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.
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Good post!
My point of view is a bit radical: since the biological species concept is operatively a non-sense among fossil taxa, we must avoid any kind of discussion involving the biological species concept in palaeontology.
Palaeontological species are, merely, unique combinations of apomorphies, and don't differ from supra-specific clades (supraspecific), contrary to what occurs in neontology.
IMHO, most (if not all) discussions in neontology that refer to the biological species concept (for example, many ecological models), when are translated to palaeontology are, simply, non-sense because the biological species cannot be determined in an unambiguous way from fossils. The palaeontological analogy cannot work.
How can we determine if the Morrison Stegosauridae were only a single or very few ecologically-generalist and morphologically-disparate species or they belonged to several eco-morphologically specialised species? How many stegosaurid specimens are known in the entire Formation? Very few, so both scenarios are possible, but, as far as I know, both cannot be falsified, and, therefore, any discussion based on them (or any argument supporting or challenging one of them) is out of science.
Palaeontology is not neontology: the Deep Time irreversibly erases a lot of informations, including the biological species.
What's unfortunate is that individual variation and sexual dimorphism are hard to "prove" with any dinosaur. Only when you have specimens coming out of your ears (Protoceratops) can you start to confidently make those kinds of assertions. As for Stegosaurus/Hespersaurus, is it possible that Hespersaurus is ancestral to Stegosaurus, given its age?
Something that's interested me lately is the idea of anagenetic change and how that affects taxonomic naming practices. For example, look at Thalassocnus, the aquatic sloth. There are currently five recognized species, and they definately form a nice morphological series from basal to derived. It's easy to ses when you see them all in sequence. But if only the most basal and most derived species were found? Now THEY look like separate genera. It's an interesting conundrum.
I actually love your summaries of arguments about taxonomy. It's a good way to catch up on the subject.
Wonderful article!
Stegosaurs had been the main subject of my senior research for nearly two years (which is complete and the abstract is submitted to GSA) and you have described the taxonomic issues in stegosaurs perfectly. It seems that Maidment was using the "Paul's lumping method" of taxonomic lumping Hesperosaurus and Wuehrosaurus as Stegosaurus, doesn't make a whole lot of sense because, as you mentioned, there a lot of distinctive morphological differences between these genuses and the genus, Stegosaurus surviving in the Early Cretaceous seems rather weird IMHO. Then again, that's less confusing compared to the descriptions of numerous Stegosaurus species found in the Morrison Formation, which is my biggest problem with stegosaurs. How is it possible for many Stegosaurus species could have coexisted with each other, despite the fact that they all pretty much have the same ecological niche as low-browsers? Perhaps different species have feed on different kinds of plants, but there's no way to be absolutely certain because no gut contents or coprolites have been found for stegosaurs (Martin, 2006), so that will remain a mystery for the moment. Not only that, but there would be too much competition (from not only from other stegosaurs, but also basal iguanodont and other ornithischian dinosaurs as well) going on that the ecosystem could not support such a large species diversity of Stegosaurus to exist. I could see maybe one or two different species of Stegosaurus and Hesperosaurs coexisting together, but six seems too much (then again, the Morrison Formation has a high diversity of enormous sauropod species with similar ecological niches).
Anyway, great article and I look forward to Part II of your stegosaur special! :)
Cheers,
Jonathan Smith
Martin, A. J. (2006). Thyreophora. Introduction to the study of dinosaurs (2nd ed., pp. 363-385). Malden, MA: Blackwell Pub..
For those not familiar with the etymology of "Thagomizer," the Wikipedia article has the story.
I more or less agree with Andrea. Genera are just bookkeeping conventions. So are species in most cases (maybe all). The only reason we have them is because working at the level of the individual is very often impractical. Unfortunately there's not much we can do about that, though.
Maybe this taxonomy talk does send some people to sleep, but I find it fascinating - particularly as it can quickly get complicated, and not only demonstrates how experts' opinions can vary but inevitably delves into the history of palaeontology. More please!
"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"
Great article, but why is that? Does having a multitude of species render a genus more successful? Would [i]Homo[/i] as a whole be perceived as doing better and filling more ecological niches than if instead of having 6 billion [i]sapiens[/i] spread out over the world, it had say, 2 billion [i]sapiens[/i] in Africa, 2 billion [i]erectus[/i] in Asia, and 2 billion [i]neanderthalensis[/i] in Europe? And this is with having a hypothetical scenario with actually different taxa, rather than merely having a rose by another name.
-snoozing- armored armadillos with optional thagomiser
Given the remarkable completeness of the USNM roadkill Stegosaurus and the lack of a parascapular spine in it, I would feel pretty confident in saying that Stegosaurus does not have such a spine. I seriously doubt they have been misidentified as tail spikes in this genus.
Great post.
People who don't find these questions interesting are not very good scientists. They tend to be more like physicists.
Thanks indeed for comments, very interesting.
Jonathan (comment 3): I don't know that even six species of stegosaur is 'too many' for the Morrison. As you note, Morrison sauropod diversity is surprisingly high with several taxa having been named in recent years (Eobrontosaurus, 1994, Suuwassea, 2004, Apatodiploplodosaurus, 2010)... and more yet to come. Remember that the Morrison is just HUGE: it spans about 10 million years and extends over 1.5 million square km. That's enough time and space to allow for about as many stegosaur species as you want.
Dan (comment 9): sorry, I may not have explained things well. I didn't mean to imply that Stegosaurus might have parascapular spines*; rather, I meant that some stegosaurs often thought to possess parascapular spines might not have had them after all (Maidment et al. (2008) suggested that the supposed parascapular spine of the Loricatosaurus priscus holotype is actually a tail spike).
* Though... whatever happened to the rumour I once heard that semi-circular parascapular 'splates' had been discovered in a Stegosaurus?
Hesperosaurus occurs lower in the Morrison than Stegosaurus but how do these various putative Stegosaurus species relate to each other stratigraphically (and geographically)?
And several relatively complete Stegosaurus specimens are now known; how do these compare to the species named on more fragmentary material?
Can they be used to augment the descriptions of some of these species?
The answers to these questions might clear up some of this
uncertainty.
There is obviously more work to be done here.
LeeB.
I really like these Phylogenetic wrestlings contrary to what you might believe Darren.
I guess, I'm one of the few people who can sit for hours (with a pint of lager or a Caol Ila) in an Airport waiting for a loved one and just 'read' and wonder about the Classification of Mammals of McKenna and Bell.
Talking about that
Do anybody know if there's a new 'Classification-book' lurking out there in the shadows ?
because this one is really falling apart (tape and all)
Darren; re comment #11: are you sure of the spelling of Apatodiploplodosaurus? (Google doesn't have it, and i make mistakes when I try to type words that long!)
"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."
Thank you! How I wish Wilson and Upchurch had never started that myth. Of course I don't think the ICZN mandates that valid genera must have diagnostic type species as well.
I do have a question that this seems like an appropriate audience for. It's common in dinosaur paleontologists to argue there can't be too many similar species in one habitat, but is that what we see in modern ecosystems?
"Or"? What about "and"? As in, "The plates were used in sexual display and thermoregulation."
Alan's rule of anatomy; Any anatomical feature that can be used for multiple purposes will, in due time, be used for multiple purposes.
"I do have a question that this seems like an appropriate audience for. It's common in dinosaur paleontologists to argue there can't be too many similar species in one habitat, but is that what we see in modern ecosystems?"
It is probably hard to get any real comparison simply because modern-day terrestrial ecosystems are so pauperized compared to just 50,000 years ago :-(
South America might be a good example, weren't there some 6 different species of glyptodonts plus a couple of 100 to 300 kilo tortoises stomping around in the Brazilian grasslands?
<< It's common in dinosaur paleontologists to argue there can't be too many similar species in one habitat, but is that what we see in modern ecosystems?>>
I think this depends on the dietary preferences and size. For example in the tropical grassland ecosystems it is common for a large number of distinct species of herbivores to be present in each size guild. Each shows slight differences in terms of feeding behavior and niche occupation but overall similarity in diet and resource exploitation. In contrast, the largest predator guild tend to be less species rich.
I would imagine the Mesozoic ecosystems dominated by endothermic stem-birds and to a lesser extant stem-crocodiles and mammaliforms be comparable. Post-Triassic among the stem-birds, I would imagine lower diversity amongst the largest predatory theropods and greater diversity among the smaller predatory theropods. The greatest diversity would be among ornithischians or herbivorous theropods with sauropods falling in the middle. I would also imagine that as in modern ecosystems the morphological disparity would greatest among the smaller sized carnivores/insectivores and herbivores.
So in conclusion the diversity of stegosaurids in the vast Morrison Fm is hardly unexpected.
Raymond,
on current classifications there were about 17 species of glyptodonts in 8 genera in South America during the latest Pleistocene (Lujanian) but only 4 species and 3 genera were tropical Brazilian.
The majority, 12 species in 6 genera were from north and central Argentina.
Their diversity decreases as you move away from there.
There were also 2 or more species of the non-glyptodont Pachyarmatherium in South America at that time and several species of large tortoise in the genus Chelonoidis.
Also the ground sloths in South America had about double this diversity but more evenly spread around the continent.
LeeB.
@Mickey: "It's common in dinosaur paleontologists to argue there can't be too many similar species in one habitat, but is that what we see in modern ecosystems?"
What do you mean for "similar species"?
As I wrote in my comment, if you refer to species identical in the skeleton but differing for other (soft) parts, this kind of question is probably a non-sense in palaeontology (a false problem, like the gender of angels). It's possible that what we consider a palaeontological species was in reality a bunch of several morphologically identical (at least in the skeleton) but biologically (=genetically/ethologically) distinct species. It's plausible (it's based on an actualistic analogy) but there is not evidence for testing such hypothesis, nor for its falsification. So, how a scientist can work with such questions?
Comment #14: Yeah I am pretty sure the name is Amphicoealis brontodiplodocus.
Are you sure that the state of S. armatus doesn't have an effect on it's family? I'm pretty sure that Stegosauridae/Stegosauria would involve the genus Stegosaurus in some capacity, so it's taxonomic status could cause problems for the family.
That's just my POV anyway.
What a lot of articulate and well thought out comments. Congratulations, you all. I suppose it falls to me to break the winning streak...
I was going to ask how inclusive the Morrison formation was, thanks Darren. It's hard to believe in specimens laid down two million years apart placed in the same biological species, so Andrea's point is appealing.
For over-bloated, massively inclusive genera, look no farther than Insecta. Varanus? Bah. I wonder how many genera (or families) our eventual procyonic successors will distribute Canis lupus familiaris among.
Finally, it's gratifying to see a Stegosaurid reconstruction with horizontal tail spikes.
Such great comments, thanks indeed everyone. The more complicated questions about stegosaur distribution in the Morrison (e.g., where the holotypes come from relative to one another, and how the scrappy holotypes compare to the few good skeletons) will take too much time to deal with properly... though I'll state quickly that some (though not all) of the Morrison 'species' are stratigraphically close to one another. I think most come from the Brushy Basin Member. As noted, Hesperosaurus is from way down in the section.
I do want to respond to Mickey's comments (comment 15)...
I don't think it was necessarily 'started' by Wilson and Upchurch: I've encountered this idea a few times when reviewing manuscripts. Authors often seem to think that the problematic status of a type genus automatically renders any 'family' (or other) name problematic, but it doesn't. Article 29 (on the formation of 'family' names) doesn't include any mandate about the type genus being valid but mostly recommends avoidance of homonymy and correct spelling etc. Article 64 states that "An author who wishes to establish a new nominal family-group taxon may choose as type genus any included nominal genus the name of which he or she regards as valid, not necessarily that having the oldest name". The validity of Stegosaurus is subjective anyway (as discussed above, it isn't a concern if you refer diagnostic specimens to S. armatus).
It's hard to find specific statements in the technical literature (he says without checking), but I've definitely encountered this opinion informally. One British palaeontologist even tried to tell me that the presence of two contemporaneous iguanodontian species in the Wealden was unlikely since big animals can't co-exist in the same habitat. This is nonsense. In the modern world, the diversity of megaherbivores is very low, and I suppose this is what inspires suggestions that prehistoric environments must have been similar. But in the only region with a reasonable megafauna (tropical Africa), there are (or, were, historically) places where you can still have as many as 6 or 7 herbivores weighing more than 1000 kg (two white rhinos, black rhino, hippo, giraffe, forest and savannah elephants). Of course, the modern world is horribly impoverished. Prior to the end-Pleistocene extinctions, the fossil record shows that at least several similar big herbivore species lived alongside one another just about everywhere.
