liquid. And while not quite so impressive as the kiddie pool-sized depression in Colorado, the traces were similar in size and shape to what the scientists got when they simulated dinosaur urination by dumping two liters of water onto sand from a height of about two and a half feet.

As with the Jurassic lakeshore that a sauropod presumably piddled upon, the only sources of fluid high enough to make the traces on the ancient sand dunes were dinosaurs. The occurrence of tracks and possible pee traces – technically termed urolites – makes for the strongest case yet. But did dinosaurs even have the anatomical equipment to urinate? To approach this question, Fernandes and colleagues turned to ostriches.


[First one, then the other. Did non-avian dinosaurs expel waste like this ostrich?]

Like all birds, ostriches eliminate waste through their cloaca – the single orifice where reproductive and excretory tracts end. But while many birds combine their dark-colored solid waste with white, semi-solid uric acid into a single messy package, ostriches expel their fluid in an impressive stream before jettisoning their feces. This method, Fernandes and coauthors concluded, could explain the traces of “an abundant falling stream of fluid” they found among the dinosaur trackways.

While no one has described a fossil of a non-avian dinosaur’s cloaca just yet, we can be confident that dinosaurs had such an arrangement. Both living dinosaurs (birds) and the closest living relatives of all dinosaurs (crocodylians) have cloacae, and so it’s highly likely that non-avian dinosaurs shared this trait. The question is whether non-avian dinosaurs expelled their solid and semi-solid waste together, like many birds, or they urinated and defecated separately as ostriches and crocodylians can.

The traces found so far hint that at least some dinosaurs urinated. Whether all non-avian dinosaurs did so isn’t clear. But at least paleontologists have a search image that will hopefully lead them to more cases of peculiar dinosaur divots. Keep that in mind should you ever be hiking over the remains of prehistoric landscapes, and watch your step. Not for worry of stepping in a dinosaur puddle, but for accidentally trampling over a rare vestige of a prehistoric bathroom break.

[And for much, much more on the science of dinosaur excrement, see Anthony Martin’s forthcoming book Dinosaurs Without Bones.]

References:

Fernandes, M., Fernandes, L., Souto, P. 2004. Occurrence of urolites related to dinosaurs in the Lower Cretaceous of the Botucatu Formation, Paraná basin, São Paulo State, Brazil. Revista Brasileira de Paleontologia. 7, 2:263-268

McCarville, K., Bishop, G. 2002. To pee or not to pee: evidence for liquid urination in sauropod dinosaurs. In: Annual Meeting of the Society of Vertebrate Paleontology. 62, 2002. Journal of Vertebrate Paleontology Supplement. 22, 3: 85 A