Lately, I’ve been wondering how gargantuan dinosaurs mated. I didn’t before, but now that the thought has occurred to me, here are some answers I found to related questions:

What evidence is there that dinosaurs used feathers in mating displays?

• Theropod dinosaurs, like T-rex and related species (especially those close to the bird lineage) had elaborate feather structures that likely served display purposes
• Microraptor and other dromaeosaurids (bipedal dinosaur family of the late Cretaceous which included deinonychus and the velociraptors) had iridescent feathers, suggesting visual signaling
• Fossils like Caudipteryx show tail feather arrangements possibly used in displays
• Many dinosaurs and their kin had crests and tail feathers in patterns consistent with sexual selection—much like the size and display of such anatomical features are used by modern reptiles and birds to attract mates and signal an individual’s fitness

Did male dinosaurs have penises?

• Most scientists believe male dinosaurs had internal phalluses (similar to modern crocodilians and some birds, such as ducks and ostriches)
• Some scientist believe that male Tyrannosauri were so fierce because their arms were too short to allow for autoerotic activity (OK, I just made that up)

• Soft tissue rarely fossilizes, so direct evidence is scarce
• Some dinosaurs likely had a cloaca (a single opening for excretion and reproduction) like most modern birds and reptiles
• The presence of a phallus is inferred based on their position between crocodilians (which have penises) and birds (which now mostly lack them-- because the gene that codes for their development is switched off, in most species, during fetal development)

What were the mating positions likely to have been for the largest dinosaurs?

• Males likely mounted females briefly from behind—perhaps with the female resting on her stomach, in the way that most birds mate.

• Weight-bearing was a major issue, especially for the largest sauropods, like diplodocus and brontosaurus (yes, that’s still a real dinosaur—they brought it back… maybe someday Pluto will be a planet again…)
• In some species, males might have balanced on hind limbs, but some paleontologists suggest "lateral positioning" where males approached from the side was the strategy for larger dinos
• Modern elephant and rhinoceros mating might provide some analogues
• Aquatic mating (using water buoyancy for support) has been proposed but remains speculative-- elephants sometimes mate in water, a behaviour I have witnessed myself

Did all dinosaurs lay eggs? 

• All dinosaurs laid eggs; this is confirmed by extensive fossil evidence:
• Eggs have been found for approximately 40-50 dinosaur species, representing most major groups
• Notable discoveries include oviraptorid, titanosaur, hadrosaur, and theropod eggs
• Egg morphology varies significantly between groups, with different shell structures and shapes

What dinosaur species likely displayed long-term care of their young?

• Oviraptorids (fossils found brooding on nests)
• Maiasaura ("good mother lizard" - evidence of extended nest care)
• Troodontids (sophisticated nest arrangements suggest active parenting)
• Many small theropods (particularly those close to bird ancestry)

Which did not?

• Some ornithischians (though evidence varies by group)
• Most ankylosaurs and stegosaurs (limited evidence of nesting behavior)
• Large sauropods (probably laid eggs and abandoned them)