Scientists are still banging out theories about how dinosaurs had sex

Dinosaur Mysteries digs into the secretive side of the “terrible lizards” and all the questions that keep paleontologists up at night.

“AND THAT’S HOW YOU MAKE a baby dino-sawr.” I wish it were as easy as Mr. DNA made it sound in the first Jurassic Park movie. Paleontologists have learned an astonishing amount about the “terrible lizards” in the past 200 years, from the colors of their feathers to the illnesses that left them with dino-sores, but there is one area of their lives that we know shockingly little about. We can be sure dinosaurs had sex to make generation after generation of reptiles, but thanks to the rarity of soft tissues in the fossil record, we are still left stumped by how they did it exactly. 

Peeking into the science of prehistoric proliferation isn’t voyeuristic. Sex is something life on Earth does with creativity and gusto. It’s the way that many organisms combine their individual genes and inevitably create new variations. These new traits—a different shade of scale, longer wing feathers, better resistance to infection—make a difference in survival and who mates with whom to pass on some of those traits. When we’re talking about evolution, we’re often just breaking down sex and its consequences. So as far as non-avian dinosaurs are concerned, we’re only just beginning to dig in.

A single fossil discovery could almost solve the problem in an instant. Paleontologists have found other prehistoric creatures whose quest for the “little death” became a more permanent one. In 2012 they reported on fossil turtles preserved in the middle of mating. The next year, a different research team reported on insects called froghoppers smooshed in the throes of arthropod passion 165 million years ago. In 2015, experts announced that 385-million-year-old armored fish had complementary fin modifications for mating, making them among the earliest known vertebrates to have penetrative sex. Which means it’s not impossible that a pair of Velociraptor or other dinosaur was preserved in flagrante in strata, maybe even with some of those mysterious soft parts.

But experts have yet to get lucky. The fossil record is incomplete and unevenly preserved, and it’s doubtful that universities or government funding agencies are going to start signing checks to search for how dinosaurs made the bed rock. Paleontologists have to work with the information they have without making museum security guards wonder why they’re taking such an interest in the back half of that Apatosaurus skeleton.

The nature of dinosaur reproductive organs is as good a place to start as any. Just last year, scientists announced that they had finally delineated the anatomy of a dinosaur’s butthole. A specimen of the horned Psittacosaurus found in Mongolian rocks from more than 100 million years ago came intact with the skin and some internal details for the area just under the tail. The parrot-beaked reptile had a cloaca—a single-use external opening at the end of the urinary, excretory, and reproductive tracts. (No wonder the term means “sewer” in Latin.) But the find confirms what paleontologists already expected based on the fact that both birds—which are living dinosaurs—and crocodiles have cloacae. And that might tell us something about what that vent held.

Dinosaur genitals certainly were not one-size-fits-all. During the Triassic, Jurassic, and Cretaceous periods, wildlife thrived in all sorts of shapes and sizes, which probably means dinosaur sex organs varied too. What was true for Psittacosaurus may not have held for Tyrannosaurus, Stegosaurus, or any other species. (In fact, famous as T. rex is, we don’t have any direct evidence of how they courted, mated, laid eggs, or nested, leaving us to hypothesize details from various relatives.) But we can be reasonably confident that non-bird dinosaurs had clitorises and phalluses, which is also a great statement to drop into the middle of cocktail party chatter.

With the exception of intersex reptiles, female alligators and crocodiles have a clitoris behind their vent, while males have a phallus. Many species of modern birds, too, have similar structures. Emus, ducks, and others have phalluses, and females of some avian species have clitorises—although the sexism prevalent in biology has kept us from drawing a full account. But the fact that the closest living relatives of Brachiosaurus and family had clitorises and phalluses hints that many prehistoric dinosaurs did too. In fact, sometimes it’s difficult to imagine how these beasts could have mated without organs to bridge the distance. While some songbirds bring their gametes into contact through a short “cloacal kiss,” it’s unlikely that amorous Ceratosaurus did the same with their huge bodies and lengthy tails.

But there’s more to sex than mechanics, of course. While we wait for a juicy fossil discovery, we can say a little bit about the moments leading up to mating in the Mesozoic. In recent years paleontologists have begun to reassess the horns, spikes, plates, and other “bizarre” structures that make long-dead reptiles endlessly fascinating. Most of these structures were once seen as weapons for attack and defense. Now, many of them seem to be biological signposts that only developed as the animals matured—sexual selection signals that were meant to be read by potential mates and rivals. So, an Ankylosaurus dotted with bony armor from its eyelids to its tail club didn’t evolve that look to only defend against tyrannosaur teeth: that was Late Cretaceous fashion brought about by season after season of dinosaur-mating choices.

Researchers have even found some of the places where singles flaunted their assets. They’ve used several fossil sites in Colorado to document where Allosaurus-like dinosaurs scraped at the ground with their taloned hind feet, scratching and kicking to impress other members of their kind just as puffins and other birds do today. Dinosaur displays were probably as unique and varied as their species were, but these tracks indicate that some big carnivores preferred a soil-spattered shuffle to start the romance. Perhaps, as fossil track expert Anthony Martin mused in his book Dinosaurs Without Bones, one day an expert or amateur will find tracks of a mating pair that will help us break down the dance steps like an archaic TikTok video.

As long as we have to rely on rare clues from a fragmentary record, the sex lives of dinosaurs will always be incomplete. Whatever we might still learn is held close by the rock. Rather than being a silly aside, however, the question of how dinosaurs reproduced is part of their enduring success story, a truly vital part of ancient lives that we can just barely touch through tooth and bone. They didn’t make it for more than 200 million years without doing something right. 

We hope you enjoyed Riley Black’s column, Dinosaur Mysteries. Check back on PopSci+ in March for the next article.