Image: Pachycephalosaurus ontogeny

Description: 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).
Title: Pachycephalosaurus ontogeny
Credit: Horner JR, Goodwin MB (2009) Extreme Cranial Ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus. PLoS ONE 4(10): e7626. doi:10.1371/journal.pone.0007626.g003
Author: John R. Horner, Mark B. Goodwin
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