Among the most iconic and remarkable of dinosaurs are the stegosaurs, a mostly Jurassic group of thyreophorans famous for the rows of spikes and plates that decorated their necks, backs and tails [somewhat inaccurate Stegosaurus stenops shown below. I did it many years ago].
As I’m fond of saying, the stegosaur we know best – Stegosaurus – is an atypical member of the group. It’s particularly large and possesses lots of plates and but a few spikes. Stegosaurus may also be unusual in lacking shoulder spikes (aka parascapular spines), but this is less clear that it used to be since the ‘parascapular spines’ of some stegosaurs have been reinterpreted as tail spikes.
For all their familiarity, Stegosaurus and its relatives remain poorly understood and we don’t know as much about their biology, behaviour and evolution as we might like. Many very interesting suggestions have been made: that stegosaurs were low-browsers, keeping their heads down among the ferns and cycads; that they were high-browsers able to rear up into tripodal postures; that they used their plates for visual display or thermoregulation; and that they employed their tail spikes as weapons. A minor debate about the tissues lining their jaws has also appeared in the literature. All of these areas are amenable to actual testing. Remember that we’re still in the early stages of modelling the biomechanics of extinct animals, so efforts to properly analyse these issues have only recently gotten underway.
Arguments about sexual dimorphism, species-level diversity and the shape of the stegosaur family tree also make stegosaurs the source of continuing debate among experts. And as if these animals weren’t remarkable enough, recent discoveries have shown that they were more diverse than we thought. Miragaia longicollum from the Late Jurassic of Portugal, named in 2009 (Mateus et al. 2009), was a long-necked stegosaur with an incredible 17 neck vertebrae (more than all but the longest-necked sauropods). Gigantspinosaurus sichuanensis from the Late Jurassic of China, named in 1992, is remarkable in having parascapular spines that are about twice as long as its scapulae (Maidment & Wei 2006) [the mounted skeleton of Gigantspinosaurus sichuanensis - shown in a very weird (and probably impossible) crouching posture - is shown above. Note the huge parascapular spikes. The photo is by S. Maidment and previous appeared here on Dave Hone's Archosaur Musings].
In June 2009 a special ‘Symposium on Stegosauria’ was held at the Sauriermuseum Aathal, Switzerland. It featured 19 talks given by virtually all of the world’s stegosaur workers (Billon-Bruyat & Marty 2010). In September 2010, a set of submitted technical papers resulting from the symposium was published in a special issue of Swiss Journal of Geosciences (SJG). Much of this research is (in my view) extremely interesting and worthy of discussion, so in this and the following article I’ll be talking about various of the resulting discoveries and controversies. I wanted to get through it all sooner but simply was not able. Many thanks to Daniel Marty for his help. Anyway…
The Morrison stegosaur wars
Stegosaurus is one of several dinosaurs that’s more or less synonymous with the Morrison Formation of the US Western Interior. Actually, it’s not unique to the Morrison Formation, as remains from Portugal also seem to belong to this dinosaur (Escao et al. 2007) (and… read on). Relatively little known outside of the dinosaur research community is that Stegosaurus is not a singleton: numerous species have been named, differing in such features as the length and gracility of their hindlimbs and in the form and/or number of their plates and spikes. Unsurprisingly perhaps, different authors have expressed different views on which species are distinct and on which are synonymous. At one extreme, some authors have recognised numerous species perhaps divisible into several distinct genera. At the other extreme, others have regarded nearly all of them as belonging to the same species. The 2001 description of the new Morrison stegosaur Hesperosaurus mjosi has also become the source of disagreement, as we’ll see. Three papers in the SJG special issue are devoted to discussion of Morrison stegosaur taxonomy [adjacent Stegosaurus photographed at the IRSNB, Brussels].
In the longest of these, Peter Galton provides a review of all named Morrison stegosaur ‘species’ (Galton 2010). In a recent review of all stegosaurs, Maidment et al. (2008) concluded that most named Morrison stegosaurs are either nomina dubia or are subjective junior synonyms of Stegosaurus armatus Marsh, 1877 (the first Stegosaurus species to be named). This view stands in contrast to the ‘traditional’, polytypic concept of Stegosaurus: most authors have thought that Stegosaurus includes species with tall, subtriangular plates (like S. armatus) as well as species with proportionally bigger, diamond-shaped plates (like S. stenops). Some of the ‘tall plated’ species were long assumed to have eight tail spikes, but it now seems that this whole hypothesis rested on one throwaway speculation (made by Marsh in 1887). All Stegosaurus species, so far as we can tell, had four spikes (Carpenter & Galton 2001). Fact: the cluster of spikes at the stegosaur tail tip is known as the thagomizer.