As for similar herbivores in general (and not necessarily giants): as one example, look at the many similar antelopes that inhabit the same habitats in Africa. These species mostly avoid competing by feeding at different levels and choosing different microhabitats, but in some cases they do compete directly. When we look at modern and Cenozoic megafaunal assemblages, it never seems to me that Mesozoic assemblages have 'too many' dinosaurs: rather, they donât have enough!
Great post. On the subject of diversity, is it possible that there were different morphs not just between different genders but also within a particular gender, especially males? I can think of several species, from birds to fish, where there are distinct body types and behaviours associated with different reproductive or feeding strategies, but how you would detect this when faced with a fossil I cannot imagine, unless you were fortunate enough to have a mass assemblage to sort through.
Darren Naish | December 30, 2010 6:45 AM:
What are the chances a given formation (let's just take the Morrison formation for an example) would preserve at least 1 specimen for 90% of the genera of large vertebrates that were alive during the period in question?
I know very little about fossilization, but it seems to me the chances are not high. It seems to me that "not enough" dinosaurs of any given type (such as "large herbivore") is the result that should be expected.
Great post, but I would be more interested in life of these animals. For example, what about juvenile present on one photo? Were these gigantic shoulder spines of Gigantspinosaurus useful for defense, or only for display? BTW, I just see here an overlooked star syndrome - Gigantspinosaurus or Kentrosaurus are just as improbable, but popular culture shows Stegosaurus around and around.
I don't buy paleontologists' idea about too many species in one place/too long time/distribution. Anyway, multiple genera of modern mammals (eg. Felis, Panthera, Vulpes, Cervus) have distribution spanning many continents. Guilds of 10 herbivores and 5 or more carnivores in one habitat are nothing unusual now or in Pleistocene. Note, that dinosaurs were much bigger than mammals, so Stegosaurus in its time was only mid-sized browser, large browsers were sauropods.
BTW, I wonder if paleontologists would be able to distinguish such species pairs as gaur/banteng, horse/wild ass, grizzly/brown bear, wolf/coyote, red deer/fallow deer (minus antlers) if they were known from only bones and so fragmentary as most dinosaurs?
Jerzy: this whole article was about systematics and such... discussion of lifestyle and palaeobiology will follow in the next article. Do remember, though, that - as stated early on in this article (and many times beforehand!!) - we just _can't_ know as much about palaeobiology and behaviour as we might like. The data just isn't there.
On distribution of taxa... at least some dinosaur 'genera' (like Stegosaurus and Allosaurus) do seem to have had large, trans-continental distributions. But what's weird is that many didn't: a lot of the Late Cretaceous hadrosaurs and ceratopsians were extremely provincial. As for distinguishing similar species (like wolves and coyotes*), it _can_ be done with bones if you have big enough samples (and even then there are problems).
* Of your other examples, brown bears and grizzlies are conspecific, and red deer and fallow deer are notably different and easy to distinguish osteologically.
So... preparation was continued after one hundred twenty two years.
I so hate it when fossils are described before preparation is finished, and then preparation is never finished. I so loathe it!
That the reasons are most likely financial in nature only shifts my hate from scientists and preparators to bureaucrats and politicians, but it doesn't lessen it in the least. Someone, and several generations of their dead predecessors, needs to be smacked upside the head. Hard.
With sternum and furcula in Allosaurus (...in completely wrong positions...)! :-) :-) :-)
Lianmuqing. G never occurs in front of i in Standard Mandarin.
And even then it depends on your definition of "species", as does this, which is not a simple fact:
All this leads straight to your next question:
I say fuck it all.
Clades, diagnosed by autapomorphies, should be named all the way down. Most species concepts, including arguably all interesting ones, are simply not applicable to a couple of old bones. Only the ICZN forces us to pretend otherwise; only the ICZN forces us to refer every organism to a species if we want to talk about it at all. You cannot have a genus without a species, it says.
