Everything You Want to Know About Dinosaur Sexual Anatomy and Reproduction, From Their Genitalia to How They Laid Eggs

Dinosaurs had sex. Fossil nests and eggs, as well as the ways today’s birds and crocodiles reproduce, leave no doubt on that point. What has consistently stumped paleontologists, however, is just how dinosaurs made the bed rock.

It doesn’t take an expert to look at the skeleton of a Stegosaurus and realize that dinosaur sex must have been anatomically complicated. Especially for large species, two multi-ton animals would have to coordinate getting close enough to mate without falling over—or on each other. Until a lucky scientist finds two intertwined dinosaurs that perished during that special moment, we may never fully grasp the mechanics of dino sex. Nevertheless, clues from modern animals and a few exceptional fossils hint at the anatomical setup that would have determined how dinosaurs did it.

Dinosaur skeletons outline where their reproductive organs would have been. All dinosaurs had hips made up of three major bones—the blade-like ilium on top, with the pubis and ischium underneath—that left space for their urinary, excretory and reproductive tracks. On a dinosaur like Tyrannosaurus rex, for example, there is a significant space between the backward-pointing ischium and the upper hip bone where all these ducts and soft tissues would have run.

The challenge facing paleontologists has been to envision the soft tissues that existed between those bones. Birds and crocodilians helped experts outline their anatomical expectations. Both living, avian dinosaurs and crocodilians have a single external opening under their hips called a cloaca, which is Latin for “sewer.” The single external slit is the single exit for a chamber that contains the exit points for urine and feces, and also conceals the genitals when they’re not in active use. Given that the trait is shared between dinosaurs and their crocodilian cousins, paleontologists expected non-avian dinosaurs like Apatosaurus and Triceratops to have cloacae, too.

A unique fossil of the small horned dinosaur Psittacosaurus confirmed paleontologists’ expectations in 2021. Soft tissues preserved with the bones indicated that the dinosaur had a cloaca with a vertical slit, like crocodiles, just behind the lower hip bone. While not necessarily a surprise, the find confirmed that dinosaurs had a crocodilian-like orifice that closed off their genitals to the outside world when not in use.

The Psittacosaurus cloaca only preserved the external opening of the dinosaur, however. The fossil provided no insight into what dinosaur genitals looked like. Birds and crocodilians are still the best guides to what extinct dinosaurs were like, their anatomy acting as an outline for what paleontologists expect of Edmontosaurus and kin.

In crocodiles and their relatives, genitals concealed behind the cloaca come in two forms. Some crocodilians have a phallus, and others have an equally large and anatomically similar clitoris. Even experts can have a difficult time telling the two forms apart. Paired with the fact that birds that are generally thought of as more archaic, such as ostriches and waterfowl, have phalluses and clitorises, too, it’s all but certain that dinosaurs like Brachiosaurus also did.

The fact that non-avian dinosaurs had what experts often call “intromittent organs” to pass gametes makes anatomical sense. While many modern songbirds pass sperm by way of a “cloacal kiss” in which the openings of both birds touch so that their genitals are close enough, these avians do not have to worry about the big, bulky tails that were practically standard for non-avian dinosaurs. Having a phallus able to bridge the distance from one cloaca to another surely would have been important for dinosaurs from Stegosaurus to T. rex, perhaps especially among large or armored dinosaurs where attaining the best position would have been challenging.

While paleontologists can be confident that all non-avian dinosaurs had cloacae, the anatomy of dinosaur phalluses and clitorises is a total mystery. The vast diversity of dinosaur shapes and sizes would also have resulted in an array of genital configurations, influenced by everything from body size to mating behavior. According to a 2006 estimate by Steve Wang and Peter Dodson, there may have been more than 1,850 genera of dinosaurs during a span of more than 150 million years. If modern birds are any indication, then all those different dinosaurs might have had vastly different genitals and mating behaviors—meaning there was no “one size fits all” anatomical or behavioral setup for dinosaur sex. Even among birds that have intromittent organs, for example, some ducks have penetrative sex with complex genitals in which curled vaginal tracts evolved in tandem with corkscrew-shaped phalluses, while cassowaries of all sexes have a pseudo-penis, which doesn’t even work the same way a mammalian penis does for passing sperm. The vast number of non-avian dinosaurs likely had very different genital variations determined by everything from the mechanics of dinosaur sex to what dinosaurs found alluring when courting.

Future finds will no doubt alter what paleontologists hypothesize about how dinosaurs reproduced. But mating was just one part of the reproductive process that has kept new generations of dinosaurs hatching on this planet since the Triassic. A smattering of fossils have indicated what happened after mating, offering insights into the internal workings of egg-laying dinosaurs.

One of the key fossils relevant to dinosaur reproduction is a pelvis of an oviraptorosaur—one of the feather-covered, beaked dinosaurs that were relatively close cousins of dinosaurs like Velociraptor—with two eggs preserved inside. Described in 2005 by Tamaki Sato and colleagues, the hips show that the female oviraptorosaur had died just before laying those eggs. This fortuitous discovery illustrated that at least some dinosaurs had a mix of bird- and crocodilian-like reproductive features.

While female birds have only one oviduct—thought to be an adaptation related to becoming light enough to fly—the presence of two eggs in the dinosaur suggested the presence of two oviducts, as in crocodilians. But the fact that there were only two eggs indicated the dinosaur laid a small number of eggs at a time, more similar to birds and less like reptiles that lay bigger egg clutches all at once. Instead of laying a large clutch all at once, like a crocodilian, the dinosaur only laid two eggs each round and arranged those pairs around the nest. (Oviraptorosaurs have famously been found preserved on top of nests that seem to show a ring of paired eggs.) The dinosaur had two functioning oviducts like a crocodile, in other words, but produced one egg at a time in each, more similar to a bird, indicating that dinosaurs were not exactly like either group of their modern relatives but had traits of both.

Abnormal eggs from some of the largest dinosaurs bolster the connection. In 2022, Harsha Dhiman and colleagues reported on a strange titanosaur egg found in India. The egg had two yolks inside, separated by another internal shell, from a mishap during development. The egg was likely pushed backward toward the part of the oviduct where shells form, a second egg forming around the first. Birds, more so than other reptiles, sometimes produce eggs like this, hinting that egg-laying dinosaurs made their eggs more like birds than turtles. Scientists aren’t sure whether all dinosaurs shelled their eggs one at a time or could create clutches, as crocodiles do, but eggs with multiple shells hint at a more bird-like process that’s likely been in place since the days of the earliest dinosaurs in the Triassic.

We still don’t know much about dinosaur sex. From possible positions to anatomy, mysteries abound. But the subject has moved beyond silly speculation. A better understanding of dinosaur evolutionary relationships has given paleontologists a framework from which to hypothesize about different aspects of dinosaur reproduction, and those ideas have been tested by discoveries in the fossil record. Future finds and analyses will undoubtedly flesh out some of the remaining unknowns. We are only just beginning to discover some of the most intimate secrets of dinosaur lives.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.