Comments on Stegosaurus Marsh, 1877 (Dinosauria, Ornithischia): proposed replacement of the type species with Stegosaurus stenops Marsh, 1887 (Case 3536) 3

Publication Type:Journal Article
Year of Publication:2011
Authors:Demirjian, V
Journal:Bulletin of Zoological Nomenclature
Volume:68
Issue:3
Start Page:215
Pagination:215-216
Date Published:09/2011
Type of Article:Comment
ISSN:0007-5167
Full Text

I am writing in support of the petition (Case 3536) by Galton to replace Stegosaurus armatus Marsh, 1877 with S. stenops Marsh, 1887 as the type species of Stegosaurus Marsh, 1877.

Maidment et al. (2008) diagnosed Stegosaurus on the basis of the following autapomorphies: (1) Quadrate-squamosal-paroccipital process articulation over- hangs the retroarticular process of the lower jaw; (2) postzygapophyses on posterior cervical vertebrae are elongated posteriorly and overhang the back of the centrum; (3) transverse processes on anterior caudal vertebrae (except for caudals one and two) project ventrally rather than laterally; (4) large, rectangular acromial process of the scapula; (5) supra-acetabular process diverges at an angle of 90 degrees from the anterior process of the ilium; and (6) medial process present on the posterior iliac process of the ilium. They also noted that Stegosaurus armatus (= Stegosaurus sensu Carpenter et al. 2001 of my usage) differs from all other stegosaurs in having: (1) edentulous portion of the dentary anterior to the tooth row and posterior to the predentary; (2) dorsally elevated postzygapophyses of the cervical vertebrae; (3) bifurcated summits of the neural spines of the anterior and middle caudal vertebrae; (4) unexpanded posterior end of the pubis; and (5) dermal ossicles embedded in the skin on the underside of the cervical region. They referred all stegosaur taxa from the Morrison Formation (except Stegosaurus sulcatus, S. longispinus, and Hesperosaurus mjosi) to S. armatus.

Of the autapomorphies cited for STEGOSAURINAE (=Stegosaurus) and Stegosaurus (= S. armatus) by Maidment et al., only two characters can be observed in the holotype of Stegosaurus armatus (YPM 1850): transverse processes on anterior caudal vertebrae (except for caudals one and two) project ventrally rather than laterally and bifurcated summits of the neural spines of the anterior and middle caudal vertebrae. As acknowledged by Galton (2010), the presence of transverse processes on anterior caudal vertebrae (except for caudals one and two) that project ventrally rather than laterally is not confined to YPM 1850 and other specimens referred to S. armatus by Maidment et al. (e.g. USNM 4934, YPM 1853) but is also found in Hesperosaurus mjosi and Stegosaurus longispinus. The caudals of YPM 1850 exhibit bifurcated summits of the neural spines of the anterior and middle caudal vertebrae (Carpenter & Galton, 2001, fig. 4.4G; Galton, 2010, fig. 1b), an autapomorphy of Stegosaurus armatus according to Maidment et al., but as Galton demonstrated, this character is also present in Stegosaurus ungulatus (YPM 1853, YPM 1858), S. stenops (USNM 4934, DMNS 2818), S. longispinus (UW 20503), and the holotype of Hypsirophus discursus (AMNH 5731). Using the updated list of synapomorphies for Stegosauria, and STEGOSAURIDAE provided by Mateus et al. (2009, supplementary information), a stegosaurian placement of S. armatus is supported by the presence of two parasagittal rows of plates or spines extending from the cervical region to the end of the tail (Carpenter & Galton, 2001, fig. 4.5C). YPM 1850 can be assigned to STEGOSAURIDAE based on the presence of a dorsal process on transverse process of caudal vertebrae and anterior caudal vertebrae with bulbous swellings at the top of neural spines (Carpenter & Galton, 2001, figs 4.4D, F).

Using the criteria outlined by Galton regarding the autapomorphic structure of dermal armor for Morrison stegosaur species, Stegosaurus ungulatus, S. stenops, S. longispinus, and Hesperosaurus mjosi differ from each other in the form of the dermal armor, as well as characters of the femur and ilium, as noted by Galton. However, except for fragments of a large dermal plate, no dermal armor is preserved in the holotype of S. armatus, so YPM 1850 lacks any dermal characters that would distinguish it from S. ungulatus, S. stenops, S. longispinus, or Hesperosaurus mjosi.

In a recent abstract, Mossbrucker et al. (2009) indicated that the holotype of Stegosaurus armatus is distinguishable from other Morrison stegosaurs by the presence of unusually robust neural spines, based on recent preparation of the holotype at the Morrison Natural History Museum (MNHM). However, this character is likely to be a product of individual variation within a species, and the results of Mossbrucker et al. have not yet been published. Thus, STEGOSAURINAE (= Stegosaurus sensu Maidment et al., 2008) comprises three valid genera, Hespero- saurus, Stegosaurus, and Wuerhosaurus; Stegosaurus sensu Carpenter et al., 2001 (= Stegosaurus armatus sensu Maidment et al., 2008) comprises three valid species (Stegosaurus ungulatus, S. stenops, and S. longispinus), with Stegosaurus armatus, Hypsirophus discursus, Diracodon laticeps, and Stegosaurus sulcatus referable to Stegosaurus sensu stricto (restricted to S. stenops, S. longispinus, and S. ungulatus) as nomina dubia. I provisionally agree with Galton in considering S. armatus a nomen dubium and restricting it to YPM 1850 until the results of Mossbrucker et al. are published and YPM 1850 is fully described.

Additional references

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: Biological Sciences, 276: 1815–1821.

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Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith