Pachycephalosaurs share a host of derived features, most of them cranial (Figure 6.13). Figure 6.14 maps general trends in pachycephalosaur evolution. The most primitive pachycepha-losaurs on the cladogram are Asian, suggesting an Asian origin for the group. Because all but two pachycephalosaurs are from the Late Cretaceous, we infer that they underwent considerable evolution during that time.
There was an early tendency to thicken the skull roof as well as develop nerves for the sense of smell. The increased dome heights in some later forms may reflect the evolution of head-butting from simple pushing to complicated head-to-flank or head-to-head butting, to ritualized display.
Figure 6.13. Cladogram of Marginocephalia emphasizing the monophyly of Pachycephalosauria. Derived characters include: at 1, thickened skull roof, frontal excluded from orbital margin, tubercles on caudolateral margin of squamosal, thin, plate-like basal tubera, double ridge-and-groove articulations on dorsal vertebrae, elongate sacral ribs, caudal basket of fusiform ossified tendons, ilium with sigmoidal border, medial process on ilium, pubis nearly excluded from acetabulum, tubercles on squamosal, broad expansion of squamosal onto occiput, free ventral margin of the quadratojugal eliminated by contact between jugal and quadrate.
Figure 6.14. Cladogram showing evolutionary changes within selected pachycephalosaurs. Derived characters include: at 1, broad parietal bone, broad medial process on ilium; at 2, nasal and postorbitals incorporated into dome.
The broad girth of pachycephalosaurs is clearly derived relative to the narrower more primitive girths seen in most other ornithischians. It suggests a backward migration and enlargement of the digestive tract to occupy a position between the legs and under the tail. As was the case for thyreophorans (see introduction to Part II: Ornithischia; and Chapter 5), simple styles of chewing must have combined with fermentation-based digestion to increase the nutrition available to pachycephalosaurs from the plants they ate.
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