The first of November was yesterday, but since it was Sunday (when so few people are online), I decided to wait until today to announce the Pick Of The Month award for October. There were 23 blog posts covering PLoS ONE articles aggregated on ResearchBlogging.org in October. A couple of those were re-posts of older stuff, but several were really excellent.
Distinguishing the skulls of juveniles and adults of the same species, and sometimes different species, can be a prickly thing in the fossil record. The result is that paleontology is littered with juvenile fossils that have been considered adults at some time or another.
Yesterday in the open access journal PLoS ONE, paleontologists Jack Horner and Mark Goodwin published a long-awaited paper positing synonymy for a trio of iconic (and melodiously-named) dinosaurs. The bone-headed dinosaurs Pachycephalosaurus, Stygimoloch, and Dracorex are all one and the same animal, according to their work. The latter two are juvenile stages, whereas Pachycephalosaurus represents a full adult.
How is this possible? The animals look so different, right?
The article in question is the one by Horner, J., & Goodwin, M. (2009). Extreme cranial ontogeny in the Upper Cretaceous dinosaur Pachycephalosaurus.
Extended neoteny and late stage allometric growth increase morphological disparity between growth stages in at least some dinosaurs. Coupled with relatively low dinosaur density in the Upper Cretaceous of North America, ontogenetic transformational representatives are often difficult to distinguish. For example, many hadrosaurids previously reported to represent relatively small lambeosaurine species were demonstrated to be juveniles of the larger taxa. Marginocephalians (pachycephalosaurids + ceratopsids) undergo comparable and extreme cranial morphological change during ontogeny.
Cranial histology, morphology and computer tomography reveal patterns of internal skull development that show the purported diagnostic characters for the pachycephalosaurids Dracorex hogwartsia and Stygimoloch spinifer are ontogenetically derived features. Coronal histological sections of the frontoparietal dome of an adult Pachycephalosaurus wyomingensis reveal a dense structure composed of metaplastic bone with a variety of extremely fibrous and acellular tissue. Coronal histological sections and computer tomography of a skull and frontoparietal dome of Stygimoloch spinifer reveal an open intrafrontal suture indicative of a subadult stage of development. These dinosaurs 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. We propose that Dracorex hogwartsia, Stygimoloch spinifer and Pachycephalosaurus wyomingensis are the same taxon and represent an ontogenetic series united by shared morphology and increasing skull length.
Dracorex hogwartsia (juvenile) and Stygimoloch spinifer (subadult) are reinterpreted as younger growth stages of Pachycephalosaurus wyomingensis (adult). This synonymy reduces the number of pachycephalosaurid taxa from the Upper Cretaceous of North America and demonstrates the importance of cranial ontogeny in evaluating dinosaur diversity and taxonomy. These growth stages reflect a continuum rather than specific developmental steps defined by “known” terminal morphologies.
Congratulations both to Andy and to the authors of the article. I have notified the winners and their prizes are on the way. I hope you will read Andrew’s post and perhaps post a comment of your own, and then go to the article itself to read it and post comments, notes and ratings there as well.