The question of whether dinosaurs possessed penises is complex because the soft tissues that would reveal their reproductive anatomy rarely fossilize. Dinosaurs, like their living relatives, engaged in sexual reproduction, evidenced by fossilized eggs and nests. Answers must come from inferring the anatomy of long-extinct species based on the characteristics of their closest surviving kin and extremely rare fossil findings.
The Cloaca: The Shared Archosaur Anatomy
The fundamental structure for excretion and reproduction in dinosaurs was almost certainly the cloaca, a Latin term meaning “sewer” or “drain.” This singular posterior opening is characteristic of the Archosauria, the major group of reptiles that includes dinosaurs, crocodiles, and birds. Unlike most mammals, archosaurs use this single chamber for the digestive, urinary, and reproductive systems. This anatomical arrangement means all waste and reproductive products pass through the same orifice. The presence of a cloaca does not determine whether a male possessed an external intromittent organ, such as a penis.
Inference from Living Relatives
To understand if dinosaurs had a penis, scientists look to their closest living relatives: the crocodilians and birds. Crocodilians, the closest non-avian relatives to dinosaurs, possess a single, non-retractable phallus that is permanently erect but stored internally within the cloacal chamber. This organ is composed of fibrous tissue and becomes engorged with lymph, not blood, for copulation. They use this organ for internal fertilization, suggesting that the ancestral archosaur may have also possessed such a structure.
Birds, which are modern dinosaurs, show a wide variation in reproductive anatomy. The majority of bird species, approximately 97%, have lost a functional intromittent organ and instead reproduce via a “cloacal kiss,” a brief touching of the cloacal vents to transfer sperm. However, basal bird species, such as ostriches, emus, and waterfowl, have retained a functional phallus.
This anatomical diversity in living archosaurs complicates the inference for extinct dinosaurs. If the ancestral dinosaur possessed a crocodilian-like phallus, many species may have retained it. The structure of a dinosaur’s body, particularly the massive tails and heavy torsos of species like Tyrannosaurus rex, suggests that some form of penetrative organ would have been necessary for successful mating, especially for large species that would have difficulty performing a cloacal kiss.
Rare Fossil Discoveries and Direct Evidence
Direct evidence of dinosaur reproductive soft tissue is extremely limited. However, one exceptional find has offered a glimpse into a dinosaur’s rear anatomy: the remarkably preserved cloacal vent of a small, horned dinosaur called Psittacosaurus. This fossil, dating back to the Early Cretaceous period, provided the first clear look at the external opening of a non-avian dinosaur.
The Psittacosaurus cloaca shares features with both crocodiles and birds. It had distinct lateral lips and was highly pigmented with melanin, suggesting it may have been used as a visual display for courtship. The fossil also showed evidence of possible scent glands on either side of the vent, an anatomical trait also seen in living crocodilians.
While the fossil offered no preservation of the internal reproductive organs, its crocodilian-like structure led researchers to infer a likelihood of internal fertilization. Scientists suggested that the animal’s reproductive mechanics were likely closer to those of crocodiles than to most birds, implying the potential presence of a stored, eversible phallus. This rare finding suggests that at least some groups of dinosaurs may have possessed a penis.