The PhyloCode will allow using specimens that do not belong to an already named species as specifiers for new clade names.
In the meantime, I recommend what most "dinosaurologists" are already doing: monospecific genera.
Uh... what? Are physicists "not very good scientists"?
At least Drosophila (1,450 species... I repeat: 1,450 species) is now being split. Get used to Sophophora melanogaster -- the ICZN rejected the proposal to make it the new type species of Drosophila.
Stegosaurus armatus is the type species of Stegosaurus. If S. armatus is a nomen dubium, so is Stegosaurus.
Stegosaurus is the type genus of Stegosauridae. If S. is a nomen dubium, so is Stegosauridae.
Being above the family group of ranks, Stegosauria does not have a type. It is not required to contain Stegosaurus, and it cannot be a nomen dubium.
Keep in mind that "nomen dubium" does not mean "invalid for the purposes of nomenclature" or anything like it. It's a mere practical problem, and nomenclature is about principles, not about taxonomic practice. The ICZN barely even mentions the term "nomen dubium"; it has no standing in it.
Kentrosaurus is featured in every single dinosaur book for children that has more than 10 pages.
I'm not sure if Gigantspinosaurus has even been validly published. There are photos of it on the Internet (duh), but not in print!
Depending on the species concept, wolves, red wolves, and coyotes are one, two, or three species. Or maybe 10 if you split the wolves further.
@Darren
Waiting for paleobiology.
"a lot of the Late Cretaceous hadrosaurs and ceratopsians were extremely provincial."
Similar to modern medium-sized ungulates. Many antelope, deer and wild goats/sheep have very small ranges.
BTW, I would agree that hadrosaurs and ceratopians are oversplit - their crests and frills evolved as easily as horns and antlers of modern ungulates.
"As for distinguishing similar species (like wolves and coyotes*), it _can_ be done with bones if you have big enough samples (and even then there are problems)."
Yes, but we know them in flesh beforehand and know how many species exist and who belongs to whom. It would be different if one had situation similar to dinosaurs - limited number of partial specimens of various age and sex, and not know how many species there should be.
Suppose a biologist from future knows nothing about a wolf, coyote, or any live canids. He has as his only study material fragmented bones of Canis from Western North America. Would he, after studying bones, arrive at the correct conclusion that there exist two species: bigger gray wolf and smaller coyote? Or, would he think there is only one variable species? Or split young wolves and coyotes into one species and adults in another? Or believe in three or more sympatric species?
How would similar study come about red deer and fallow deer in Europe? Or, say, wapiti and sika in Eastern Asia?
"brown bears and grizzlies"
My mistake, naturally, black bears and grizzlies.
"I'm not sure if Gigantspinosaurus has even been validly published. There are photos of it on the Internet (duh), but not in print!" (from David Marjanovic)
I'm fairly certain of have its description in my archive somewhere...maybe it's just a review of Asian stegosaurs, now that I think about it. Hmph.
A part of me thinks that dinosaurs like ceratopsids are oversplit, but another part thinks that genus specifiers are useful in that they give a shorthand for the animal. Pachyrhinosaurus could mean the boss-nosed centrosaurine from this time period and this locality. Styracosaurus cold mean the spike-frilled ceratopsid from this time period and this other locality...I don't know if that makes any sense to anyone but me.
A couple of brief things...
Gigantspinosaurus is too valid. Figures were provided by Peng et al. (2005) and Maidment et al. (2008).
Wolves: the idea that the Eastern wolf or Eastern Canadian wolf warrants specific status (as Canis lycaon) is becoming more popular (for more see Grewal et al. (2004) and Kyle et al. (2006)). Red wolves are apparently mostly part of this taxon. In Asia, lineages termed C. himalayensis and C. indica appear to be outside of the C. lupus + domestic dog clade (Aggarwal et al. 2007).
Refs - -
Aggarwal, R. K., Kivisild, T., Ramadevi, J. & Singh, L. 2007. Mitochondrial DNA coding region sequences support the phylogenetic distinction of two Indian wolf species. Journal of Zoological Systematics and Evolutionary Research 45, 163-172.
