If you knew nothing at all about dogs, but you were presented with a lineup of the skeletons of a variety of breeds from chihuahua to bulldog to German shepherd to mastiff, you could be excused for thinking they were different species. Their skeletons seem to be so different, yet we know they are all just varieties of one subspecies, Canis lupus familiaris, that have been created through artificial selection. Paleontologists, on the other hand, do not have breeder's records and must think carefully about what distinguishes one species of dinosaur from another. A new study by Jack Horner and Mark Goodwin in the journal PLoS One suggests that some dinosaurs previously thought to be separate species, even genera, were really just the growth stages of one species of dinosaur.
The dinosaurs that are the focus of the new study are three "bone-heads," or pachycephalosaurs: Pachycephalosaurus, Stygimoloch, and Dracorex. These were bipedal ornithischian dinosaurs that had hard bony domes on their heads, often complemented with an array of spikes. Dracorex was small with a relatively flat head with small spikes, Stygimoloch was mid-sized with a small bony dome and huge horns, and Pachycephalosaurus was large with a large bony dome and relatively small horns. Together these dinosaurs appear to represent a growth series from juvenile to adult, all grouped together as Pachycephalosaurus, and the evidence can be found in the makeup of the bones.
Even though bones are hard they are not static things. They are constantly being remodeled; the change may be difficult to see from day to day but bone is still constantly being reabsorbed and laid down. The same processes happened in these dinosaurs, allowing for major modifications of the skull.
Looking at the microscopic structure of the skull bones, Horner and Goodwin found that the horns on the skulls they examined started off small, grew large, and then were reorganized as smaller structures around the edge of the solid dome of the skull. The young dinosaurs were not born with adult ornamentation but grew into it over time. Why large spikes were a juvenile characteristic and a bony dome was an adult characteristic, however, is still unknown.
Extreme changes in skull shape during growth can also be seen in hadrosaurs, where what were considered "small" species turned out to be juveniles of already known species, and in horned dinosaurs. In fact, at this year's Society of Vertebrate Paleontology meeting, Horner and paleontologist John Scannella proposed that Triceratops is a growth stage of the larger horned dinosaur presently known as Torosaurus. This hypothesis has yet to be fully supported, but it does seem that many Cretaceous ornithischian dinosaurs underwent major anatomical changes during their lifetimes. No doubt this area of research will generate much discussion and debate in the years to come.