Pachycephalosauria

    Known in vernacular as the "bone-heads," the pachycephalosaurians are members of a bizarre group of ornithischians with greatly thickened skulls. It has been thought that the derived members came in two families,  Homalocephalidae and Pachycephalosauridae, and this still may be the case once more remains are found, but no support has been found for Homalocephalidae in recent analyses. "Homalocephalids" were known as "flat-headed," while the pachycephalosaurids are "dome-headed."  Within Pachycephalosauridae the members are classified by skull characters (as that's often all there is to go on), including closure of certain holes.
    Whom the closest relatives of pachycephalosaurians are is a subject that has long been debated.  In the past, stegosaurians, ankylosaurians, and ornithopods have been suggested. Today, they fit relatively comfortably with the ceratopsians in Marginocephalia.
    Pachycephalosaurians are reliably known only from the Cretaceous of the northern continents; a supposed large Gondwana taxa, known as Majungatholus, has turned out to be an unusual thick-headed abelisaurid, Majungasaurus (oddly, there are what have been described as possible tiny pachycephalosaurians from the early Norian [LTr] of India, although the odds aren't great, as getting bone expansions on the skull has been a popular evolutionary option for hundreds of millions of years among dozens of groups; wait for the paper, I guess). They are typically small bipeds with large skulls relative to body size, and a variety of bony ornamentation decorating the skull. They are known from rather poor remains, usually only thickened frontoparietal domes for pachycephalosaurids (these, the frontal and parietal, being the bones involved in the domes); as a result, certain parts, like hands, have not yet been discovered. They are often reconstructed with five fingers, a reasonable number considering the basal number of manual digits in Cerapoda is five, and bipedal cerapodans typically have either four or five. "Homalocephalids" are currently known from better skeletal material than pachycephalosaurids.
    The purpose of the immensely thick skulls has long been debated. The actual brain of a pachycephalosaurian is very small in relation to the size of the skull (although the parts relating to the sense of smell are expanded, suggesting a keen snout), so the massive skull was not for holding a large brain. For several decades, the common interpretation has been that the animals used them to fight amongst themselves in the manner of bighorn sheep. "Homalocephalids" would have pushed each other, and pachycephalosaurids would have rammed each other. There are some problems with this idea, including the fact that the round domes of pachycephalosaurids may not have allowed good contact, and could have sent the heads of ramming opponents twisting in directions detrimental to the health of the neck. Thus, the commonly-illustrated proposal has lost favor to the idea that the domes were mainly for display, but it is making a comeback, with flank-butting and/or head shoving preferred over "domes-at-ten-paces".
    With the proposal of sheep-like head butting came the proposal the animals lived like bighorn sheep as well, in herds in upland areas. Neither idea is improbable or contradictory to known evidence; for example, North American pachies are usually known only from worn skull remains, suggesting the bones traveled a long distance to the lowlands. Males and females have been tentatively identified from Stegoceras skull remains.
    Because pachycephalosaurian teeth bear a passing resemblance to those of troodontids, an inaccurate illustration of a Troodon tooth led to the idea Troodon was a pachycephalosaurian and the same as Stegoceras. In old works this group may be called Troodontidae. Fittingly enough, the genus Yaverlandia, once thought to be an Early Cretaceous pachycephalosaurid, is actually a troodontid.
    Current pachy research is divided into two camps, which in the mid -2000s seemed to publish a big paper a year largely reversing what the other side said (this isn't strictly true, but sometimes it feels like this). The rhetoric has reached the "overheated" stage, and a third-party review by someone who knows these animals is in order, but I think there's only two groups of pachy researchers at this point. What I've got is a sort of mismatch guaranteed to upset partisans of both groups. A new monkey wrench is the possibility that "flat-headed" pachies are fully-domed taxa that haven't hit puberty. The flagship sequence for this new interpretation is spiky Dracorex going to small domed and more spiky Stygimoloch going to less spiky but big domed Pachycephalosaurus.

<--Pachycephalosauria
      |--Wannanosaurus
      `--+--Goyocephale
           `--+--Homalocephale
                `--Pachycephalosauridae
                     |--Colepiocephale
                     |--Stegoceras
                     `--+--+--Amtocephale
                          |    |--Hanssuesia
                          |    |--Prenocephale
                          |    |--Sphaerotholus
                          |    `--Tylocephale
                          `--Pachycephalosaurus

Pachycephalosauria: Being given a generic name ending with a conjugation of "-tholus" has been unlucky for pachies: Stenotholus turned out to be Stygimoloch (and then Pachycephalosaurus), Ornatotholus is a juvenile Stegoceras, Majungatholus turned out to be an abelisaurid, Gravitholus is dubious, and Sphaerotholus was challenged not more than a few months after being coined. The moral of the story is use "-cephale" instead.
    The old Homalocephalidae is now considered a lineage of progressively more derived animals. Several of them are interesting in their possession of a dentary fang similar to that of the heterodontosaurids. 

Pachycephalosauria i.s.: 

Pachycephalosauridae:

Pachycephalosauridae i.s.:

Pachycephalosaurinae:  All animals at this level are known to have closed the supratemporal fenestrae (holes that would have been found on the top-rear of the skull) except for Tylocephale, which is incomplete in that area.

Pachycephalosaurinae i.s.:

Pachycephalosaurini: