Asked by: Barry Cull, Taumarunui, New Zealand
Probably a bit like giraffes. We can’t be sure because reproductive organs don’t fossilise, and no fossils of dinosaurs caught in the act have ever been found. We don’t even know for certain that dinosaurs had penises, although it’s likely. Birds are descended from dinosaurs and the most primitive bird lineages have penises, including ducks and ostriches.
An animal with a penis, and eggs with waterproof shells, suggests internal fertilisation. The problem is how a female Brontosaurus managed to avoid being crushed beneath the weight of a 16-tonne male, or a male stegosaurus escaped impalement on the female’s dorsal plates. It would certainly have been a precarious business, but not (ahem) insurmountable.
The actual moment of copulation was probably very brief, to minimise the mechanical stresses on both partners. The male may have thrown one leg over the female’s tail and used a relatively long, extensible penis to reach the female’s genitalia.