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A five-inch-long impression of the baby ankylosaur Propanoplosaurus marylandicus. The head is the triangular shaped portion near the top, and the right forelimb can be seen to the left. From Stanford et al., 2011.

Think of a dinosaur. Any dinosaur. Got it? Good. Now, chances are the dinosaur in your head is an adult animal. In books, movies and just about every other medium in which dinosaurs appear, adult animals almost always represent the dinosaurian lineage. To some extent, that’s because baby dinosaurs have been difficult to find and identify, but over the past few decades paleontologists have been working up a deeper understanding of the early lives of many dinosaur species. The latest find to be announced, just published in the Journal of Paleontology, offers some insight into what armored dinosaurs looked like shortly after they hatched from their eggs.

The study, by Ray Stanford, David Weishampel and Valerie Deleon, does not describe an actual skeleton, but rather the impression of a tiny dinosaur body found in the roughly 112-million-year-old rock of Maryland. It takes a moment of looking at the fossil to pick out the parts. The top surface of the little dinosaur’s head and the underside of part of the body are preserved as a five-inch-long impression in the rock. You may have even seen this fossil if you visited the Smithsonian’s National Museum of Natural History—the fossil has been on display in a small case dedicated to discoveries of local dinosaurs.

Stanford and colleagues have given this tiny dinosaur a name: Propanoplosaurus marylandicus. The name is a tribute to the fact that the arrangement of skull bones in the small specimen closely resembles that seen in Panoplosaurus, which was a later armored dinosaur that belonged to a subgroup called the nodosaurids. Nodosaurids lacked the famous tail clubs seen in some other ankylosaurs, but the backs of many species were decked with arrays of spiky armor.

The fact the a new dinosaur species has been named on the basis of a juvenile skeleton impression is unusual. Paleontologists have been wary of coining new dinosaur names on the basis of baby or juvenile individuals. It is often difficult to figure out which juvenile skeleton goes with which adult skeleton, especially in a case like this, when there are no actual juvenile bones to study. Furthermore, the remains of adult nodosaurid dinosaurs from the East Coast have been rare, incomplete and difficult to diagnose.

Paleontologists have previously identified an ankylosaur called Priconodon from around the same place and time period, but the authors of the new study point out two problems with making a connection between this dinosaur and the hatchling impression. First, Priconodon is known only from teeth—there are no corresponding parts that can be compared between the adult and hatchling. Second, paleontologists are not even sure that Priconodon is a valid dinosaur name since the remains of the animal contain no distinctive features that would allow it to be consistently identified. Perhaps the adult teeth and the hatchling mold really do represent the same species of dinosaur, but at the moment, it is impossible to be sure. Given the rarity of East Coast dinosaurs—and Eastern ankylosaurs in particular—just what the adorable little nodosaurid grew up to look like is probably going to remain a mystery for some time to come.

References:

Stanford, R., Weishampel, D., & Deleon, V. (2011). The First Hatchling Dinosaur Reported from the Eastern United States: Propanoplosaurus marylandicus (Dinosauria: Ankylosauria) from the Early Cretaceous of Maryland, U.S.A. Journal of Paleontology, 85 (5), 916-924 DOI: 10.1666/10-113.1