Huge long-necked Rapetosaurus dinosaurs that lived on Madagascar had adult-sized babies

An artist's illustration comparing the sizes of a newborn Rapetosaurus (second from left) to other large-bodied mammalian newborns.
An artist's illustration comparing the sizes of a newborn Rapetosaurus (second from left) to other large-bodied mammalian newborns.PHOTO: REUTERS

WASHINGTON (Reuters) - The babies of a huge, long-necked dinosaur called Rapetosaurus that lived on the island of Madagascar did not just sit in a nest and look cute. They were born ready for action.

Scientists on Thursday (April 22) announced the discovery of fossils of a baby Rapetosaurus the size of a big dog that apparently starved to death during a drought several weeks after hatching from its soccer-ball-sized egg.

Unlike many animal babies, particularly humans, the hatchling Rapetosaurus had adult proportions, meaning it likely did not need significant parental support and was actively foraging for plants rather than waiting for momma to feed it.

Such babies are known as "precocial", as opposed to"altricial" offspring that have different body dimensions from adults, cannot get around by themselves and require considerable parental support for food and protection, Macalester College paleontologist Kristi Curry Rogers said.

"The main conclusion was that this is the first evidence for a truly precocial dinosaur: one that matured rapidly and without parental care," Adelphi University paleontologist Michael D'Emic added.

Rapetosaurus lived about 67 million years ago, not long before the demise of the dinosaurs, and was the largest creature in Madagascar at the time.

The baby, known from a partial skeleton including limb bones, pelvic bones, fingers, toes and several vertebrae, was probably around 1.2m long from head to tail and weighed between 23kg and 40 kg when it died.

An adult Rapetosaurus was probably around 12m long, a little more than a school bus, and weighed around 16 tons, roughly 2.5 times as much as an elephant, Dr Curry Rogers said. It was a medium-sized member of a dinosaur group called titanosaurs, which included the largest land animals ever on Earth.

Titanosaurs were part of a larger assemblage of dinosaurs called sauropods, known for their long necks, long tails and voracious appetite for plants.

Dr D'Emic said the researchers looked at the microstructure of the baby's bones to see preserved cavities that once held cells, blood vessels and nerves. The density and organization of those indicated a rapidly growing individual, and there was even evidence for bone repair, suggesting an active lifestyle and fast metabolism, he added.

Examples of precocial animals today include most lizards, snakes and reptiles, certain birds and some large mammals including wildebeest.

"Precocial young can avoid predation on their own, and there is a much smaller chance of the entire brood succumbing to predation at once," Dr Curry Rogers said.

The research was published in the journal Science.