Anyway, the main thrust of Galton’s new paper is to note his various disagreements (and, to be fair, agreements) with Maidment et al.’s (2008) proposals. If the latter authors are correct, S. armatus exhibited “a wide range of variation postcranially”, and apparently this is not the case in the only stegosaur known from many individuals (namely, Kentrosaurus aethiopicus from Tanzania) [adjacent stegosaurs photographed at the Sauriermuseum, Aathal; images courtesy of D. Marty. The upper image depicts 'Moritz' the Hesperosaurus].
Goodbye Stegosaurus, hello Diracodon?
The problem with S. armatus being the type species for Stegosaurus is that its type specimen (YPM 1850: a few vertebrae, a dorsal plate and fragments of pelvis and femur) seems to be non-diagnostic*. Maidment et al. (2008) avoided this problem both by regarding other Stegosaurus species as similar enough to the S. armatus holotype to warrant inclusion within S. armatus, and by relying on various of the specimens belonging to these ‘species’ for their diagnosis of S. armatus. If, like Galton, you don’t think that those other species are ‘enough alike’ S. armatus to be included within it, then S. armatus stands alone. Accordingly, the name Stegosaurus itself is a nomen dubium. What to do?
* Though not everyone agrees on this. Mossbrucker et al. (2009) described how new preparatory work has revealed the presence of unusually robust neural spines in YPM 1850, and they suggest that this makes S. armatus a diagnosable taxon. We await further news.
One solution could be to give up on the name Stegosaurus altogether and provide a new generic name for the remaining valid species. Or, re-use an old name: Diracodon Marsh, 1881 is available. In the interests of promoting stability, it would not be wise to get rid of the name Stegosaurus; the more sensible solution is to elect a new species to be the type, with S. stenops Marsh, 1887 being the best choice (Galton 2010). Galton is wrong, however, in saying that the dubious status of S. armatus poses a problem for the names Stegosauridae and Stegosauria, since the ICZN (= International Commission on Zoological Nomenclature) does not mandate that higher-level taxa need to be based on currently recognised genera.
Of the Stegosaurus species named in addition to S. armatus, Galton (2010) suggests that S. ungulatus Marsh, 1879 is valid and diagnosable, and so is S. stenops Marsh, 1887. S. sulcatus Marsh, 1887 – notable for the massively expanded bases of its tail spikes – was regarded by Maidment et al. (2008) as a nomen dubium classified only as Stegosauria indet. Galton (2010) argues that this taxon is also likely diagnostic (the enlarged tail spike bases are suggested to be autapomorphic) [Stegosaurus and Allosaurus mount shown below photographed at the Denver Museum of Nature and Science].
S. longispinus – notable for its transversely flattened tail spikes with their subtle constrictions part-way along the shafts – was also regarded as a nomen dubium by Maidment et al. (2008), and is also resurrected as a valid species by Galton (2010). The caudal vertebrae of S. longispinus are unusual, being vertically compressed and with rounded (rather than hexagonal) articular surfaces (some people suggest that these differences are big enough to indicate that S. longispinus might not belong in Stegosaurus at all).
In addition to these Stegosaurus species, Galton (2010) also notes that Hypsirophus discurus Cope, 1878 (named for vertebrae and a rib fragment) might also be valid. Small details present on its vertebrae are different from the corresponding regions of other stegosaur vertebrae.
Hesperosaurus and Wuerhosaurus: to sink or not to sink?
By far the most argued about Morrison stegosaur is Hesperosaurus mjosi. Described by Carpenter et al. (2001) as a close relative of Dacentrurus, Hesperosaurus was reinterpreted by Maidment et al. (2008) as a species of Stegosaurus. Galton (2010) argues that H. mjosi is substantially different from Stegosaurus and definitely warrants recognition as a distinct genus. Unsurprisingly, the same argument is put forward by Kenneth Carpenter (2010) in another of the special issue’s articles.
As Carpenter (2010) explains, Hesperosaurus differs from Stegosaurus sensu stricto in details of skull shape, in neck length (13 cervical vertebrae versus 10 in Stegosaurus sensu stricto) and body length (13 dorsal vertebrae versus 17 in S. stenops), in the shape of its vertebrae, scapulocoracoid and pelvis, and in plate shape (low and oval versus tall and sub-triangular in Stegosaurus sensu stricto). I’m inclined to agree with Carpenter (2010) that these many differences do make Hesperosaurus ‘different enough’ to deserve its own genus, and this is especially true when we see how very similar many traditionally recognised Mesozoic dinosaur genera are to one another (skeletally, at least) [skeletal reconstructions of S. stenops and H. mjosi compared, from Carpenter (2010). The Stegosaurus is © G. Paul].
