However, a fossil from China of a Psittacosaurus is so well preserved that the opening the Labrador-size dinosaur used to pee, poop and reproduce is visible, allowing paleontologists to study it for the first time.
While it doesn't offer any concrete answers on how dinosaurs may have procreated, it does give some hints.
"We don't have any dinosaur fossils where you can be confident they've been caught in the act," said Jakob Vinther, a paleontologist and senior lecturer at the University of Bristol's School of Earth Sciences.
What we know is "based on natural history where we compare it to living groups of animals."
While most mammals have separate holes for bodily functions, many other animals -- including birds and reptiles -- have just one and it's known as the cloaca.
The fossilized cloaca confirms that dinosaurs had one but it doesn't look like that of any other living animals.