Paleobiology
The adaptive significance of the skull dome has been heavily debated. The popular hypothesis among the general public that the skull was used in head-butting, as sort of a dinosaurian battering ram, was first proposed by Colbert 1955. This view was popularized in the 1956 science fiction story "A Gun for Dinosaur" by L. Sprague de Camp. Many paleontologists have since argued for the head-butting hypothesis, including Galton 1970 and Sues 1978. In this hypothesis, pachycephalosaurs rammed each other head-on, as do modern-day mountain goats and musk oxen.
Anatomical evidence for combative behavior includes vertebral articulations providing spinal rigidity, and the shape of the back indicating strong neck musculature. It has been suggested that pachycephalosaurs could make their head, neck, and body horizontally straight, in order to transmit stress during ramming. However, in no known dinosaur can the head, neck, and body be oriented in such a position. Instead, the cervical and anterior dorsal vertebrae of pachycephalosaurs show that the neck was carried in an "S"- or "U"-shaped curve.
Also, the rounded shape of the skull would lessen the contacted surface area during head-butting, resulting in glancing blows. Other possibilities include flank-butting, defense against predators, or both. The relatively wide width of pachycephalosaurs (which would protect vital internal organs from harm during flank-butting) and the squamosal horns of the Stygimoloch (which would have been used to great effect during flank-butting) add credence to the flank-butting hypothesis.
A histological study conducted by Goodwin & Horner 2004 argued against the battering ram hypothesis. They argued that the dome was "an ephemeral ontogenetic stage", the spongy bone structure could not sustain the blows of combat, and the radial pattern was simply an effect of rapid growth. Later biomechanical analyses by Snively & Cox 2008 and Snively & Theodor 2011 concluded, however, that the domes could withstand combat stresses. Lehman 2010 argued that the growth patterns discussed by Goodwin and Horner are not inconsistent with head-butting behavior.
Goodwin & Horner 2004 instead argued that the dome functioned for species recognition. There is evidence that the dome had some form of external covering, and it is reasonable to consider the dome may have been brightly covered, or subject to change color seasonally. Due to the nature of the fossil record, however, it cannot be observed if color actually played a role in dome function.
Longrich, Sankey & Tanke 2010 argued that species recognition is an unlikely evolutionary cause for the dome because dome forms are not notably different between species. Because of this general similarity, several genera of Pachycephalosauridae were incorrectly lumped together. This is unlike the case in ceratopsians and hadrosaurids, which had much more distinct cranial ornamentation. Longrich et al. argued that instead the dome had a mechanical function, one which was important enough to justify the resource investment, such as combat.
Read more about this topic: Pachycephalosauria