Grewal, S. K., Wilson, P. J., Kung, T. K., Shami, K., Theberge, M. T., Theberge, J. B. & White, B. N. 2004. A genetic assessment of the Eastern wolf (Canis lycaon) in Algonquin Provincial Park. Journal of Mammalogy 85, 625-632.
Kyle, C. J., Johnson, A. R., Patterson, B. R., Wilson, P. J., Shami, K., Grewal, S. K. & White, B. N. 2006. Genetic nature of eastern wolves: past, present and future. Conservation Genetics 7, 273-287.
Maidment, S. C. R., Norman, D. B., Barrett, P. M. & Upchurch, P. 2008. Systematics and phylogeny of Stegosauria (Dinosauria: Ornithischia). Journal of Systematic Palaeontology 6, 367-407.
Peng, G., Ye, Y., Gao, Y., Shu, C.-K. & Jiang, S. 2005. Jurassic Dinosaur Faunas in Zigong. Sichuan Scientific and Technological Publishing House (Zigong).
"I don't think it was necessarily 'started' by Wilson and Upchurch: I've encountered this idea a few times when reviewing manuscripts."
I know I first heard it from their Titanosaurus paper that famously dropped Titanosauridae and made Lithostrotia. Since then people have complained about it in relation to Ceratops and other taxa.
"Stegosaurus armatus is the type species of Stegosaurus. If S. armatus is a nomen dubium, so is Stegosaurus.
Stegosaurus is the type genus of Stegosauridae. If S. is a nomen dubium, so is Stegosauridae."
Find me the ICZN rule that says this.
Thanks for the replies on biodiversity of "ecologically similar" taxa.
that may be an awkward pose for the stegosaur skeleton...but its also the most plausible-looking Godzilla ever.
Happy New Year, to you and your family and friends.
C. indicus of course. Latin words in -cis and -nis are masculine.
Actually, you're right -- it depends on just how undiagnostic the type specimen is. Or at least that would make sense; the ICZN simply doesn't mention it.
Outside the glossary ("A Latin term meaning 'a name of unknown or doubtful application'."), the term "nomen dubium" only occurs twice in the ICZN:
List of changes in the introduction to the 4th edition:
"8. If the existing name-bearing type of a species-group taxon is indeterminate, so that the correct application of the name to a particular taxon is doubtful (i.e. the name is a nomen dubium), an author should request the Commission to set it aside and designate a neotype."
(That's interesting. It's completely contrary to current practice.)
Art. 75.5:
"75.5. Replacement of unidentifiable name-bearing type by a neotype. When an author considers that the taxonomic identity of a nominal species-group taxon cannot be determined from its existing name-bearing type (i.e. its name is a nomen dubium), and stability or universality are threatened thereby, the author may request the Commission to set aside under its plenary power [Art. 81] the existing name-bearing type and designate a neotype.
Example. The holotype of the ammonite species Cycloceras laevigatum M'Coy, 1844 lacked important diagnostic features. Upon request the Commission under its plenary power set aside the type status of this specimen and designated a neotype (Opinion 1720 (1993))."
That's it.
Not a scientist, so can't really contribute to the interesting parts of the discussion, but I've always loved stegosaurs. They were my favorite dinosaur when I was a kid, better than Triceratops! I still adore Robert Bakker's vision of the stegosaur from The Dinosaur Heresies, inaccurate and hypothetical as it may be: "A five-ton ballet dancer with an armor-plated tutu of flipping bony triangles and a swinging war club." (Chapter 11: Mesozoic Arms Race, p. 233)
It's nice to see that they're still being talked about, illustrated and examined.
Question: Aside from size and the allometric changes accompanying it, and aside from the fact that modern paleontologists suppose they walked with legs straight under and had their plates upright instead of flat on the back, what differences do they have from armadillos? I confess that it seems to me they were really giant armadillos.
The animal in Bakker's illustration was Stegosaurus stenops which he separates into it's own genus Diracodon.
Speaking of which if S. armatus is invalid I would think that the next best species to use for the type should be S. ungulatus as every source I have read (that dates from later than the 1970's) considers that animal to be just a junior synonym of S. armatus.
