A bit of new news: Peter Galton, master of all things ornithischian, has just proposed a revision to what we call Stegosaurus, by setting aside the type species Stegosaurus armatus Marsh (1877) in favor of the more complete Stegosaurus stenops Marsh (1887). This follows Galton’s original (2010) indication that he would seek this revision, hot on the heels of Susannah Maidment’s revision of Stegosaurus, in which she referred virtually all meaningful material from Stegosaurus stenops Marsh (1887) and Stegosaurus ungulatus Marhs (1879) as well as Hesperosaurus mjosi Carpenter et al. (2001), the previous other “good” species, to Stegosaurus armatus, leaving only Stegosaurus armatus and Stegosaurus mjosi to stand, with Wuerhosaurus homheni from Asia (Dong, 1973) as Stegosaurus homheni.
Galton’s argument is based essentially on Maidment et al. (2008)’s diagnosis of Stegosaurus and Stegosaurus armatus on the basis of a number of features (12 out of 17) not actually present in the holotype specimen of Stegosaurus armatus, but due to the referral of the bulk of Morrison Stegosauridae to the hypodigm. Galton (2010) notes that the variation now validated for Stegosaurus armatus is not apparent in other stegosaur taxa (such as Kentrosaurus aethiopicus) while one of the 17 “autapomorphies” is also present in Stegosaurus mjosi, and uses these two criteria to argue that the referral is unfounded. Galton then argues that if the species is a nomen dubium, it cannot substantiate use of Stegosaurinae, Stegosauridae, Stegosauroidea, or Stegosauria, but this is fallacious: One of these, Stegosauria, is not based on nomenclature consistent with the scope of the ICZN, and does not require a type species to fixate it (it just helps); the other names, while they do require a type species, do not require the type species to NOT be a nomen dubium to be useful or used. As I stressed here, all a nomen dubium seems to be, in the ICZN, is a “doubtful name;” use of the nomen dubium as a type does not engender higher level taxa, although it is frequently implied that it should. The degree of the problem is thus over-stressed by Galton on this point, and needlessly so.
The name Stegosauridae, and all coordinated-rank taxa, follow the type species; thus, there will always be a Stegosauridae, and it will always include Stegosaurus armatus. What it may not include is Stegosaurus stenops or Stegosaurus ungulatus, if either of them fall out phylogenetically closer to other taxa than armatus does, rather than form a monophyletic group. In this case, we are given several options:
1. Rename those now-outlying taxa, such that they are now not members of Stegosauridae.
2. Designate a new types species to ensure the taxon Stegosauridae corresponds to a monophyletic one that includes the inferred stegosaurids stenops and ungulatus, even if it no longer includes armatus.
3. Designate a new type specimen for armatus to ensure the taxon Stegosauridae corresponds to a monophyletic one that includes the inferred stegosaurids stenops and ungulatus, and this time especially includes armatus.
4. Do nothing at the moment, and wait for substantive phylogenetic analysis including the restricted armatus type that will either distinguish the problem, or solve it, and then follow taxonomy from that point forward.
It is substantive in its own regard that doubt to the association of various species to a single essential complex on the basis of a single partial tail specimen and single plate (YPM 1850, holotype of Stegosaurus armatus Marsh, 1877) is enough to argue against referral.
As I may have argued, the validation of referral should be based on substantive similarity of material. If this is in doubt, then the material should not be referred. In this way, I disagree with Maidment et al. (2008), although Galton (2010) does not make this point, making an appeal to diversity instead to account for the distinction of species. But Galton is correct in at least one major manner: YPM 1850, and thus armatus, seem insufficient to correctly diagnose the complex that is Stegosaurus. Galton’s choice for a new type species, S. stenops, is based on Galton’s agreement with the large number of features it possesses of Maidment et al. (2008)’s complex diagnosing Stegosaurus and Stegosaurus armatus. It becomes clear that much of this is based on USNM
4923 4934, holotype of Stegosaurus stenops Marsh (1887) which is a nearly complete and articulated specimen preserved on a single block and available from both sides. Stegosaurus ungulatus Marsh (1879) is an earlier named, valid taxon according to Galton, based on a complex of several specimens of at least 30% completeness that totals most of the skeleton. Galton does not distinguish between these two taxa for his choice of type species in 2010, although the record in 2011 may permit him this, so I am unaware of why one and not the other.
