This is the page of Prehistoric Life with the least number of surely-valid genera (merely two): Stegoceras and Prenocephale. All the other animals linked with The Other Wiki are either possible synonyms (Homalocephale, Stygimoloch, Dracorex), or were once considered pachycephalosaurs but aren't actually. The ones named in this intro have been the most common non-stock pachycephalosaurs in media after Pachycephalosaurus (even though Dracorex is a very recent addition).
The other Stego: Stegoceras
Rule of Cool is a merciless thing. It doesn’t matter if you are the most abundant, complete, well-known, or even the first discovered dinosaur within your group: if you aren’t cool enough, another cooler relative will take you the stock-role in pop-consciousness.
Stegoceras matches perfectly all this. By far the most abundant, complete, well-known, and even the first discovered dinosaur within its group, which should be renamed Stegocerates rather than Pachycephalosaurs. 2.5 m long, Stegoceras was just half the length of Pachycephalosaurus, but shared the same Friar-Tuckish face, having a smooth dome bordered by a collar of tubercles. But its dome was less-prominent, “only” 1 inch thick, and its nose lacked those spikes Pachycephalosaurus had. In short, the milder version of Pachycephalosaurus.
Both pachys lived in Late Cretaceous North America, but the smaller one was slightly earlier, as usual among Late Cretaceous dinosaurs. Was discovered in year 1924, but originally thought a weird ornithopod; the placement in the current group was made just after the discover of Pachycephalosaurus twenty years later. And don’t confound it with Stegosaurus, please. As a pachy, “the other Stego” was a small, two-legged animal with a heavy head and the “body armor” limited to its skull.
The other stego has also the distinction to be the only pachycephalosaur from which many individuals are known, not just one or two, and the only whose body-frame is known with sureness, to the point to be used as a model for other relatives: when you watch the body, legs, arms, neck and tail of a pachycephalosaur, you’re arguably watching those of Stegoceras. In dinosaur books, the other stego is often treated as the effective stock pachycephalosaur, unlike TV programs which will ever prefer the namesake of the family. For infos about its possible lifestyle, see Pachycephalosaurus.
Domeheads and flatheads in Asia: Prenocephale & Homalocephale
Pachycephalosaurs are very rare things. Few species have been described so far, almost always from the Late Cretaceous, note and they are either North American, or Asian. While the North American ones are more spectacular (Pachycephalosaurus), or more abundant (Stegoceras), the Asian ones are nonetheless very interesting; the two most classic ones were both discovered in the 1970s in Mongolia. Their names make a sort of pun if pronounced together: Prenocephale and Homalocephale.
Prenocephale aptly means “prominent head”; was very similar to Stegoceras, size and period included, but had a shorter snout, different tubercles, and a higher dome. Like most boneheaded dinosaurs, only skull material is known, but its first skull is so well preserved that even osseous canals for the passage of blood-vessels are distinguishable!
Homalocephale was also similar in size, but is known from several pieces of its skeleton as well other than the skull. Its name means flat head, and with reason: it has indeed a flat head, making it very unpachycephalosaur-looking. Actually its skull structure was clearly pachycephalosaurian, with slighty thickened skull-roof and bony tubercles very similar to those of Prenocephale. This detail led in 2010 the hypothesis that Homalocephale was just the juvenile form of Prenocephale with a not-yet developed dome.
The unusual wideness of the Homalocephale pelvis led also speculation about a possible viviparity (aka giving birth to live offspring). There is no proof of this, as well as in every other non-avian dinosaur: and remember that modern dinosaurs (birds) and their closest relatives (crocs) are all egg-laying animals.
Cool names in America: Stygimoloch & Dracorex
Many other pachycephalosaurs have received the suffix -cephale (meaning head in Greek): Goyocephale (flat-headed like a Homalocephale), Tylocephale (with the tallest dome among pachycephalosaurians), Alaskacephale (found in Alaska), etc. Some others end with -tholus (meaning "dome"): ex. Gravitholus ("heavy dome"), Ornatotholus ("ornated dome", sometimes synonimized with Stegoceras), Sphaerotholus ("spherical dome", once believed a North-american species of Prenocephale). But two boneheads have gained much cooler names: Stygimoloch and Dracorex. The former means Spiky Devil from the Death River, the latter Dragon King. Both lived in USA alongside Pachycephalosaurus and, surprisingly, are known only from one skull or little more.
Stygimoloch was discovered in the eighties: Stegoceras-sized, was the only known pachycephalosaur with spikes developed into true horns, and its dome was tall and narrow. Dracorex was found only in 2006: also of similar size, had an almost-as-spiky skull coupled this time with a flat head.
Even though much more developed, the spiky ornamentation of both was very similar to Pachycephalosaurus. Basing on this detail, some have proposed in the 2000s that the “devil” and the “dragon” are just different immature stages of Pachycephalosaurus, with Dracorex being the most immature growth-stage, Stygimoloch the intermediate one, and Pachycephalosaurus the fully-mature form.
Talking about Dracorex: as its mythical-sounding genus-name was not enough, its complete scientific name is: Dracorex hogwartsia. Our "harrypottersaur" is also one of the few real dinosaurs portrayed in the TV series Primeval, even though in a quite fanciful way, with an actual dragon-like crest on its back.
The never-never pachys: Micropachycephalosaurus & Yaverlandia
Remember Majungatholus, that pachycephalosaur which revealed to be the horn of a giant theropod? This was not an isolated case. Yaverlandia from Early Cretaceous England, was once mentioned as the “most ancient pachycephalosaur”, but its only remain (a tiny skull-dome with two small thickenings above) has been reclassified as a bird-like theropod.
Many things might deceptively resemble pachy domes and lead experts in error; the fact that pachycephalosaurs included some of the tiniest dinosaurs (ex. Wannanosaurus was only two feet long) has also contributed to this. Still another piece of bone has been attributed to another virtually-unknown pachycephalosaur, which could get nonetheless a mention in the Guinness Book Of Records… as “the longest dinosaur name”: Micropachycephalosaurus note , “tiny thick-headed lizard”. The ironical thing is, this was one of the smallest dinos that ever lived, only 50 cm/1.5 ft long. And was more likely a very primitive ceratopsian rather than a true pachycephalosaur.