Dinosauria by Weishampel David B

Author:Weishampel, David B.
Language: eng
Format: epub
Publisher: University of California Press

TWENTY-ONE

Pachycephalosauria

TERESA MARYASKA

RALPH E. CHAPMAN

DAVID B. WEISHAMPEL

Pachycephalosauria, often called dome-headed or thick-headed dinosaurs, is a group of bipedal ornithischians with thickened bones of the skull roof. Pachycephalosaurians retain a few primitive ornithischian features, including a short premaxilla, premaxillary dentition, pronounced heterodonty, small leaflike cheek teeth, a mandible with a low coronoid process and a moderately developed retroarticular process, and an ischium without an obturator process. The group is restricted to the Late Cretaceous, with the exception of two Early Cretaceous species, Yaverlandia bitholus and Stenopelix valdensis. Pachycephalosauria, as treated in this chapter, includes 14 genera, all but one (Stegoceras) of which are monospecific. The number of specimens per species varies from one in most species to several dozen in Stegoceras validum. Complete skeletons and skulls of Pachycephalosauria are scarce, but pachycephalosaurian domes or skull fragments are commonly found in western North America (see, e.g., Giffin 1989; Goodwin 1990; D. B Brinkman et al. 1998); the geographic distribution of the group is a wide one and it occurs throughout the Northern Hemisphere, mostly in western North America and central Asia, but also in Europe (table 21.1). A single species (Majungatholus atopus), previously reported as a pachycephalosaur from Gondwana (Sues and Taquet 1979), is not a member of this clade, but is instead a theropod according to recent discoveries (Sampson et al. 1998).

The ancestry of Pachycephalosauria has long been thought to be an open question; however, a number of derived cranial and pelvic characters link Pachycephalosauria with Ceratopsia (Maryaska and Osmólska 1974; Sereno 1986, 1997, 2000).

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