page_title: Pachycephalosaurus
section_title: Growth
hierarchical_section_title: Pachycephalosaurus / Paleobiology / Growth
caption_attribution_description: English: Cranial ontogenetic sequence of Pachycephalosaurus wyomingensis with morphological landmarks highlighted in color. Scale bar = 5 cm. The ontogenetically oldest adult, AMNH 1696, in (A) dorsal and (B) right lateral views. A younger adult, UCMP 556078 (cast) with inflation of the frontoparietal dome+lateral cranial elements and mature nasal and squamosal nodal ornamentation in (C) dorsal and (D) right lateral views. MPM 8111, a partial skull of "Stygimoloch" in (E) dorsal and (F) left lateral views (reversed) illustrates the high narrow frontoparietal dome, squamosal nodes and horns characteristic of the subadult growth stage. Landmarks on the dorsal skull of MPM 8111 in orange (anterior) and red (posterior) constrain the position of the dome. The youngest growth stage in this cranial ontogenetic series is "Dracorex", TCNI 2004.17.1 (cast) in (G) dorsal and (H) right lateral views. The position of the squamosal horns and nasal nodes are consistent in these four pachycephalosaurid skulls, which increase in overall length and size from youngest (G,H) to oldest (A,B). Polski: Sekwencja zmian ontogenetycznych czaszki Pachycephalosaurus wyomingensis. Pasek skali = 5 cm.
context_page_description: Pachycephalosaurus is a genus of pachycephalosaurid dinosaurs. The type species, P. wyomingensis, is the only known species. It lived during the Late Cretaceous Period of what is now North America. Remains have been excavated in Montana, South Dakota, Wyoming and Alberta. It was a herbivorous creature which is primarily known from a single skull and a few extremely thick skull roofs, though more complete fossils have been found in recent years. Pachycephalosaurus was one of the last non-avian dinosaurs before the Cretaceous–Paleogene extinction event. Another dinosaur, Tylosteus of western North America, has been synonymized with Pachycephalosaurus, as have the genera Stygimoloch and Dracorex in recent studies.
  Like other pachycephalosaurids, Pachycephalosaurus was a bipedal herbivore with an extremely thick skull roof. It possessed long hindlimbs and small forelimbs. Pachycephalosaurus is the largest-known pachycephalosaur. The thick skull domes of Pachycephalosaurus and related genera gave rise to the hypothesis that pachycephalosaurs used their skulls in intra-species combat. This hypothesis has been disputed in recent years.
context_section_description: Dracorex and Stygimoloch were first supposed to be either juvenile or female Pachycephalosaurus at the 2007 annual meeting of the Society of Vertebrate Paleontology. Jack Horner of Montana State University presented evidence, from analysis of the skull of the single existing Dracorex specimen, that this dinosaur may well be a juvenile form of Stygimoloch. In addition, he presented data that indicates that both Stygimoloch and Dracorex may be juvenile forms of Pachycephalosaurus. Horner and M.B. Goodwin published their findings in 2009, showing that the spike/node and skull dome bones of all three 'species' exhibit extreme plasticity, and that both Dracorex and Stygimoloch are known only from juvenile specimens while Pachycephalosaurus is known only from adult specimens. These observations, in addition to the fact that all three forms lived in the same time and place, led them to conclude that Dracorex and Stygimoloch were simply juvenile Pachycephalosaurus, which lost spikes and grew domes as they aged. A 2010 study by Nick Longrich and colleagues also supported the hypothesis that all flat-skulled pachycephalosaur species were juveniles of the dome-headed adults, such as Goyocephale and Homalocephale. The discovery of baby skulls assigned to Pachycephalosaurus that were described in 2016 from two different bone beds in the Hell Creek Formation have been presented as further evidence for this hypothesis. The fossils, as described by David Evans and Mark Goodwin et al are identical to all three supposed genera in the placement of the rugose knobs on their skulls, and thus the unique features of Stygimoloch and Dracorex are instead morphologically consistent features on a Pachycephalosaurus growth curve.
caption: Growth series showing reduction of spikes and growth of dome with age, according to Horner and Goodwin
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