On that note, it’s increasingly widely recognised that our view of how similar or how distinct a species must be before it warrants separation as a ‘genus’ is arbitrary and definitely not consistent across groups. As Greg Paul has pointed out in many of his writings, Corythosaurus-like lambeosaurines and Centrosaurus-like centrosaurines, for example, exhibit less skeletal variation than do species included within, say, Varanus. Ergo, one could make the argument that all those lambeosaurines should be lumped into the same genus (and this is exactly what Paul does) [adjacent composite shows but a little of the morphological variation present within Varanus. The V. exanthematicus in the middle is by Shizhao and is from wikipedia; the prasinoids at the bottom were photographed by El Cattivo86, image is from here. The Komodo dragon was photographed at ZSL London Zoo].
The contrary argument is that – compared to many other ‘genera’ – Varanus encompasses too much disparity and is overdue for splitting. My conclusion on this matter is that we need to totally give up on the idea that ‘genera’ (and, for that matter, species) need to be consistent across clades: these Linnaean categories are human constructs, employed for convenience. Clades are real, but whether any given clade represents a ‘family’, a ‘genus’ or even a ‘species’ is down to opinion, and my advice is that research communities should decide among themselves as to the taxonomy they want to follow. When considering Mesozoic dinosaurs, the community has (mostly) agreed that we’re happy with relatively small ‘genera’, not over-bloated, massively inclusive ones on par with Varanus. Seen within this framework, Hesperosaurus is indeed ‘distinct enough’ for a genus, and this is true even if it’s recovered as the sister-taxon to Stegosaurus sensu stricto.
Several new Hesperosaurus specimens have recently been discovered and are currently under study, and it’s hoped that the new information they provide will help resolve the phylogenetic status of this stegosaur.
Carpenter (2010) also disagrees with another taxonomic decision made by Maidment et al. (2008). They argued that Wuerhosaurus homheni from the Valanginian-Albian Lianmuqing Formation of China should also be reclassified as a species of Stegosaurus because “the elements preserved are identical in most respects to those of Stegosaurus armatus” (pp. 13-14), and because Wuerhosaurus grouped together with Stegosaurus sensu stricto in a cladistic analysis. Yet, as Carpenter (2010) says, the known elements of Wuerhosaurus look quite different from those of Stegosaurus sensu stricto (one example: the dorsal plates of Wuerhosaurus are low and sub-rectangular, and unlike the tall, sub-triangular plates of Stegosaurus sensu stricto) [UPDATE: see Susie Maidment's comment below: comment # 47]. As with Hesperosaurus, again a good argument can be made that Wuerhosaurus is ‘distinct enough’ to get its own genus [Brian Franczak's reconstruction of Wuerhosaurus is shown below. Brian, oh Brian, where are you now?].
As should by now be obvious, Galton’s (2010) and Carpenter’s (2010) conclusions on Morrison stegosaur taxonomy differ quite substantially from Maidment et al.’s (2008). I don’t know who’s ‘right’. I’m not personally a fan of the rather uncritical mass lumping practised by Maidment et al. (2008), but at the same time I do appreciate that individuals within a species are not carbon copies of one another. Indeed, osteological variation within species can be enormous (examples within non-domesticate Tetrapoda: Homo sapiens, Ursus arctos, the Dinornis moa).
Why care? What does it all mean?
Seriously, well done you if you’ve read this through to the end: I know it’s the sort of stuff that sends a lot of people to sleep. As I hope I’ve said before, taxonomic shufflings of the sort discussed here are often assumed to be meaningless exercises in trivia. But in fact they’re really very important if we want to get a handle on phylogeny, diversity, community ecology and such.
Let’s look at what the competing taxonomic proposals mean for the success and duration of Stegosaurus. If Maidment et al. (2008) are right about Wuerhosaurus being synonymous with Stegosaurus, then this taxon survived from the Late Jurassic into the Early Cretaceous: a duration of between c. 17 and c. 37 million years (the uncertainty is due to the poorly constrained age of the Lianmuqing Formation). A duration of this sort is not impossible, but it would be exceptional given that most dinosaur genera have durations of just a few million years. It would also mean that Stegosaurus occurred across North America, Europe and China. In short, this taxonomic decision makes Stegosaurus truly exceptional, and arguably among the ‘most successful’ of dinosaurs ever.
