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.
"It is very unique. Most cloacas form a kind of slit. Sometimes it's a vertical split, sometimes it's a smiley face, sometimes it's a sour face. This thing has a V-shaped structure with a pair of nice flaring lips and there's not a living groups of animals that have morphology like that," Vinther said. "It is somewhat similar to crocodiles but still unique."
The study, which published in the journal Current Biology on Tuesday, said that large, pigmented lobes on either side of the opening could have harbored musky scent glands, as seen in living crocodiles and alligators.
What's more, the outer margins of the cloaca are highly pigmented with melanin. While they don't know for sure what color it was, it likely would have contrasted with the dinosaur's pale underbelly, Vinther said.
This distinctive pigmentation could mean the vent was used to display and signal, similar to living baboons and some breeding salamanders.
The fossil is displayed at the Senckenberg Museum of Natural History in Frankfurt, Germany, but was found in a fossil-rich area of Liaoning in northern China.
Vinther had worked on the fossil before in 2016, reconstructing the dinosaur's color patterns, and it was only at the end of that study that he realized that the cloaca was really well preserved, he said.
In animals with cloacal vents, the genitals are tucked inside the body and haven't been preserved so it's not known whether this particular dinosaur was male or female.
Most birds, the only living relative of dinosaurs, mate by "cloacal kissing" -- by pressing together their openings. Some paleontologists think dinosaurs may have mated like this.
Vinther, however, believes that this dinosaur would have had a penis -- the fossilized opening is more similar to a crocodile's, which do, and there are some birds, like ostriches and ducks, which also have penises.
"From what we can see, this cloaca would not have been suitable for cloacal kissing," Vinther said. "It looks like it would have been penetrative sex."