From movies to museum displays, the dinosaurs we most often see are fully mature animals. There are a few good reasons for this. The first is that the skeletons of adult dinosaurs are among the most impressive specimens in the whole of the fossil record, but it is also true that the bones of juvenile dinosaurs are relatively rare. Scientists are still learning about how dinosaurs grew up, and a long-forgotten discovery has shown how one of the most famous dinosaurs changed as it matured.
During the early 20th century, paleontologist Earl Douglass collected numerous dinosaur remains from what is today Dinosaur National Monument in northern Utah. Among the bones retrieved from the 150-million-year-old work were several skulls of Diplodocus, including one from an adult, one from a subadult and one from a juvenile—but the juvenile skull was never formally described. Now paleontologists John Whitlock, Jeffrey Wilson and Matt Lamanna have undertaken that task, and what they have found is that juvenile Diplodocus substantially differed from adults.
The skulls of adult Diplodocus are very strange. The nasal openings are placed far back on the skull near the eyes, and the front of the snout is squared-off with a row of peg-like teeth sticking out at the front. In the juvenile skull, by contrast, the snout is more rounded, and the teeth extend much further back along the jaws. As the authors of the new study suggest, this may indicate that adult and juvenile Diplodocus ate different things. While the square snout of the adults would allow them to indiscriminately crop low-lying vegetation, the juveniles would have been better-adapted to selectively browse on softer leaves. This would have been important as juvenile Diplodocus would require a large amount of high-quality foods to grow, and so their mouth shape may have allowed them to more easily pluck up the more nutritious foods they needed. Once they grew to adult size, however, this need for high-quality plant food would be reduced, and so the skull became reshaped to reflect the different feeding habits of adults.
Whitlock, J., Wilson, J., & Lamanna, M. (2010). Description of a Nearly Complete Juvenile Skull of Diplodocus (Sauropoda: Diplodocoidea) from the Late Jurassic of North America Journal of Vertebrate Paleontology, 30 (2), 442-457 DOI: 10.1080/02724631003617647