[D] Stygimoloch spinifer [Su] [jG]
Galton & Sues, 1983
Cretaceous Late Maastrichtian
Ornithischia Genasauria Cerapoda Marginocephalia Pachycephalosauria Pachycephalosauridae
Hell Creek Formation, Montana, North Dakota, Lance Formation, Wyoming, US
Pachycephalosaurus (Brown& Schlaikjer, 1943) = Tylosteus (Leidy, 1872)
Pachycephalosaurus > Pachycephalosaurus wyomingensis (Gilmore, 1931) >> Troodon wyomingensis (Gilmore, 1931) >>> Tylosteus ornatus (Leidy, 1872), Pachycephalosaurus grangeri (Brown & Schlaikjer, 1943) Pachycephalosaurus reinheimeri (Brown & Schlaikjer, 1943)
Pachycephalosaurus (Brown& Schlaikjer, 1943) > Pachycephalosaurus wyomingensis (Gilmore, 1931) > Dracorex hogwartsia [Juvenile] (Bakker, Sullivan, Porter, Larson & Saulsbury, 2006) > Stygimoloch spinifer [subadult] Galton & Sues, 1983 >> Stenotholus kohleri (Giffin, Gabriel & Johnson, 1987)
Dracorex hogwartsia (juvenile) and Stygimoloch spinifer (subadult) are reinterpreted as younger growth stages of Pachycephalosaurus wyomingensis (adult). This synonymy reduces the number of pachycephalosaurid taxa from the Upper Cretaceous of North America and demonstrates the importance of cranial ontogeny in evaluating dinosaur diversity and taxonomy. These growth stages reflect a continuum rather than specific developmental steps defined by ‘‘known’’ terminal morphologies. (Horner JR, Goodwin MB ,2009)
Based only on parts of squamosals but can be characterized by a prominent squamosal shelf, ontogenic closure of the supratemporal fenestrae, and massive horn-cores on the squamosals.
A new skull of Stygimoloch spinifer (MPM 8111) from the Upper Cretaceous Hell Creek Formation of North Dakota is the most complete specimen discovered to date. It allows much of the skull and braincase of this unusual pachycephalosaurid to be described for the first time and confirms a suite of diagnostic characters for the species.
The skull is long with a vaulted, transversely narrow frontoparietal dome and a robust squamosal forming a prominent posterior shelf The shelf is ornamented by three to four large, low- angle horns and multiple clusters of smaller bony nodes. The orientation of the squamosal is preserved along an unambiguous contact with the frontoparietal suture, allowing definitive determination of the orientation of the squamosal horns.
These cranial features indicate a different mode of agonistic behavior than previously suggested for Stegoceras and Pachycephalosaurus. The high, narrow dome of Stygimoloch spinifer is not suited for head-butting, and the orientation of its squamosal horns and ornamental nodes strongly suggest display functions.
Several additional specimens are described and referred to Stygimoloch spinifer. Abstract from: MB Goodwin, EA Buchholtz, and RE Johnson, Journal of Vertebrate Paleontology, 1998, 18(2): 363-375