Imagine if a human baby went from cuddly-infant-size to rude-teenager-size in just a couple of months, and then walked out the door, fully independent. That's a rough approximation of what a mysterious dinosaur, unfortunately named Rapetosaurus, used to do, according to a new study published today in Science.
At about 50 feet in length, with an extremely long neck and elephant-like legs, Rapetosaurus is big, but on the smaller side within its family — the titanosaurs. The titanosaurs were the largest dinosaurs that ever lived and the last sauropod group remaining at the end of the Cretaceous Period, 65 to 70 million years ago, just before all dinosaurs went extinct.
Rapetosaurus was among the world's last dinosaurs
Sauropods — a long-necked, long-tailed, vegetarian group of dinosaurs — were long known for growing incredibly in size between birth and adulthood. But scientists needed a skeleton from a very young sauropod in order to better understand this phenomenal growth, and just how fast it happened. The study's authors — led by Kristina Curry Rogers of Macalester College, who discovered the first adult Rapetosaurus skeleton in 1996 — were the first to get their hands on a baby Rapetosaurus preserved in a fossil-rich rock formation in northwestern Madagascar.
This paper is accompanied by some delightful illustrations. This one shows how big the Rapetosaurus from the study was when it died. About the size of a labrador!
The researchers used two techniques to study the bones of the baby dino: bone histology (in other words, a microscopic-level look at the bone cells) and X-ray computed tomography (essentially a run-of-the-mill CT scan). This Rapetosaurus was estimated to weigh between 5.5 and 9.5 pounds at birth. Only a couple of months later, when the baby dino appears to have starved to death, it weighed about 88 pounds. That's quite a growth spurt. "The evolution of these fast growth rates in these colossal dinosaurs [is] one of the greatest achievements in the history of evolution," Stephen Brusatte, a paleontologist at the University of Edinburgh who did not take part in the study, told The Verge.
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Not only did the dinosaur grow extremely fast, researchers believe that it didn't require much help from its parents to get around or fend for itself. (Its starvation was drought-related, not caused by overzealous teenage rebellion!) Despite the precipitous speed of the dino's early growth, in fact, the shape and proportion of its limbs appear to remain a constant. "[These bones] basically look like miniature versions of much bigger Rapetosaurus bones," says Rogers, adding that this characteristic is "common in animals that don't rely on significant parental care, and have to get moving quickly after birth or hatching."
The young Rapetosaurus was about half the size of a newborn African elephant when it died (and a thirtieth of its size at birth). It's wild that an adult Rapetosaurus would be about three times the size of an adult elephant, yet it's babies are so much tinier!
The baby Rapetosaurus' weight-bearing bones also exhibited signs of bone remodeling — a lifelong process in which old bone tissue is resorbed from the skeleton and replaced. Rogers explained that animals don't traditionally begin bone remodeling until they are capable of moving around completely on their own.
Finally, the researchers examined the dinosaur's growth plates, looking to compare their cartilage deposits to those of various modern animals. Animals that require a lot of parental care have very thick and irregular cartilages, but the cartilage deposits in the Rapetosaurus' growth plates were very thin, just like those of modern bird species that require little parenting.
Greg Erickson, a professor of anatomy and vertebrate paleobiology at Florida State University, said the team's conclusions about the Rapetosaurus are solid. He was impressed with the team's choice of analyzing bones at the microscopic level to figure out the dinosaur's size at hatching, a method that's already been used on modern animals. Erickson also agrees that the Rapeotosaurus was probably independent at birth, saying the researchers proved their case well. "It would be hard to imagine these giant parents taking care of these tiny babies," he adds.
This illustration shows how small a Rapetosaurus hatchling is, as well as how the Rapetosaurus femur bone retains the same shape throughout its life. Also a silhouette of a woman for some reason.
The scope of this study isn't huge — it's simply an interesting glimpse into the life of one weird dinosaur. However, Rogers said, the paper's methods will be useful for other scientists working with baby dinosaur fossils. Erickson agreed, saying Rogers' team presented a set of methods with "lots of gems," and suggesting that the same methods could be used to look at the growth process of even larger sauropods.