How Did Dinosaurs Reproduce? Eggs, Nests and the Evidence for Parents

No dinosaur gave birth to live young; every known species laid eggs with hard or leathery shells, like the birds and crocodiles that are their nearest living relatives. The eggs were smaller than people expect. The largest known, from a giant oviraptorosaur in China, are 60 centimetres long, and the eggs of the biggest sauropods, animals of 50 tonnes, were the size of a grapefruit, because an eggshell thick enough to support a larger egg would be too thick for the embryo to breathe through or break out of. Shells are found in a range of textures, smooth, ridged and pebbled, and their microscopic structure sorts them by family; in 2020 the discovery of soft-shelled eggs in two early groups suggested that the first dinosaur eggs were leathery, like a turtle's, and that hard shells evolved more than once. Eggs are known from every continent, and fragments are so common in some Cretaceous rocks of France and India that they were sold as curiosities before anyone knew what they were.

Dinosaurs laid in clutches, and the arrangement of the clutch shows how they laid. Oviraptorosaurs and other theropods laid pairs of elongated eggs in a ring or spiral, one pair at a time from paired oviducts, with the centre left clear, apparently for the parent to sit in. Sauropods dug pits, laid twenty to forty round eggs in a heap, and covered them with earth or vegetation, as crocodiles and some birds do today; a site in Argentina called Auca Mahuevo holds thousands of such nests across several square kilometres, where titanosaurs returned to the same floodplain to lay year after year for centuries. The duck-billed hadrosaurs made mounds in colonies, with nests spaced about an adult's length apart, and at Egg Mountain in Montana the nests of Maiasaura, the good mother lizard, contain hatchlings whose worn teeth and unfinished leg joints show they had been eating in the nest for weeks, fed or led to food by adults.

The brooding oviraptorosaurs settle one question directly: the adults sat on the clutch with their arms spread over the eggs in the posture of a nesting bird, and since the arms of these animals carried long feathers, the feathers were presumably the incubator. Chemistry adds detail. The isotopes in eggshell record the temperature at which the egg formed, and they show that oviraptorosaur eggs were laid at body temperatures like a bird's, while sauropod eggs were incubated at the cooler temperatures of a buried nest, warmed in some cases by volcanic ground. The evidence for care after hatching:

• •Adults fossilised on nests, in Mongolia, China and Montana, in brooding postures
• •Hatchlings in nests with worn teeth, showing they were fed there rather than leaving at once
• •A Psittacosaurus adult from China buried with 34 juveniles, perhaps a creche, perhaps a nest destroyed by a mudflow
• •Trackways of adults walking with juveniles, and bone beds in which a herd of one species includes all ages
• •Bird-like incubation temperatures for the feathered theropods, and reptile-like burial for sauropods

Dinosaurs hatched small and grew fast. A sauropod hatchling weighed a few kilograms and reached tens of tonnes in perhaps thirty years, adding several kilograms a day at its peak; Tyrannosaurus went from a chick to a nine-tonne adult in about twenty years, with a teenage growth spurt visible in the rings of its bones; and the hadrosaurs reached adult size in less than a decade. Rapid growth meant that predators could not afford to wait for their prey to get big, and it also meant that most dinosaurs died young: the majority of individuals in a bone bed are juveniles, and few of the giants lived past thirty. That a fifty-tonne animal started life inside an egg the size of a grapefruit, a scaling that no mammal approaches, is among the reasons sauropods could get so large, since no mother ever had to carry or bear a giant.

How dinosaurs mated is unknown, since the soft parts do not fossilise and the mechanics of a sauropod pair remain a matter of engineering conjecture; a single fossil of a Psittacosaurus with a preserved cloaca, described in 2021, is the only direct evidence of the anatomy involved. Whether they had the sex-determining chromosomes of birds or the temperature-dependent sex of crocodiles is undecided, and the answer bears on whether a cooling or warming climate could have skewed their sex ratios. And the courtship displays that the feathers and crests of many species suggest, scrapes in Colorado rock interpreted in 2016 as the nest-scraping dances of theropods, and the coloured crests of the hadrosaurs, are inferred from their living descendants, the birds, whose behaviour is now read backwards into the animals from which it came.

Telling male from female dinosaurs is usually impossible from a skeleton, but one tissue offers a clue. Female birds that are about to lay grow a special layer called medullary bone inside the hollows of their leg bones, a store of calcium they draw on to make eggshell, and it disappears after laying. In 2005 researchers reported the same kind of tissue inside the thigh bone of a Tyrannosaurus rex from Montana, and concluded that this individual was a female that died while producing eggs. Similar tissue has since been reported in a few other dinosaurs, including Allosaurus.

The finding does more than identify one skeleton's sex. When medullary bone turns up in animals whose growth rings show they were still growing, it means those dinosaurs began breeding before they reached full adult size, as many fast-growing reptiles do and most mammals do not. That fits the picture of short, risky lives in which starting early paid off. Scientists treat each case cautiously, since some bone diseases can produce tissue that looks similar, and chemical tests have been used to strengthen the identification in the best examples.

How long a dinosaur egg took to hatch was guesswork until researchers looked at the teeth of embryos. Teeth grow by laying down dentine in daily layers, and the faint lines between those layers, known as von Ebner lines, can be counted under a microscope, just as they can in the teeth of living crocodiles and mammals. Because dinosaur embryos began forming teeth well before hatching, counting the lines in a fossil embryo gives a minimum time spent in the egg, to which an estimate is added for the weeks before the first tooth appeared.

A study published in 2017 applied the method to embryos of two plant-eaters. Protoceratops, a sheep-sized horned dinosaur with small eggs, took roughly three months to hatch, and the duck-billed Hypacrosaurus, whose eggs were nearly the size of a football, took nearly six. Those figures are close to what reptiles need and about twice as long as birds would take for eggs of the same size. A long incubation means a nest exposed for months to predators, floods and cold, and some researchers have suggested it left non-bird dinosaurs at a disadvantage beside faster-breeding birds after the asteroid struck, though that remains a hypothesis.

Bird eggs come in many colours, from blue to speckled brown, and studies of fossil eggshell suggest that some dinosaurs laid coloured eggs too. In 2018 researchers reported traces of the same two pigments that colour modern bird eggs in the eggshells of several dinosaurs, including blue-green colouring in the eggs of an oviraptorosaur, a feathered relative of the birds.

In living birds, coloured and speckled eggs are usually laid in open nests, where colour helps camouflage the eggs or signal something to the parents, while birds that nest in holes often lay white eggs. Finding coloured eggs in dinosaurs supports the idea that some of them, at least, laid their eggs in open nests and tended them, rather than burying them completely as crocodiles do.

All dinosaurs laid eggs, small relative to the adult because shell thickness limited their size, in clutches arranged by family: theropods in rings that a feathered parent sat on at bird-like temperatures, sauropods in buried heaps warmed by the ground, hadrosaurs in colonies where hatchlings stayed and were fed. Brooding adults, fed nestlings and mixed-age herds show parental care in at least some groups, growth from hatchling to giant took a few decades at most, and how they mated and determined sex is still unknown.