Dinosaurs

The tiny preserved bones of a Rapetosaurus hatchling.

K. Curry Rogers, M. Whitney, M. D'Emic, and B. Bagley

Babies of giant dinosaurs were surprisingly self-sufficient

In 2012, paleontologist Kristina Curry Rogers was digging through drawers of miscellaneous crocodile fossils at the State University of New York at Stony Brook when she saw something both familiar and unfamiliar: the bones of a giant dinosaur, in miniature. Even out of context, she realized instantly that they had come from a class of long-necked plant-eaters called titanosaurs. “They looked exactly like the bones I’d spent 15 years of my life studying,” she says. That similarity, along with clues from bone structures inside the fossils, has now yielded the first clear evidence that hatchlings of at least some of the largest dinosaurs were essentially self-sufficient, scaled-down adults.

The bones, of a dinosaur called Rapetosaurus krausei, were a rare find. Researchers have found later-stage juveniles and fossilized eggs of sauropods—the group of long-necked dinosaurs to which Rapetosaurus and other titanosaurs belong—but none that had just hatched. Evidence from fossilized embryos in a nesting site in Argentina hinted that another titanosaur species was precocial: self-sufficient right after hatching. And thanks to their massive size as adults, “it’s always seemed a bit weird to think of titanosaurs having much in the way of parental care,” says Thomas Holtz, a paleontologist at the University of Maryland, College Park. But until now, there was no way to tell for sure.

The bones were unearthed in Madagascar in 1998 and 2003 but were misclassified until Curry Rogers, who works at Macalester College in Saint Paul, noticed them. She and her colleagues started by measuring the proportions of the terrier-sized creature and found that they closely matched those of adults. Then the team examined cross-sections of the fossils to determine how it grew. One key finding was a hatching line: a shift in the internal structure of the bone that in modern lizards and crocodilians indicates when an animal emerged from its egg. That feature enabled the researchers to estimate both the juvenile’s size when it emerged from its egg and its age when it died. In the dinosaur’s short life—between 39 and 77 days long—it grew by an order of magnitude, from about 3.4 kilograms to 40 kilograms, the team reports on p. 450.

Having adultlike body proportions at such a young age indicates that this species left the egg able to walk and probably feed itself—unlike duck-billed dinosaurs, which were probably nest-bound and relied on their parents for food at that age. In another sign of an active life, the young titanosaur’s bones had been remodeled. That means the tiny dinosaur was constantly keeping its bones the right shape as it grew—evidence that they had undergone the biomechanical stress of walking around.

ADAPTED BY G. GRULLÓN/SCIENCE

The bones also hint at an unpleasant death. Cartilage at the ends of bones stops growing in starving animals—and in the Rapetosaurus specimen, the preserved calcified cartilage regions are extremely thin. That fits with what researchers know about the site, a drought-ridden and stressful environment at the time the young dinosaur lived there. “It’s very exciting,” says Holly Woodward, a paleohistologist at Oklahoma State University, Tulsa. “There’s direct evidence of starvation for the mode of death.”

Holtz says other titanosaurs probably shared Rapetosaurus’s growth strategy. But he cautions against concluding that all long-necked giants were born self-sufficient. “To most people, all sauropods look alike: tiny at one end, tiny at the other, big in the middle.” he says. “But that doesn’t mean there wasn’t a lot of variation within them.”