This way if S. stenops does turn out to be different enough to be it's own genus it won't cause the confusion I have seen since Iguanodon's type was changed.
"Actually, you're right -- it depends on just how undiagnostic the type specimen is. Or at least that would make sense; the ICZN simply doesn't mention it."
See? Yet because of Wilson and Upchurch, everyone assumes the ICZN says undiagnostic species can't have diagnostic genera and family-level groups based on them.
""8. If the existing name-bearing type of a species-group taxon is indeterminate, so that the correct application of the name to a particular taxon is doubtful (i.e. the name is a nomen dubium), an author should request the Commission to set it aside and designate a neotype."
(That's interesting. It's completely contrary to current practice.)"
Yup. This is all why I argued on the DML that the ICZN should reject Galton's proposal. Not only does armatus' status not matter for any higher taxa, the proper thing to do would be to propose a neotype for it, not make stenops the type species.
A well written and informative post. I suspect the debate regarding the taxonomic relationships of the Stegosauria is going to run and run. The state of disarticulation of the plates in many specimens combined with their fragmented, worn and often crushed appearance only adds to the confusion in my opinion.
:-o
You're kidding, right?
One look at their skeletons, and you can tell which one is a dinosaur and which one a mammal.
"At least Drosophila (1,450 species... I repeat: 1,450 species) is now being split. Get used to Sophophora melanogaster -- the ICZN rejected the proposal to make it the new type species of Drosophila."
If you type "drosophila not sophophora" in pubmed, you find that the reaction of the molecular biology community to this was basically to laugh, shrug and to continue on publishing their D. melanogaster papers. ;)
Laurence Crosson (#37)--
Aside from the fact that Armadillos are mammals and Stegosaurs are dinosaurs, and that Stegosaurs are MUCH bigger...
--The erect plates on the backs of Stegosaurs mean that there is no armor on most of the skin surface, and so is very different from the surface-covering armor of Armadillos. Another "family" of dinosaurs, the Ankylosaurs, are more like Armadillos in this regard than Stegosaurs are.
--Armadillos don't have "thagomizers" at the ends of their tails. (Some Glyptodonts -- an extinct group of armored mammals related to Armadillos -- did have tail "clubs," spiky in some species.)
--Armadillos are more or less omnivorous, and they have simple, peg-like, teeth. Stegosaurs are thought to have been primarily plant-eaters, and have teeth that can do a bit of cutting: slicing and dicing their plant food.
---
Lots of different animals have independently evolved armor. Among dinosaurs, in addition to the Stego- and Ankylosaurs, some Titanosaurs (members of the long-necked Sauropod "family" of dinosaurs) also had armor plates over at least parts of their bodies. Among mammals, Pangolins (more closely related to cats and dogs than to Armadillos!) are about as thoroughly armored as Armadillos, but their armor scheme is very different in its details.
Frankly, I'm not surprised. It would most likely have been easier for everyone if the ICZN had accepted the petition.
One way or another, Drosophila needs to be split.
I haven't had time to read the comments yet, but I certainly intend to. Apologies if this has been covered or is just ignorant; my two cents:
Sexually selected traits seem to be more rapidly altered than naturally selected traits, in bits and pieces I dimly recall from reading science articles (genes controlling surface features like hair and eye color in humans showing rapid change, etc).
If the plates are read as sex appeal or simply as species signifiers for environments where more than one species of stegosaur is found, then I imagine they could change radically in size and shape over time. So we could see a species fundamentally the same but with great variation in that one trait over a geologically short period of time.
In that case, if plate shape is the only significant difference between two species, I'd suggest lumping. But as the post indicates, some species have other pretty dramatic differences (number of vertebra) and different genera should be used for them. Says this guy.
-
Oh no!!!! Now they're going to cause Stegosaurus to explode, just like poor Iguanodon!
Nice summary of the situation Darren. I do take exception to the following, however:
"..uncritical mass lumping practised by Maidment et al. (2008).."
