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".
    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 (supposed tiny pachycephalosaurians known from domes from the early Norian [LTr] of India are more likely some kind of offbeat archosauriform). 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 (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.
    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.

           |     `--+--Hanssuesia
           |          `--Stegoceras
                          `--+--"Stegoceras" breve
                                         |     `--Prenocephale
                                              |     `--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 combination of immature fully domed forms, and 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


Taxon or Taxa: Time/Place: Comments:
Wannanosaurus yansiensis Hou, 1977 Campanian-?early Maastrichtian (LK) of China Wannanosaurus is a small animal known from partial skull and postcranial remains. It's hard to place, as the known specimens are not fully grown.
Colepiocephale lambei Sullivan, 2003 (originally Stegoceras lambei Sternberg, 1945) early middle Campanian (LK) of Alberta Colepiocephale has a dome which looks somewhat triangular in top view, and has an oblique slope in lateral view. The name literally means "knuckle head" (which leaves me waiting for Curleyia howardi-"nyuck, nycuk, nyuck!").
Hanssuesia sternbergi Sullivan, 2003 (originally Troodon sternbergi Brown and Schlaikjer, 1943) late middle-early Campanian (LK) of Alberta and Montana Hanssuesia has a very round dome in most views.
Stegoceras: Lambe, 1902  S. validum (type) Lambe, 1902 (including Stegoceras browni [Wall and Galton, 1979]) late middle-early late Campanian (LK) of Alberta and Montana Known from a partial skeleton and dozens of partial skull domes, this animal is one of the more common Judithian dinosaurs. Like most pachycephalosaurians, it is small (for a nonavian dinosaur).  Its premaxillary teeth are wideset and its muzzle wide compared to Prenocephale, indicating different feeding preferences (possibly indicating that Prenocephale was a more selective forager).
A flat-headed specimen first named Stegoceras browni, and later given its own genus Ornatotholus, was previously (sometimes) regarded as the only North American "homalocephalid". It now is thought to represent a juvenile individual of Stegoceras.
S. novomexicanum is known from a nearly complete dome and a couple of partial domes, very similar to standard-issue Canadian Stegoceras.
S. novomexicanum Jasinski and Sullivan, 2011 middle late Campanian (LK) of New Mexico
Goyocephale lattimorei Perle, Maryanska, and Osmolska, 1982 late Santonian-early Campanian (LK) of Mongolia Goyocephale is one of the better known pachycephalosaurians, with a skull and most of the skeleton known. It is known to have had a dentary fang. As a "flat-head", is it too juvenile?  Unlike some others, though, there apparently isn't an obvious candidate for the adult form.
Homalocephale calathocercos Maryanska and Osmolska, 1974 early Maastrichtian (LK) of Mongolia Like Goyocephale, a partial skull and postcranium are known, showing this animal to have been a tubby beast. An interesting feature, besides the knobbly head, is the greatly expanded lateral processes on the tail vertebrae, which when combined with the relatively wide-set hip bones could allow for the guts to continue back behind the hips for a short distance.
There's a chance that it's the same thing as Prenocephale; Homalocephale is flat-headed, the type is immature, and the two are from the same formation. On the other hand, the skull ornamentation differs, as well as the jaws and teeth, and the (grand total of two) specimens of interest are approximately the same size.
Tylocephale gilmorei Maryanska and Osmolska, 1974 (?Prenocephale) ?middle Campanian (LK) of Mongolia This animal is known from an incomplete skull. It could be the same as Prenocephale.
"Stegoceras" breve Lambe, 1918 late middle-early late Campanian (LK) of Alberta Based on juvenile remains (frontoparietal domes), this material was long assigned to S. validum. It may represent a species of Stegoceras, an example of Prenocephale or its relatives, or its own genus.
Amtocephale gobiensis Watabe, Tsogtbaatar, and Sullivan, 2011 ?Santonian (LK) of Mongolia Amtocephale is a small early pachycephalosaurid, known from a frontoparietal dome (the dome is rather eroded on top, which makes it look excessively knobby).
Acrotholus audeti Evans, Schott, Larson, Brown, and Ryan, 2013 late Santonian (LK) of Alberta Evans et al. are boldly challenging the Curse of -tholus with this entry, a fairly derived (fully domed) yet chronologically early form. It is based on the requisite frontoparietal dome, with another assigned as the paratype.
Prenocephale prenes Maryanska and Osmolska, 1974 early Maastrichtian (LK) of Mongolia Prenocephale is known from material including a skull similar to that of Stegoceras, but with heavy ornamentation and closed supratemporal fenestrae. 
Alaskacephale gangloffi Sullivan, 2006 late Campanian (LK) of Alaska This is based on a squamosal like that of Dracorex, but lacking the long spikes. Another juvenile?
Pachycephalosaurus wyomingensis Brown and Schlaikjer, 1943 (originally Troodon wyomingensis Gilmore, 1931) (potentially including
Dracorex hogwartsia [Bakker, Sullivan, Lucas, Larson, and Saulsbury, 2006], Stenotholus kohlerorum [Giffin, Gabriel, and Johnson, 1987], and Stygimoloch spinifer [Galton and Sues, 1983])
late Maastrichtian (LK) of Wyoming, South Dakota, and Montana First, there was Pachycephalosaurus. (Well, not quite. First there was Tylosteus ornatus [Leidy 1872], which is usually thought to be Pachycephalosaurus and was taxonomically suppressed as such, although the Dracorex authors proposed it was more like Dracorex - so maybe Just Another Juvenile.)  (And then, of course, Pachycephalosaurus started as a species of Troodon because the teeth are vaguely similar and the idea was in fashion at the time). It had a big round dome, a knobby nose, and knobby squamosals. If you think of pachycephalosaurs as bald people, it had fairly close-cropped "hair" around and behind the "ears."
Then, there was Stygimoloch. It had a tall narrow dome and clusters of stocky spikes flanking it. Think Larry from The Three Stooges. It quickly swallowed Stenotholus, which means "narrow dome" and tells you what part of the skull its authors had.
Then there was Dracorex, a flat-headed dragon-looking thing, or something like a thescelosaur in a Halloween mask of Stygimoloch. It was odd because it some of the same facial features as Pachycephalosaurus and Stygimoloch, but appeared much more basal because it lacked a dome and had big supratemporal fenestrae. It was known to not be fully grown, but I for one thought it was close to adulthood because of the well-developed spikes. After all, in ceratopsids spikes don't come in strongly until the animal is nearly fully grown. But what if the spikes weren't the important part?
At about the same time Dracorex was published, researchers were starting detailed investigations of dinosaur growth. One thing leads to another, and suddenly it appears that not only is Dracorex based on a juvenile, and Stygimoloch based on a subadult, but that Dracorex grows into Stygimoloch and Stygimoloch grows into Pachycephalosaurus. In this, a flat-headed spiky juvenile grows a tall narrow dome as a subadult, then grows a great big dome as an adult. Meanwhile, the spikes are reabsorbed, becoming blunt nodes (except for the snout nodes, which stay about the same). Alternatives are also possible (mix-and-match growth stages and taxa); time and more specimens will tell.
The largest known pachycephalosaurian, at over five meters in length, Pachycephalosaurus had a set of nodules at the rear of the skull and at least four short spikes on the snout, possibly for display or rooting. It is rather rare, and known primarily from partial skulls. The incompleteness of the published remains may contribute to a history of widely-varying size estimates. In particular, those who like their dinosaurs large must be pleased with the 8 m length estimate in the Illustrated Encyclopedia of Dinosaurs, which sticks out like, well, a pachycephalosaur nearly twice as long as other estimates. I have no idea where this came from; perhaps there was a typo, or a rumor in the dead of the night.
Sphaerotholus: Williamson and Carr, 2002 (?Prenocephale) S. goodwini (type) Williamson and Carr, 2002 middle late Campanian (LK) of New Mexico Based on "bowling ball"-type domes, these are derived pachycephalosaurids. Beyond that, I am confused by the current state of pachy taxonomy.
S. buchholtzae Williamson and Carr, 2002 late Maastrichtian (LK) of Montana

