When you think about dinosaurs, does the creature’s anus come to mind? It was certainly not addressed directly in Jurassic Park, a 1993 documentary about paleontological experimentation.
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The anus is soft tissue, not bone, and thus unlikely to be preserved by the fossil record. But Slate author Riley Black directs us to this scientific study about the subject. Researchers Phil R. Bell, Michael Pittman, Thomas G. Kaye, and Christophe Hendrickx have carefully studied the Psittacosaurus, for which a few soft tissue samples remain preserved to modern times. They conclude that this dinosaur had a cloaca similar to that of crocodilian species. From their abstract:
Here, we describe the outer morphology of the only known non-avialan dinosaur cloaca, preserved in an exceptional specimen of the early-diverging ceratopsian dinosaur Psittacosaurus. We clarify the position of the cloaca with respect to the ischia and caudal vertebrae and document the scales immediately adjacent to the abdomen and tail. We find that the cloaca is from a near-sexually mature subadult individual and is most similar to the cloaca of crocodylians, to the exclusion of lepidosaurians and birds.