It wasn't 'uncritical' at all - I examined pretty much every stegosaur specimen known from anywhere in the world. My lumping conclusions are based firstly on how different the other stegosaur genera were to each other relative to how different 'Hesperosaurus' and Stegosaurus are to each other, and secondly the results of my cladistic analysis, in which 'Wuerhosaurus', 'Hesperosaurus' and Stegosaurus formed a clade. I think half the problem is that a lot of stego research is very American-centric, so that some people only look at the American stuff, and never look at anything from the rest of the world (at least, not first hand). When you have two different taxa and that's all you look at, you're bound to see the differences between them. But when you throw in a lot of other taxa too, you are better able to decide if the differences are important or not.
With regard to 'Wuerhosaurus', I have examined the specimen first hand, and I can assure all your readers that Carpenter's observation that the plates are low and long is incorrect: the plate is just broken dorsally, as I stated in my 2008 paper. It's just the base of the plate that remains, so we really don't know what shape it was. However, it is strongly transversely compressed, like the plates of Stegosaurus. Perhaps more importantly, the iliosacral block is indistinguishable from that of Stegosaurus (and also 'Hesperosaurus').
I accept a lot of people disagree with my conclusions regarding Stegosaurus, but I maintain that I'm the only person who's examined all of the material. I urge people to go take a look for themselves.. and make up their own mind.. If you disagree after that, then fair enough. But please, please, don't base your taxonomic conclusions on grainy photographs from photocopies of 1980s Vertebrata Palasiatica articles.
In insects, it is not only raw species numbers that make a genus unmanageable; rather the genus' diagnosability. Large genera such as Tipula (Diptera: Tipulidae, ca 2800 species), Megaselia (Diptera: Phoridae, about 1500 species) and Chrysis (Hymenoptera: Chrysididae, slightly over 1000 species) survive partly because it takes lots of work to reconstruct the phylogenies to base eventual splitting on, but also because they (to my knowledge) seem to be monophyletic. Drosophila does indeed need to be split, but that is not because of its size. It is because it has repeatedly been found to be horrendously paraphyletic, and because further lumping would create loads of secondary homonyms.
As for my derogatory use of the term "physicist", it is not based on disregard of the physical sciences. Ernest Rutherford famously once said that all scientists either are physicists or stamp collectors. My previous comment was an almost verbatim quote from Luis Alvarez; except his contempt was for palaeontologists.
My point, buried under massive layers of intertextuality, is that under the over-simplistic classification of Rutherford the stamp collector mentality is the one best suited for answering complex biological questions. I apologize for any misunderstandings I might have caused.
What do you mean?
And nomenclature isn't a biological question (though phylogeny, ontogenetic variation, sexual dimorphism etc. of course are).
Sigh! J'aime la taxonomie!
Allen Hazen and David MarjanoviÄ. Thank you for kindly replying to my zoologically naïve question. The intended meaning was something like this: If evolution were cyclical, then what creature living today would a future Stegosaur evolve from (and back into)?
Unfortunately, this question makes so little sense that I can't answer it, because evolution is as far from cyclical as it could possibly be. It's not an arrow either; it doesn't have a direction at all. It's not even on the same page. I'm trying to say I don't know how to imagine a cyclical kind of evolution.
Mutations happen; some mutations make reproduction in the current environment more difficult and thus tend to die out; some others make reproduction in the current environment easier and thus tend to spread. That's all.
If the environment goes through cyclical variations, evolution will tend to follow. The question is, what is the closest analogue?
Is stegosaurus larger than gigantspinosaurus just want to know and can dacentrosarus be just as large thank you.
But even if the environment goes through cyclical variations, there are many cases where there's more than one solution to the same problem. There is no reason to expect that the same solution would emerge every time. Evolution builds on what is already there, plus random -- unpredictable -- mutations.
In short, there is no reason to assume that there ever will be such a thing as "a future stegosaur".
I guess David is not big on cloning prehistoric animals from DNA. I agree that may be difficult even with soft tissue from a T-Rex. There are, of course, many reasons for convergent evolution and the re-sprouting of similar variations on the evolutionary bush. While mutations are random, the environment limits the successful ones to a narrow range. In any case, what I am hoping for is ideas about analogues. Dollo's Law might amount to just a rule even as Cope's "Law" has. In geology cyclical change is a well accepted fact and the Milankovic cycles are well-known. If there won't be a future stegosaur, what did it evolve from?