<p>Pachycephalosaurus wyomingensis Gilmore, 1931 dinosaur (reconstruction) from the Cretaceous of western USA (public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA).
</p><p>The pachycephalosaurs were bipedal, herbivorous dinosaurs with distinctively thick-boned skull caps.  They are often referred to as the bone-headed dinosaurs (or dome-headed dinosaurs, or thick-headed dinosaurs).  The largest species and namesake of the group is Pachycephalosaurus wyomingensis Gilmore, 1931, a famous component of Montana’s Hell Creek Formation (Maastrichtian Stage, upper Upper Cretaceous, ~65-70 m.y.).  Not surprisingly, the thick bony skull caps are the most commonly found remains of this species (&amp; of all pachycephalosaurs).  The snout and back of skull were covered in knobby spikes.
</p><p>Pachycephalosaurs are usually inferred to have engaged in head-butting (as do modern bighorn sheep), with the thickened skull caps functioning as stress cushions to avoid brain damage.  Some workers suggest that head-to-flank pushing behavior was more likely in many species.  In Pachycephalosaurus wyomingensis, the bone layer above the brain cavity got to be nine inches thick, plus skull caps are known with scars.  It’s safe to conclude that this particular species did engage in head-butting.
</p><p>Classification: Animalia, Vertebrata, Dinosauria, Ornithischia, Genasauria, Cerapoda, Marginocephalia, Pachycephalosauridae
</p><p>Stratigraphy: Hell Creek Formation &amp; Lance Formation, near-uppermost Upper Cretaceous
</p><p>Locality: Wyoming &amp; Montana &amp; South Dakota, USA
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<hr><p>References:
</p><p>Maryanska, T.  1990  Pachycephalosauria.  pp. 564-577 in  The Dinosauria.  Berkeley.  University of California Press.
</p><p>Forster, C.A. &amp; P.C. Sereno.  1997.  Marginocephalians.  pp. 317-329 in  The Complete Dinosaur.  Bloomington, Indiana.  Indiana University Press.
</p><p>Sues, H.-D.  1997.  Pachycephalosauria.  pp. 511-513 in  Encyclopedia of Dinosaurs.  San Diego.  Academic Press.
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Horner, J.  2001.  Dinosaurs Under the Big Sky.  Missoula, Montana.  Mountain Press Publishing Company.  195 pp.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
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