For more than a century, paleontologists have been confident that all dinosaurs reproduced by laying eggs. After all, no dinosaur gave birth to live young (nor do their modern bird descendants), and nesting sites found from Montana to Mongolia indicate that prehistoric dinosaurs laid clutches of sturdy eggs. Slowly, though, new research has begun to change that picture of dino reproduction.
Up until now, paleontologists thought that all dinosaurs laid hard-shelled eggs. A recent study by University of Calgary paleontologist Darla Zelenitsky and colleagues found that some dinosaurs, like the 73 million-year-old horned dinosaur Protoceratops and the 215 million-year-old, long-necked dinosaur Mussaurus, laid soft-shelled eggs similar to the leathery eggs of some modern reptiles. By mapping out these findings onto the dinosaur family tree, the paper proposes the unexpected idea that all dinosaurs originally initially laid soft-shelled eggs. Over time, at least three different lineages independently evolved hard-shelled eggs.
This research could help to explain why dinosaur eggs are harder to find than many paleontologists would expect because softer eggs would be less likely to fossilize. And working out which dinosaurs laid which types of eggs is important for answering big questions about dinosaur parental care. That’s because no typical dinosaur nest exists. Some species laid lots of round, hard eggs in a pile. Others laid eggs two-by-two and arranged them carefully. Some eggs are spheres. Some are cone-shaped. And as is the case with modern birds, different egg types relate to the ways adult dinosaurs behaved.
“Even among only the hard eggs of dinosaurs, there are considerable differences in the architecture of the eggshell,” says University of Calgary paleontologist Darla Zelenitsky. “Such varied eggshell structure indicates vastly different nest styles, incubation methods, and times between egg-laying and hatching.”
A picture of dinosaur parenting
One matter of debate is whether dinosaur parents stuck around and guard their eggs, or, like today’s sea turtles, laid them and then left the offspring to fend for themselves. The answer seems to vary by species.
For example, consider the parrot-like dinosaurs called oviraptorids. Paleontologists have found the gorgeous skeletons preserved in a position where they seem to be sitting over nests of eggs. “It’s tempting to call this brooding, like living birds,” says San Diego Natural History Museum paleontologist Ashley Poust, “but we’re still unsure if that was part of their behavior.” Still, the details would indicate that the dinosaurs constructed their nests with care. Scientists know from previous finds that oviraptorids laid two eggs at a time in a clutch of 30 or more. “This means that the mother would have to stay with or at least return to the nest, lay her pair of eggs, arrange them carefully in the circle, and bury them appropriately every day for two weeks to a month,” Poust says.
Those eggs would have taken months to hatch. While experts are still searching for definitive evidence, parent dinosaurs may have sat with these nests until the hatchling babies pushed their way out of the shells. Also, Zelenitsky notes, researchers have found a large number of oviraptorosaur nests with adult dinosaur skeletons nearby. “These dinosaurs were completely obsessed with their eggs,” she says.
Oviraptorosaurs were not alone. The shovel-beaked dinosaur Maiasaura, which means “good mother lizard,” got its name in part from Marion Brandvold’s discovery of a nest containing baby dinosaurs too developed to be newborns. In the excavations and analysis that followed, Maiasaura became one of the earliest and best examples of dinosaurs watching over their offspring for an extended period after hatching.
Yet not all dinosaurs were doting parents. For example, the soft eggs that Protoceratops and Mussaurus would have laid, according to the new study, had to be covered so they wouldn’t dry out but were too thin to support the weight of a parent. The dinosaurs that laid soft-shelled eggs would’ve made nests to cover their hatchlings-to-be, but probably didn’t do anything more than watch over the nest area.
The biggest dinosaurs might have done little to look after the next generation. “We have shown parental care in distantly-related dinosaurs,” Poust says, “but for some groups, like sauropods, we don’t have evidence of post-laying care.” Sauropods include the long-necked giants like Apatosaurus and Brachiosaurus. Paleontologists have found their expansive nesting grounds, including some sites where dinosaurs laid eggs in areas that were warm with geothermal activity, perhaps to incubate the offspring. But researchers have no evidence that the parents stuck around.
“Long-necked dinosaurs buried their eggs carefully,” Poust says, “but like turtles, the evidence points to little further care—a strategy of lay ‘em and leave ‘em.”
This image doesn’t quite evoke the tender nurturing of The Land Before Time. But it makes sense biologically. “If giant dinosaurs were nesting in colonies like seagulls and parents remained there until hatching,” Zelenitsky says, “food resources for the parents would likely dwindle fast.” The daily food requirements of large adult dinosaurs may have prevented them from looming over their nests until hatching day. And this, in turn, might help to answer another thorny question.
At various sites around the world, paleontologists have found bonebeds containing young dinosaurs of the same species. A trio of Triceratops, an array of Alamosaurus, and a squad of Sinornithomimus appear to indicate that young dinosaurs of various species grouped together as they navigated their youth.
Why? More eyes offer a better chance to spot predators, for example. Adolescent dinosaurs forming cross-species social groups makes sense given what we know about how harsh life in the Mesozoic could be. Even among Maiasaura, who received better-than-average parental care, nearly 90 percent of the hatchlings died within the first year. If young dinosaurs could last through those first 365 days, and grow large enough not to be a snack for larger carnivores, they stood a better chance at survival.
Perhaps some dinosaurs employed additional parenting strategies, but researches can’t say because they have yet to find the evidence. In this case, living birds might offer some examples of what to look for. Some birds nests collectively, with multiple mothers laying in one nest, Poust says. Did the likes of Allosaurus do the same? We know, too, that some birds are brood parasites, meaning they leave their eggs in the nest of a different species, counting on other parents to raise their young. It’s not out of the question that some non-avian dinosaurs might have tried the technique first, getting another species to take care of their hungry youngster.
“It’s just a matter of finding the right fossil,” Poust says, “or re-examining old evidence with new eyes.”