These Baby Dinosaurs Were Born With Adult-Like Proportions

UA 9998 and friends. Image credit: D. Vital

Newly recovered fossils from a baby titanosaur suggest that the dinosaur would stay the same shape throughout its life, even as it grew to the size of a bus. These findings are reported in the journal Science.

Titanosaurs like Rapetosaurus krausei were the biggest land animals to ever walk the Earth. Multiple fossils from the 50-foot-long adults have been recovered and studied, but until recently we knew very little about hatchlings and juveniles. Then, paleontologists recovered the remains of an itsy-bitsy R. krausei from a rock formation in Madagascar. They found bones from the dinosaur’s hips, front legs, back legs, spine, and tail—all tiny. Of course, tiny is relative. The baby, which the researchers named UA 9998, appears to have been just a few weeks old, 88 pounds, and just over a foot tall when it died—and yet it was a mere speck compared with its mother.

Mom and baby, with shadowy human for scale. Image credit: Raul Martin and Kristina Curry Rogers

And the researchers did compare UA 9998 with adults like its mother. They cut slices from one of the baby dinosaur’s leg bones and examined them under the microscope. They also scanned all the bones using X-ray computed tomography.

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

The researchers found that aside from their diminutive scale, the baby’s bones were astonishingly like those of full-grown adults. Young though it was, the baby had sturdy bones and would have been independent enough to run around on its own. UA 9998 was proportioned and built exactly like an adult. Had it lived, it would have expanded. Compare that to, say, baby birds, or even human infants—floppy, dependent creatures that bear only a passing resemblance to their parents.

You might not guess it to look at them, but other dinosaurs like theropods and ornithischians had parent-offspring relationship more like ours. The sturdy little remains of UA 9998 suggest to paleontologists that titanosaur parents might have been a little less involved.