Did Dinosaurs Look After Their Young? Nests, Colonies and a Fossil Caught Sitting
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In 1978 a fossil hunter in Montana found a nest containing the bones of fifteen juveniles whose teeth were already worn from feeding and whose legs were not developed enough to walk. They had been in the nest for weeks, they were eating, and somebody had been bringing them food. The animal was named Maiasaura, the good mother lizard, and the find began a shift in how the whole group is imagined, from reptiles that abandoned their eggs to animals whose behaviour looked much more like a bird's.
The nesting ground in Montana
The site, at what became known as Egg Mountain, was found by Marion Brandvold and worked by Jack Horner and Robert Makela from 1978. What it produced was not one nest but a colony: bowl-shaped mounds of earth roughly two metres across, spaced about seven metres apart, which is the body length of an adult, arranged over a wide area and repeated through several layers of sediment, meaning the same ground was used season after season. Each held up to about twenty eggs arranged in a spiral, and some contained hatchlings and older juveniles. Two details carried the argument. The teeth of the juveniles showed wear, so they were eating solid plant material while still in the nest, and their leg joints were not fully formed, so they could not have gathered it. Something brought food to them, which is parental care by any definition, and the spacing of nests one adult apart implies adults moving among them.
The oviraptor rehabilitation
The other decisive evidence came from correcting a seventy-year-old mistake. In 1923 an expedition to Mongolia found a small theropod lying on a clutch of eggs, assumed the eggs belonged to the abundant Protoceratops nearby, and named the animal Oviraptor, egg thief, on the theory that it had been raiding the nest when it died. In 1993 an expedition to the same region found an embryo inside one of those eggs, and it was an oviraptorid. The eggs were its own. Several specimens have since been found in the same posture, crouched over a clutch with the arms spread back and around it, which is precisely how a modern bird broods, and the arms in question carried feathers. The animal named for stealing eggs turned out to have died protecting them, and the name cannot be changed under the rules of zoological nomenclature.
What the eggs and nests show
Fossil eggs are abundant, and the variation in them maps onto different strategies:
- •Egg shape and shell porosity separate eggs buried in mounds of vegetation, which need to exchange gas through damp material, from eggs brooded in the open, which have thinner, less porous shells
- •Clutch arrangement varies from rough piles to careful spirals and paired rows, the last reflecting an animal laying two eggs at a time, as birds do and crocodiles do not
- •Colonial nesting grounds are known for hadrosaurs, sauropods and several theropods, with one site in Argentina covering several square kilometres of sauropod eggs
- •Some nests are clearly covered and some are clearly not, which corresponds to whether adult body weight would have crushed them
- •Eggshell chemistry has been used to estimate incubation temperature, and growth lines within the shell and inside embryonic teeth give incubation times, which for the species measured run to three to six months, far longer than any bird
Who sat on the nest
A study published in 2008 by David Varricchio and colleagues tackled a question that seems unanswerable and is not. In living animals, the amount of parental care correlates with the ratio of adult body mass to clutch mass, and the pattern differs by who does the caring: species with male-only care, which includes many ground-nesting birds such as emus and rheas, have unusually large clutches for the adult's size. Measured against that relationship, the oviraptorids and troodontids found brooding fall into the male-care group. A separate line of evidence supports it, since medullary bone, a special calcium reserve present in female birds forming eggs, has been identified in some dinosaur fossils and was absent in the brooding specimens tested. The tentative conclusion is that paternal care came first, that it was inherited by birds, and that the maternal incubation typical of most modern birds is the later arrangement.
Not all of them
The picture is not uniform, and the initial enthusiasm overcorrected. Sauropods are the clearest counterexample: an enormous site at Auca Mahuevo in Argentina shows thousands of eggs laid in shallow scrapes over many seasons, with no evidence of nest attendance, no arrangement suggesting a brooding adult, and a hatchling the size of a rabbit emerging from a parent weighing thirty tonnes, which makes sustained care physically awkward and probably unnecessary in an animal producing hundreds of offspring. The likeliest reading across the group is a spectrum, with small feathered theropods brooding like birds, hadrosaurs feeding altricial young in colonies, and the giants laying many eggs and leaving. That spread is exactly what modern reptiles and birds show, and the useful correction to the older view is not that all dinosaurs were good parents but that the question has different answers for different animals and can actually be tested.
The takeaway
A colonial nesting ground in Montana found in 1978 produced juveniles with worn teeth and undeveloped legs, meaning adults brought food to the nest, and specimens of oviraptorids crouched over their own clutches in a brooding posture corrected a 1923 misidentification of an egg thief. Egg shape, shell porosity and clutch arrangement distinguish buried nests from brooded ones, clutch size relative to adult mass suggests the brooding parent was male, and sauropod sites show enormous numbers of eggs with no sign of attendance.