All of the big dinosaurs were small once. From eggs the size of basketballs grew 100-ton behemoths. But how did they go from so wee to so huge? Scientists are hunting for clues among fossils that are many millions of years old, and this week they’re just a little bit closer to solving some of the big (and little) mysteries.
A new study describes the remains of a rare baby titanosaur, the largest class of dinosaur ever to roam the planet. When it died, the baby weighed about 90 pounds and was about a foot tall at the hip. It had hatched out of an egg maybe six weeks earlier, weighing about 7.5 pounds. The discovery answers a lot of questions about how huge dinosaurs grew up from relatively small eggs, but the biggest mystery here is why there were questions in the first place. Why are baby dinosaur fossils so rare? An adult female would likely have laid hundreds of eggs in her lifetime — of these, maybe two would survive long enough to reproduce themselves, assuming a stable overall population. That means that dinosaur babies, the population of a malthusian, jurassic world, would have died all the time.
The answer likely lies in the way dinosaurs died. Dinosaur eggs are rare because they either hatched and broke or got eaten and broke. It would be a strange event that would bury a nest of eggs in the muck, intact for preservation and fossilization. Similarly, most baby dinosaurs that died before adulthood were likely taken by predators. Rather than leaving a complete skeleton, the babe would be reduced to a pile of broken, scattered bones, difficult to identify and piece back together.
But there’s also this: When you’re looking for multi-ton giants, the bones of smaller offspring can easily go unnoticed. And that’s exactly what happened in the case of the Rapetosaur in question. Paleontologist Kristina Curry Rogers has been studying the species, a moderate-sized titanosaur found in what is now Madagascar, since 2001. It wasn’t until she dug through a collection of smaller bones, which were assumed to have belonged to turtles and crocodiles, that she realized that many of them appeared to be miniature versions of those of the Rapetosaurus parent.
The fossil record is biased towards the big in more ways that one. Bigger bones are more durable, and therefore preserved more easily. They stick out of rocks for longer before they are swept away by erosion, and they’re easier to spot. Humans also look for the big ones more, either because they capture our imaginations or fetch higher prices on fossil markets. But the little guys will share many secrets with those willing to put in the effort to look for them. This baby Rapetosaurus, for example, tells us a lot about how the biggest dinosaurs grew up to be so big. The young dinosaur’s bones are proportioned in a very similar way to those of adults, which indicates that it was likely able to walk and feed independently right after birth.
This makes sense. It’s hard to imagine a beast the size of a fire truck doing much parenting to a creature the size of a newborn human. (Elephants can walk on day one.) It’s also different from what’s been seen in other dinosaur species, which appear to have been attentive parents. The Rapetosaurus also grew up quick, doubling in size three times in just a couple months (imagine a human baby weighing 90 pounds at two-months-old). Growing quickly was probably its main defence against predators — a race against time to get big enough to fend them off before getting eaten.
The researchers suggest that this particular dinosaur baby likely died of starvation in drought-stressed Madagascar. This may have been the fate of some of his dino-brothers and dino-sisters, too. Their little skeletons may still be out there, waiting to give themselves to scientific discovery, waiting for some human to realize what’s been in front of them all along.