Turning now to Morrison taxa, if the Late Jurassic ecosystem preserved in the Morrison Formation was – say – home to as many as six (or more) stegosaur species, one might make all kinds of inferences about stegosaur success across time, and about resource competition among Morrison herbivores. The presence of many species would show that several lineages were thriving at this time, it would show that North America was globally important in the evolution of this group, and it would indicate that several contemporaneous low-level browsers were either competing, or carving up the niches in the Morrison ecosystem [the cartoon below illustrates this view of high diversity. The stegosaurs shown were not necessarily contemporaneous; Hesperosaurus in particular is from very low down in the Morrison Formation and was thus comparatively old. Larger versions of the cartoons shown here can be seen on the Tet Zoo facebook page].
Now picture how different things are if all the Morrison stegosaurs belong to just two species [as shown below, using Maidment et al.'s (2008) taxonomy]. Stegosaurs as a whole must be perceived as doing less well overall, North America is just another place where stegosaurs are found (and not necessarily an ‘important’ place in terms of their evolution), and Morrison palaeo-environments were not as packed with ecologically similar low-level browsers as they would be in the multi-species model.
So, who is right: are there lots of species, or just one or two? (I’ve already explained above how I think that the inclusion of Wuerhosaurus within Stegosaurus is incorrect). The great frustration that arises whenever such issues are discussed – and, believe me, they’re discussed all the time in palaeontology – is that they can never be definitely resolved in the absence of huge samples and/or of molecular data. Specimen-level phylogenetic analyses can help, but they don’t necessarily allow you to make clear decisions about where one ‘species’ ends and another begins. Nevertheless, someone should do this work with the Morrison stegosaurs to see what happens (does, for example, S. longispinus fall within the clade that includes S. ungulatus and S. stenops, or does it go elsewhere and hence not belong to Stegosaurus?).
Detailed, statistically compelling morphometric studies can be used to test the ‘closeness’ of individual specimens (and, hey, maybe someone should do work of this sort on the Morrison stegosaurs as well) but you can still argue that such tests aren’t watertight: some modern species are just about skeletally indistinguishable from close relatives while, conversely, some species contain appreciable and confusing amounts of variation, as noted above. Because we remain constrained in terms of what information we have available on Mesozoic dinosaurs, a ‘diagnostic species approach’ seems the most appropriate one: if you find characters that look to be diagnostic, and if you can’t convincingly show that these are best explained by sexual dimorphism, ontogeny or deformation, you should assume that you’re dealing with species. Agree or disagree? Let me know.
More on stegosaurs to come next… don’t worry, it will not involve taxonomy or phylogeny.
Stegosaurs have been mentioned on a few previous occasions on Tet Zoo. Please see…
- A most atypical stegosaur
- Where the scelidosaurs and iguanodontians roam
- Patagonian Mesozoic Reptiles, a book review
Refs – –
Billon-Bruyat, J.-P. & Marty, D. 2010. Preface: Symposium on Stegosauria proceedings. An international conference on stegosaur finds of the world organized by the Sauriermuseum Aathal (8th and 9th June 2009, Aathal, Switzerland). Swiss Journal of Geosciences 103, 139-141.
Carpenter, K. 2010. Species concept in North American stegosaurs. Swiss Journal of Geosciences 103, 155-162.
– . & Galton, P. M. 2001. Othniel Charles Marsh and the myth of the eight-spiked Stegosaurus. In Carpenter, K. (ed) The Armored Dinosaurs. Indiana University Press (Bloomington and Indianapolis), pp. 76-102.
Escaso, F., Ortega, F., Dantas, P., Malafaia, E., Pimentel, N., Pereda-Suberbiola, X., Sanz, J. L., Kullberg, J. C., Kullberg, M. & Barriga, F. 2007. New evidence of shared dinosaur across Upper Jurassic Proto-North Atlantic: Stegosaurus from Portugal. Naturwissenschaften 94, 367-374.
Galton, P. M. 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.
Maidment, S., Norman, D., Barrett, P., & Upchurch, P. (2008). Systematics and phylogeny of Stegosauria (Dinosauria: Ornithischia) Journal of Systematic Palaeontology, 6 (04) DOI: 10.1017/S1477201908002459
– . & Wei, G. 2006. A review of the Late Jurassic stegosaurs (Dinosauria, Stegosauria) from the People’s Republic of China. Geological Magazine 143, 621-634.
Mateus, O., Maidment, S. C. R. & Christiansen, N. A. 2009. A new long-necked ‘sauropod-mimic’ stegosaur and the evolution of plated dinosaurs. Proceedings of the Royal Society B doi:10.1098/rspb.2008.1909.
Mossbrucker, M. T., Bakker, R. T. & Prueher, L. 2009. New information regarding the holotype of Stegosaurus (Marsh 1877). In Symposium on Stegosauria Abstracts. Sauriermuseum, Aathal, Switzerland, p. 9.