That all said, if it increases the stability of the clades based on them, switching type species can be useful. But in this case, the type species of Stegosauridae (and its coordinate taxa ending in -inae and -oidea, et al.) does not need to be a “nomen validum” to be useful. This is largely done when the material used is insufficient to be the basis of any nomenclature, and this has yet to be determined. Maidment et al. (2008), in fact, showed the opposite, using YPM 1850 to assert the ability to differentiate this taxon from other stegosaurids. This only fails if, as Galton argues, stenops and ungulatus cannot be differentiated from armatus, but can from one another and other taxa, variation Maidment (et al., 2008; 2010) argue is individual and thus variable. I side with Galton on this matter, but note one solution is to designate a new type specimen of armatus, not necessarily a new type species for Stegosaurus. While the historical value of Stegosauridae is high, the likelihood that it will not include putative stegosaurids like stenops or ungulatus has not been supported in systematic analyses where armatus was included but itself not including the complex of Morrison stegosaur specimens. Thus the importance of a change in type species seems … premature. And indeed, a type specimen re-designation would seem more favorable, although unlike some taxa, most early-collected specimens of stegosaurid in the Morrison have ended up as types of one species or another, resulting in a plethora of Morrison Formation Stegosauridae.
Update: I wrote the last paragraph, and its request to substantiate the argued synonymies on the basis of phylogenetic work, without having read the supplemental material that accompanies Maidment et al. (2008). Thanks to Mickey Mortimer (my nemesis), this has been rectified.
Maidment et al. (2008) included in the coding for their Stegosaurus armatus OTU — as also seen in Mateus et al. (2010) — the bulk of stegosaurid specimens from the Morrison, including a specimen from the Lourinhã Group of Portugal. There is no statement following regarding the separation of codings per respective named species, although I suppose the matrix began this way. In the matrix, two characters (and the only two in the matrix) are shown with variable representation in Stegosaurus, the presence or absence of scapulocoracoid fusion and a coracoid foramen or notch; while the former may be ontogenetic, the latter is more ambiguous. Maidment et al. (2008) do not include putativelt discriminatory characters in their matrix, including those raised by Gilmore (1914) and Galton & Upchurch (2004). These features, including the presence of a ventral sacral keel, relative length of braincase characters, features of the distal bulbous expansion of the sacral and caudal neural spines, and the relative sizes and shapes of the base of the caudal plates and spines, are absent and their features, coded on an individual basis, might discriminate taxa therein presumed a priori to be synonymous. [References are updated below.]
Carpenter, K., Miles, C. A. & Cloward, K. 2001. New primitive stegosaur from the Morrison Formation, Wyoming. pg.455-484 in Carpenter (ed.) The Armored Dinosaurs. Indiana University Press (Bloomington).
Dong Z.-m. 1973. [Dinosaurs from Wuerho.] Institute of Paleontology and Paleoanthropology Memoir 11:45–52. (in Chinese)
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.
Galton, P. M. 2011. Case 3536. Stegosaurus Marsh, 1877 (Dinosauria, Ornithischia): Proposed replacement of the type species with Stegosaurus stenops Marsh, 1887. Bulletin of Zoological Nomenclature 68(2):127-133.
Galton, P. M. & Upchurch, P. 2004. Stegosauria. pg. 343–362 in Weishampel, Dodson & Osmólska (eds.) The Dinosauria (2nd edition). University of California Press (Berkeley).
Gilmore, C. W. 1914. Osteology of the armoured Dinosauria in the United States National Museum, with special reference to the genus Stegosaurus. United States National Museum Bulletin 89:1–143.
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.
Maidment, S. C. R. 2010. Stegosauria: A historical review of the body fossil record and phylogenetic relationships. Swiss Journal of Geosciences 103:199-210.
Marsh, O. C. 1877. A new order of extinct reptilia (Stegosauria) from the Jurassic of the Rocky Mountains. American Journal of Science (3rd series) 14:34–35.
Marsh, O. C. 1879. Notice of new Jurassic reptiles. American Journal of Science (3rd Series) 18:501–505.
Marsh, O. C. 1887. Principal characters of American Jurassic dinosaurs. Part IX: The skull and dermal armor of Stegosaurus. American Journal of Science (3rd series) 34:413–417.
Mateus, O., Maidment, S. C. R. & Christiansen, N. A. 2009. A new long-necked ‘sauropod-mimic’ stegosaur and the evolution of the plated dinosaurs. Proceedings of the Royal Society, B 276:1815-1821.