For more than a century, paleontologists have believed that all dinosaurs reproduce by laying eggs. After all, no dinosaurs ever gave birth to living cubs (nor did their modern bird descendants), and nesting sites from Montana to Mongolia indicate that prehistoric dinosaurs laid clutches of strong eggs.
However, gradually new research began to change this pattern of dinosaur breeding.
Until now, paleontologists believed that all dinosaurs lay eggs in hard shells. A recent study by University of Calgary paleontologist Darla Zelenitskaya and her colleagues found that some dinosaurs, such as the 73 million horned protoceratops dinosaur and the 215 million long-necked Mussaurus dinosaur, lay soft-shell eggs similar to the leathery eggs of some modern reptiles. Comparing these findings to the dinosaur family tree, the authors of the article put forward the unexpected idea that all dinosaurs originally laid eggs with soft shells. Over time, at least three different genera have independently developed hard-shell eggs.
This study could help explain why dinosaur eggs are harder to find than many paleontologists would expect, because softer eggs are less likely to petrify. And figuring out which dinosaurs laid which types of eggs is important in answering the big questions about dinosaur parenting. This is because there is no typical dinosaur nest. Some species laid many round, hard eggs in a heap. Others laid eggs in pairs and laid them out neatly. Some eggs are spheres. Some of them are tapered. As with modern birds, different types of eggs are associated with the behavior of adult dinosaurs.
"Even among the hard dinosaur eggs alone, there are significant differences in the architecture of the eggshell," says University of Calgary paleontologist Darla Zelenitskaya. "This varied shell structure indicates very different nesting styles, incubation methods and the time between egg-laying and hatching."
Dinosaur rearing picture
One controversial issue is whether the parents of the dinosaurs stayed behind and guard their eggs, or, like today's sea turtles, laid them and then left the offspring to their fate. The answer seems to depend on the species.
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For example, consider the parrot-like dinosaurs called oviraptorids. Paleontologists have found magnificent skeletons preserved in such a position as if they were sitting over nests of eggs. “It's tempting to call it brooding like living birds,” says San Diego Natural History Museum paleontologist Ashley Post, “but we're still not sure if this was part of their behavior.” Yet the details indicate that dinosaurs built their own. nest with care. Scientists know from previous finds that oviraptorids lay two eggs at a time in a clutch of 30 or more eggs. “This means that the mother would have to stay with the nest, or at least return to it, lay her pair of eggs, arrange them neatly in a circle, and bury them every day for two weeks to a month,” says Post.
It would take months to hatch from these eggs. While experts are still looking for definitive evidence, the parent dinosaurs may have sat with these nests until the cubs hatched from their shells. Also, Zelenitskaya notes, the researchers found a large number of oviraptorosaur nests with adult dinosaur skeletons nearby. “These dinosaurs were completely obsessed with their eggs,” she says.
The Oviraptorosaurs weren't alone. The shovel-billed dinosaur Mayazaura, which means “good mother lizard,” derives its name in part from the discovery by Marion Brandwold of a nest containing young dinosaurs too evolved to be newborns. In the excavation and analysis that followed, Mayazaura became one of the earliest and finest examples of dinosaurs observing their offspring long after hatching.
However, not all dinosaurs were loving parents. For example, the soft eggs that the Protoceratops and Mussaurus would lay, according to new research, had to be coated to keep them from drying out, but too thin to support the parent's weight. Dinosaurs that laid soft-shell eggs had to make nests to cover their future young, but probably did nothing other than watch the nest.
The largest dinosaurs may have done little to look after the next generation. “We have shown parental concern in distantly related dinosaurs,” says Post, “but for some groups, such as sauropods, we have no evidence of post-masonry withdrawal.“Sauropods include long-necked giants such as Apatosaurus and Brachiosaurus. Paleontologists discovered extensive nesting sites for them, including some places where dinosaurs laid their eggs in areas that were warm with geothermal activity, perhaps to incubate offspring. But the researchers have no evidence that the parents stayed close.
"The long-necked dinosaurs buried their eggs carefully," says Post, "but like the turtles, the evidence suggests they cared little for them - a strategy of laying and leaving them behind."
This image does not quite resemble Earth's tender upbringing before time. But it makes biological sense. “If giant dinosaurs nested in colonies like gulls and the parents stayed there until they hatched,” Zelenitskaya says, “the food resources for the parents would probably be quickly depleted.“The daily food requirements of large adult dinosaurs may have prevented them from hovering over their nests until the day of hatching. And this, in turn, could help answer another burning question.
In various places around the world, paleontologists have found bone cells containing young dinosaurs of the same species. The three Triceratops, the Alamosaurus massif, and the order of the Sinornithomimus seem to indicate that young dinosaurs of various species grouped together as they roamed their youth.
Why? For example, the more eyes you have, the more likely you are to spot predators. Teenage dinosaurs forming interspecific social groups make sense given what we know about how rough life in the Mesozoic could have been. Even among the Mayasaurs receiving higher-than-average parental care, nearly 90% of the pups died within the first year. If young dinosaurs could last those first 365 days and grow large enough not to be a snack for larger predators, they would have a better chance of survival.
Some dinosaurs may have used additional parenting strategies, but researchers can't tell because they have yet to find evidence. In this case, live birds may offer some examples of what to look for. Some birds nest collectively, with several mothers lying in the same nest, says empty. Do people like Allosaurus do the same? We also know that some birds are brood parasites, which means they leave their eggs in a nest of another species, relying on other parents to raise their young. It is possible that some non-avian dinosaurs may have tried this technique first, forcing another species to take care of its hungry cub.