The history of pachycephalosaurs is mostly a story of domes. Even though some skeletons have been uncovered over the years, the most commonly-found part of these bipedal Cretaceous herbivores is the thickened, decorated skull. As a result, much of what we know about these dinosaurs comes from skull fragments, and this can sometimes seed confusion about which fossils represent new species and which are individuals of already-known dinosaurs.
Take the partial pachycephalosaur skull UCMP 130051, for example. In 1990, paleontologist Mark Goodwin described the skull–discovered in the Judith River Formation of Montana–as an adult of the previously-known dinosaur Stegoceras. The skull was large for a Stegoceras, and lacked the array of nodes commonly seen on the back shelf of the skull but was otherwise matched the anatomy of the common pachycephalosaur. But when paleontologist Robert Sullivan wrote a review of known Stegoceras material in 2003, he thought that UCMP 130051 was distinct enough that it belonged to a new kind of pachycephalosaur he named Hanssuesia sternbergi.
Now the story of UCMP 130051 has taken another turn. In the latest issue of the Journal of Vertebrate Paleontology, Ryan Schott and David Evans argue that the skull is really an adult Stegoceras after all. After reconstructing a Stegoceras growth series with juvenile and subadult specimens, Schott and Evans found that UCMP 130051 more closely resembled younger Stegoceras than other skulls Sullivan attributed to Hanssuesia. UCMP 130051 was just a bit bigger and lacked the nodes on the back of the skull that characterized younger individuals–the rest of the anatomy was “indistinguishable” from Stegoceras.
Exactly why UCMP 130051 was missing the set of bumps seen on younger Stegoceras fits into a wider debate about how much dinosaurs changed as they grew up. The “Toroceratops” controversy is the most prominent example, perhaps matched by the longer debate over “Nanotyrannus“, but pachycephalosaurs also form a facet of discussion. In 2009, Jack Horner and Mark Goodwin proposed that the dome-headed dinosaurs Dracorex and Stygimoloch were really just younger individuals of the contemporary dinosaur Pachycephalosaurus. This proposal required drastic changes to the dinosaur’s skull during its life, including forming a dome, growing long skull spikes, and then resorbing those spikes. The transformation must have been spectacular.
While not quite as drastic as in the transition from the spiky “Stygimoloch” form to adult Pachycephalosaurus, Schott and Evans found that Stegoceras probably went through similar changes. In their study, which focused on the ornamented squamosal bones at the back of the skull, younger individuals had prominent nodes that varied in size and shape. In UCMP 130051, though, those bumps were missing, indicating that they were resorbed when Stegoceras reached adulthood. And while they are tentative about this identification, Schott and Evans point out that some Stegoceras specimens–including UCMP 130051–appear to have resorption pits on the surface of the bone; an indicator that their skull ornaments were changing shape as they dinosaurs reached skeletal maturity. Stegoceras didn’t undergo the same back-and-forth horn growth suggested for Pachycephalosaurus, but the change in those little skull nodes hint that the dinosaur went through a more subdued change as it reached full size.
But the new study by Schott and Evans isn’t just about how young Stegoceras changed into adults. By reconstructing the dinosaur’s growth series, the paleontologists also discovered clues that may help paleontologists parse the ever-growing number of dinosaur species, as well as what all that crazy headgear was for. While young Stegoceras showed a high degree of variation in the shape and number of ornaments on their squamosal bones, for example, the dinosaur’s retained the same general “ornamental pattern” throughout their lives. This means that isolated squamosal bones can be useful in identifying pachycephalosaurs known only from partial skulls (and there are quite a few of them).
Of course, one of the biggest mysteries about pachycephalosaurs is why they had domes and spikes in the first place. Depending on who you ask, the ornaments were used to help the dinosaurs recognize members of their own kind, as sexual signals, as weapons or some combination of these. Schott and Evans prefer a mosaic approach to the problem. The fact that even the youngest Stegoceras specimens had recognizable, diagnostic ornaments on their squamosal bones, the researchers argue, indicates that these bumpy adornments probably acted as species recognition signals. They don’t seem to have any role in defense, and the fact that dinosaurs grew these signals before sexual maturity means that they probably weren’t advertisements for mates. If this is true, though, the question is why adult specimens would lose the display structures so late in life.
Then there’s the dome. Young Stegoceras, Schott and Evans point out, were relatively flat-headed. Thick domes developed as the dinosaurs grew up, and previous studies of Stegoceras skulls hinted that the rounded structures were capable of taking quite a shock. (Some pachycephalosaur fossils may even preserve damage from bouts gone awry.) Paleontologists are not agreed on this point, but it is possible that these dinosaurs really did butt heads. This idea, combined with the fact that domes grew as the dinosaurs approached reproductive and skeletal maturity, might mean that domes were sexual signals, and possibly even used in competitions to garner mates. Frustratingly, though, testing these ideas is extremely difficult. We can’t observe the animals themselves, and can only approach these aspects of their lives indirectly through the detail of fossilized bone. We know more about pachycephalosaurs than ever before, but the evolution of their bizarre features remains contentious.
Schott, R., Evans, D. (2012). Squamosal ontogeny and variation in the pachycephalosaurian dinosaur Stegoceras validum Lambe, 1902, from the Dinosaur Park Formation, Alberta. Journal of Vertebrate Paleontology, 32 (4), 903-913 DOI: 10.1080/02724634.2012.679878