Pachycephalosauria i.s.

Taxon or Taxa: Time/Place: Comments:
Gravitholus albertae (?N.D.) Wall and Galton, 1979 late middle or early late Campanian (LK) of Alberta Based upon a solitary, enigmatic, and probably pathological dome, this large-domed animal probably resembled Hansuessia.
"Microcephale" (N.N.) Sereno, 1997 LK of Alberta Yet to be described, this animal is apparently based on remains including very tiny skull caps.
Texacephale langstoni (?N.D.) Longrich, Sankey, and Tanke, 2010 late Campanian (LK) of Texas Texacephale is known from a pair of domes from a basal pachycephalosaurian.
?"Palaeoscincus" latus (N.D.) Marsh, 1892 late Maastrichtian (LK) of Wyoming This indeterminate tooth taxon could be some sort of pachycephalosaurian.


"T." bexelli Bohlin, 1953 ?late Campanian (LK) of China Currently a memorial to the practice of assigning pachycephalosaurian species to Troodon, this derived pachycephalosaurian requires a new name.
"T." edmontonensis (N.D.) Brown and Schlaikjer, 1943 late middle Campanian- Maastrichtian (LK) of Alberta A rare and poorly known animal, this creature lived alongside Pachycephalosaurus. Its remains, frontopartietal domes, were long assigned to Stegoceras, but are not generically distinct. It may be referable to Prenocephale.


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