Enhanced Morrison Stegosaur Diversity

[DD1991]

It's interesting to note that the ICZN has not yet published the results of a vote on Case 3536 (proposal to designate Stegosaurus stenops as the type species of Stegosaurus). (Last time I checked the June 2013 issue of the BZN online, no opinions of the ICZN were posted online yet.)

When I first read Galton's proposal to change the type species for Stegosaurus, I was starting to have some reservations about Case 3536 because part of the holotype of Stegosaurus armatus (YPM 1850) was still under preparation at the Morrison Museum of Natural History. I discussed Galton's proposal with Matthew Mossbrucker (Morrison Museum of Natural History)] and Mossbrucker informed me of an abstract discussing the diagnosability of S. armatus based on recent preparation of YPM 1850* (Mossbrucker et. al. 2009). But I felt compelled to support the proposals in Case 3536 because Susie Maidment and other workers might criticize for not ruling out the possibility that differences between S. armatus, S. stenops, and S. ungulatus could be a product of individual variation (Demirjian 2011). Later on, I (Demirjian 2012) published another comment in the Bulletin of Zoological Nomenclature urging the ICZN to address the priority of S. ungulatus over S. stenops in a final decision on Case 3536 because Stegosaurus ungulatus is based on sufficient material of diagnostic value to be considered valid, even if the lectotype (YPM 1853**) is less complete than the holotype of Stegosaurus stenops. Maybe some members of the ICZN know that Galton took the misguided step of preferring S. stenops over S. ungulatus as the candidate for the new type species of Stegosaurus because it was founded on the most complete holotype specimen of any Stegosaurus species in the Morrison, so the ICZN may have to settle on a compromise decision to make S. ungulatus as the type species of Stegosaurus.

The change of type species for Stegosaurus may circumvent questions about the conspecificity of S. armatus with either S. ungulatus or S. stenops, but it will have no impact on other Morrison stegosaur species, as Hesperosaurus is readily distinguished from Stegosaurus proper in having oval plates and a short, broad skull, and S. longispinus and S. sulcatus may warrant separate generic status if the presence of long tail spikes in S. longispinus and the presence of a parascapular/parapelvic spike on the shoulder region in S. sulcatus are confirmed as non-ontogenetic as per a histological analysis of UW 20503 and USNM 4937. Nonetheless, Morrison dino gurus are gradually beginning to appreciate the newly recognized generic diversity of the Morrison stegosaurs.

*YPM 1850 also contains some non-stegosaur specimens catalogued under the same number as Stegosaurus armatus, including an allosaurid tibia; the non-stegosaur elements need to be given a new museum catalogue number so that YPM 1850 is restricted to the stegosaur elements.

**YPM 1853 consists of material from two separate individuals found at the same quarry; archival research by Carpenter and Galton (2001) at the YPM shows that not all the spikes catalogued under YPM 1853 were found in situ with the tail vertebrae, as some of the spikes were found scattered apart at Reed Quarry 12 at Como Bluff. As a matter of fact, USNM 7414 (also from Reed Quarry 12) has some cervical vertebrae overlapping with the cervicals assigned to YPM 1853, further confirming that YPM 1853 is a composite of two individuals. Therefore, four of the spikes catalogued under YPM 1853 may belong to the same individual as USNM 7414 since the remaining spikes were found in situ with the tail vertebrae of YPM 1853.

Demirjian, V. (2011) Comments on Stegosaurus Marsh, 1877 (Dinosauria, Ornithischia): proposed replacement of the type species with Stegosaurus stenops Marsh, 1887 (Case 3536). Bulletin of Zoological Nomenclature, 68(3): 215-217.

Demirjian, V, 2012. Comment on Stegosaurus Marsh, 1877 (Dinosauria, Ornithischia): proposed replacement of the type species with Stegosaurus stenops Marsh, 1887 (Case 3536). Bulletin of Zoological Nomenclature 69 (1): 63-64.

Carpenter K, Galton PM (2001). "Othniel Charles Marsh and the Eight-Spiked Stegosaurus". In Carpenter, Kenneth(ed).The Armored Dinosaurs. Indiana University Press. pp. 76–102.

Mossbrucker, M. T., Bakker, R. T., and Prueher, L., 2009, 'New information regarding the holotype of Stegosaurus (Marsh 1877).' Symposium on Stegosauria Abstracts, Aathal, Switzerland, p. 9.