Even the largest dinosaurs of all time started their lives breaking out of small eggs. Now, a rare fossil of a baby titanosaur suggests that once these precocious youngsters hatched, they were on their own in a harsh landscape.
The bones, found in 70- to 66-million-year-old rock in Madagascar, belonged to a long-necked, herbivorous dinosaur named Rapetosaurus krausei. This species was a titanosaur, the group that includes the largest known dinosaurs, as well as celebrities like Apatosaurus—the sauropod formerly known as Brontosaurus.
Macalester College paleontologist Kristina Curry Rogers named Rapetosaurus in 2001, and she has continued to study the armor-studded titan ever since.
While looking through a collection of turtle and crocodile bones, Curry Rogers kept coming across tiny fossils that belonged to a baby Rapetosaurus. The rare specimen provided a perfect opportunity to examine a part of dinosaur life that experts still know little about.
“Our record of the earliest lives of sauropods up to this point has been limited to a few rare discoveries,” such as embryos in eggs and a few juveniles, Curry Rogers says. What baby sauropods were like and how they lived has been obscured by this gap, making the newfound Rapetosaurus bones a very lucky find.
“We wanted to glean everything we could from these bones, from anatomy and body size estimates to details of bones under the microscope and with micro-CT scanning,” Curry Rogers says.
As reported this week in Science, all those details have now painted an unprecedented picture of how these dino babies started life.
“Deep inside all of the bones in our sample, there is a subtle distinction in bone organization that we identified as a ‘hatchling line,’” Curry Rogers says. This line represents a recovery period after the stress of pushing out an egg.
From there, the paleontologists estimated the dinosaur’s size at birth as well as its size at death. Given the way bone grows, they estimate that the baby Rapetosaurus perished between 39 and 77 days after birth.
Amazingly, from a start of 3.4 kilograms at hatching, the Rapetosaurus was about 40 kilograms when it died a few months later.
“That’s like going from Chihuahua to a great Dane in six weeks,” says Des Moines University paleontologist Sarah Werning, who was not involved in the study.
The proportions of its bones also look a lot like those found in adults, and signs of the bone remodeling itself under stress show that the young animal was moving around in a pretty active way. Together, all these clues “illustrate that Rapetosaurus probably fended for itself after hatching, without much parental care,” says Curry Rogers.
The small dinosaur’s bones might even give away what killed it. Details of the preserved cartilage suggest that it starved to death, which fits with the idea that its prehistoric habitat was a harsh place where annual dry seasons probably meant doom for many baby dinosaurs.
Werning agrees that the baby was likely ready to rove on its own, noting that “Rapetosaurus babies were ridiculously overengineered," and seem ready to bear heavy weights long before they had reached their massive size as adults.
“In other words,” Werning says, “they laid the foundation for a skyscraper under a single-story home, knowing they were going to keep building.”
Werning says the new study is a good reminder that dinosaurs didn’t all have the same childhoods. Some babies were running around on their own, while others stayed nest-bound for extended periods of time. Curry Rogers and her team have now created a type of guidebook for researchers who want to test whether a given dino species was independent at birth.
Curry Rogers adds that, given this result, a Land Before Time scenario with a dutiful mama sauropod looking after her young just doesn’t fit. Instead, imagine the Cretaceous Littlefoot “out in the wilds of the Maevarano Formation ecosystem, dodging hungry theropods and crocodiles!”