How many Stegosaurs in the Morrison Formation?

Fig 1.- Stegosaurus ungulatus.

Stegosaurus is one of the most iconic dinosaurs from the famed Late Jurassic Morrison Formation of North America. Artists have often depicted the plated “roofed lizard” as the formidable opponent to its nemesis, the fearsome theropod Allosaurus. The first remains of a stegosaur have been unearthed in Colorado during the Bone Wars and described by Othniel Charles Marsh in 1977 as Stegosaurus armatus. Since then, numerous specimens were found and described from different areas encompassing the Morrison Formation, in Colorado, Wyoming and Utah. Of the 11 or so names that were given to stegosaur remains from the United States, Marsh has described no less than 7 species.

Fig 2.- Hesperosaurus mjosi

But subtle differences seen in the bones can either be signatures of different species or simply reflect individual variations within the same species. How to define characteristics that are unique to a given species (autapomorphies) is easier said than done when the number of specimens is limited and when remains are fragmentary (i.e. statistics is lacking).

Fig 3.- Stegosaurus stenops

In a 2008 review of all stegosaurs, Susannah Maidment and colleagues made some very drastic cuts in the number of species present in the Morrison formation, recognizing only two valid ones: Stegosaurus armatus and Hesperosaurus mjosi that they renamed Stegosaurus mjosi. The other species are either junior synonyms of S. armatus or dubious (nomen dubium). However in another case of the now classic opposition between “splitters” (scientists who think there is only a narrow range of variation inside a species, thus considering more species as valid) and “lumpers” (scientists who think that there are few species with large individual variations), Peter Galton and colleagues in a 2010 paper had a closer look on the various remains from the Morrison formation and found that up to 7 species must be considered valid. To add a little twist to the story, they infer that the type species, Stegosaurus armatus, is a nomen dubium due to the fragmentary nature of the holotype that does not allow defining any autapomorphy. But if S. armatus were unvalidated, so would be the genus name, Stegosaurus. Consequently, a petition to the ICZN (International Commission of Zoological Nomenclature) was filed to make the best-known species, Stegosaurus stenops as the new type for the genus (good choice, nobody really wants our favorite plated lizard to be renamed Hypsirophus or Diracodon !).

Fig 4.- Stegosaurus longispinus, here depicted as a Kentrosaurus grade species

In view of the diversity of Ornithopods and Sauropods in the Morrison formation, it is not that surprising to see more than one species of stegosaurs living at the same time there. If we discard the species that are either too fragmentary or not properly described, we end up with some 4 or 5 different stegosaurs in North America. Among them, Hesperosaurus mjosi is the oldest, having been found close to the base of the formation. It is easily distinguishable by the plates, which are longer than taller. Stegosaurus stenops is the best known with several almost complete skeletons.  It is characterized by three unpaired alternating dermal plates just before the spikes while Stegosaurus ungulatus has three pairs of small dermal plates there. Interestingly, remains of a stegosaur from Portugal have tentatively been assigned to this particular species. Stegosaurus sulcatus with its large based tail spikes might also be another valid species.

Stegosaurus longispinus, characterized by longer tail spines or “thagomizer” is only known from materials from the rear of the animal, which were unfortunately mostly destroyed in the early 1920s. Because of the longer tail spines, some have argued that S. longispinus might have been a Kentrosaurus-like stegosaur, which would need to be redescribed as a new genus.

Note: as I am writing this, I realized this story was already covered much more eloquently and in depth by paleontologist Darren Naish a few months earlier (see his post here)…



References:

S. C. R. Maidment, D. B. Norman, P. M. Barrett and P. Upchurch. 2008. Systematics and phylogeny of Stegosauria (Dinosauria: Ornithischia). Journal of Systematic Palaeontology 6(4):367-407.

P. M. Galton, 2010. Species of plated dinosaur Stegosaurus (Morrison Formation, Late Jurassic) of western USA: new type species designation needed. Swiss Journal of Geosciences 103, 187-198.