No more Demons and Dragon Kings? Pachycephalosaurus ontogeny
One of the most influential discoveries that has radically changed our understanding of dinosaurs and their world is the realization that dinosaurs often went through dramatic physical changes as they aged. It has been well known for some time that unlike modern birds, non-avian dinosaurs took several years to reach adult size and began breeding before reaching skeletal maturity, but shared with them very rapid growth rates, resulting in animals that 'lived fast and died young'. Thanks to this growth habit, most dinosaurs that we have a significant sample size for show a particular pattern when it comes to their fossil record: hatchling and juveniles tend to be rare due to very high mortality rates (many were eaten and digested, resulting in no preservation), rapid growth rates to larger size and preservation bias that favors fossilization of large bodied, large boned animals. By comparison, there tends to be a large number of individuals that are one-half to two-thirds maximum adult size that represent animals that have reached sexual (but not skeletal) maturity, and a small number of individuals that have reached maximum adult size and skeletal maturity.
On top of all that, some dinosaurs also appear to develop unique structures like horns, domes and crests at various points during their development, and many are quite dramatic, appearing very quickly during ontogeny. No wonder then that it was not uncommon for scientists to name several species of dinosaur found at the same time and same place differentiated largely by size and display structures. And possibly the best example of this situation was Pachycephalosaurus, Stygimoloch ("Styx demon") and Dracorex ("dragon king"); found at the same time, in the same place, more closely related to each other than to other pachycephalosaurs, and differing only in size and cranial features. And then Dr. Jack Horner changed everything.
Horner has been one of the primary advocates of the hypothesis that up to a third of all named dinosaurs are actually growth stages of other species rather than separate taxa. While most well know for proposing that Torosaurus is the mature adult form of Triceratops, Horner & Goodwin (2009) determined that both Dracorex and Stygimoloch were not mature adults by examining the internal bone structure of skulls of each pachycephalosaur; both exhibit bone texture and composition consistent with juvenile and subadult age, respectively. They further found that pachycephalosaurs "employed metaplasia to rapidly grow and change the size and shape of their horns, cranial ornaments and frontoparietal domes, resulting in extreme cranial alterations during late stages of growth."
Horner's Pachycephalosaurus growth hypothesis has been essentially unchallenged by other scientists, and has been bolstered by several previous and more recent works: Goodwin et al (1998), Williamson & Carr (2002) Schott et al (2009), Schott et al (2011) and Longrich et al (2010) have all found that other known "domeless" pachycephalosaurs like Homalocephale are (like "Dracorex") juvenile individuals which develop domes as they reach adulthood, and can likely be synonymized with previously named taxa. Just this year, Goodwin & Evans (2016) described remains of juvenile Pachycephalosaurus significantly smaller than "Dracorex", and also confirmed that the characteristic horns or studs at the rear of the skull were present even at a very young age. Goodwin & Evans also give us a term (originally coined by Dr. Denver Fowler and adopted by Horner and others) to describe the very distinct growth stages that pachycephalosaurs, ceratopsians, hadrosaurids and some theropods exhibit over their lifetimes: ontigimorphs
As with many things regarding dinosaurs, our understanding of a particular situation is heavily biased by available data. One of the commonly overlooked sources of data is stratigraphy, where a fossil was found in relation to known date points and other fossils within a formation. Saurian scientific advisor Dr. Denver Fowler recently published a massive stratigraphic chart that correlates and in some cases revises the age of virtually every Cretaceous rock formation in western North America, as well as plotting the stratigraphic ranges of several dinosaur lineages. Tucked into this huge wall of data are these two entries:
Dr. Fowler has included a huge amount of data for both of these entries. The basic take-aways are:
- Precise stratigraphic data was rarely recorded when the type specimen of Pachycephalosaurus was collected, but all those we do have data for come from lower layers in the Hell Creek and other formations.
- Most specimens of "Stygimoloch" also lack precise stratigraphic data, but those we do have all cluster towards the upper layers of the formation. The only known specimen of "Dracorex" is from the 'middle part of the formation' according to its description.
- Large squamosals have never been found in the upper layers of the Hell Creek or other formations, indicating we have probably yet to find a specimen of similar maturity to the large Pachycephalosaurus skulls most commonly depicted. A theoretical 'upper Hell Creek pachy' would probably look very similar to 'Sandy', an as yet still unpublished specimen with significant post cranial remains
- A skull similar in size to those known for "Dracorex" and "Stygimoloch" collected from the same quarry that yielded a T. rex named Sue (about 15m above the bottom of the Hell Creek Formation) displays no long spikes and is most similar to known adult skulls of Pachycephalosaurus
- Pachycephalosaurus can display dramatic individual variation in cranial ornamentation, with some individuals possessing different numbers of spikes on left and right squamosal
- A skull from the middle portion of the Hell Creek formation once assigned the genus Stenotholus (later synonymized with "Stygimoloch" before the latter was synonymized with Pachycephalosaurus) displays squamosal horns shorter than those of other "Stygimoloch" individuals, but longer than those of typical Pachycephalosaurus.
Based on the limited data available, Dr. Fowler elected to retain Stygimoloch spinifer as a separate taxon in his chart. What data we do have could also suggest a very similar scenario to that of Triceratops over the course of Hell Creek time: Anagenesis from one species (Triceratops horridus) to another (Triceratops prorsus). With more data, perhaps we'll be talking about Pachycephalosaurus wyomingensis and Pachycephalosaurus spinifer in the future.
Goodwin, M. B., E. A. Buchholtz, and R. E. Johnson. (1998) Cranial anatomy and diagnosis of Stygimoloch spinifer (Ornithischia: Pachycephalosauria) with comments on cranial display structures in agonistic encounters. Journal of Vertebrate Paleontology 18:363–375.
Goodwin, M. B., and D. C. Evans. 2016. The early expression of squamosal horns and parietal ornamentation confirmed by new end-stage juvenile Pachycephalosaurus fossils from the Upper Cretaceous Hell Creek Formation, Montana. Journal of Vertebrate Paleontology.
Fowler DW. (2016) A new correlation of the Cretaceous formations of the Western Interior of the United States, I: Santonian-Maastrichtian formations and dinosaur biostratigraphy. PeerJ Preprints 4:e2554v1 https://doi.org/10.7287/peerj.preprints.2554v1
Longrich, N.R., Sankey, J. and Tanke, D. (2010) "Texacephale langstoni, a new genus of pachycephalosaurid (Dinosauria: Ornithischia) from the upper Campanian Aguja Formation, southern Texas, USA." Cretaceous Research
Schott, R. K., D. C. Evans, T. E. Williamson, T. D. Carr, and M. B.Goodwin. (2009) The anatomy and systematics of Colepiocephale lambei (Dinosauria: Pachycephalsauridae). Journal of Vertebrate Paleontology 29:771–786.
Schott, R. K., D. C. Evans, M. B. Goodwin, J. R. Horner, C. M. Brown, and N. R. Longrich. (2011) Cranial ontogeny in Stegoceras validum (Dinosauria: Pachycephalosauria): a quantitative model of pachycephalosaur dome growth and variation. PLoS One 6(6):e21092.
Williamson, T. E., and T. D. Carr. (2002) A new genus of derived pachycephalosaurian from western North America. Journal of Vertebrate Paleontology 22:779–801.
Woodward, H. N., Freedman Fowler, E.A., Farlow, J.O., Horner, J.R. (2015) "Maiasaura, a model organism for extinct population biology: a large sample statistical assessment of growth dynamics and survivorship." Paleobiology, October 2015