This is a page of Prehistoric Life with a very low number of surely-valid animals, because known pachycephalosaurs have always been very few, even less than the stegosaurs.
Excellent Skull: Prenocephale
- 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 prenes aptly means prominent head; was very similar to Stegoceras validum, 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! Some alleged Prenocephale remains were also found in North-America, but they actually don't pertain to it.
- Once, Pachycephalosaurians used to be divided in two main families, the Pachycephalosauridae (those with domed heads) and Homalocephalidae (the ones with flat heads). A third less-known family, Chaoyangsauridae, is now known to belong to the basal ceratopsian lineage. Today this distinction is not recognized anymore: the flat-heaed kinds are actually identical to the bulge-headed ones except for the shape of their skull, and now one single family of pachycephalosaurs is recognized, Pachycephalosauridae. The traditionally most-known "flathead" among the pachies is Homalocephale calathocercos. Homalocephale was similar in size to Prenocephale, but is known from several pieces of its skeleton as well other than the skull. The name Homalocephale 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. The last detail led in 2010 the hypothesis that Homalocephale was just the juvenile form of the latter, 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: remember that modern dinosaurs (birds) and their closest relatives (crocs and turtles) are all egg-laying animals, while live-bearers among the modern Amniotes are known only from therian mammals and some lizards/snakes.
Domeheads and Flatheads
- Many pachycephalosaurs have received the suffix -cephale, meaning head in Greek. Goyocephale means "decorated head", was found in Mongolia and was flat-headed like a Homalocephale but with a pair of canine-like teeth; Tylocephale means "swollen head", was also Mongolian and with the tallest dome among pachycephalosaurians; Alaskacephale found in Alaska maybe was the closest relative of Pachycephalosaurus. Some others end with -tholus, meaning "dome": ex. North American Gravitholus ("heavy dome") & Ornatotholus ("ornated dome", today synonimized with Stegoceras), as well as Sphaerotholus ("ball-like dome"), once believed the North-american species of Prenocephale.
The First Pachycephalosaur?: Yaverlandia
- Remember Majungatholus ("Majunga's dome"), that pachycephalosaur from Madagascar which revealed to be the horn of a giant theropod? This was not an isolated case. Yaverlandia from Early Cretaceous England (Isle of Wight) was once mentioned as the most ancient pachycephalosaur: but its only remain, a tiny skull-dome with two small thickenings above (its complete scientific name, Yaverlandia bitholus, means "Yaverland's double-dome") has been reclassified as a bird-like theropod. "Majungatholus", in turn, was believed the only pachycephalosaur living in the Southern Emisphere. Many things might deceptively resemble pachy domes and lead experts in error; the fact that pachycephalosaurs included some of the tiniest dinosaurs has also contributed to this. For example, Wannanosaurus from China was only two feet long (like a Microraptor) with a flat head that made it looking like a miniature Homalocephale, but, uniquely among known pachycephalosaurs, lacked any skull-protuberances: it is believed by some the actual most basal known pachycephalosaur.
- Still another piece of bone found in China in the 1970s has been attributed to another virtually-unknown pachycephalosaur from Late Cretaceous, which could get nonetheless a mention in the Guinness Book Of Records as the longest dinosaur name: Micropachycephalosaurus. note This sesquipedalian name was made combining the particle "micro" with "Pachycephalosaurus", meaning small thick-headed lizard. Indeed, it was actually one of the smallest dinos that ever lived, maybe only 50 cm/1.5 ft long, like an Anchiornis or an Epidexipteryx. But research made in the 2000s has shown it not to be a true pachycephalosaur, but more likely a very primitive late-surviving ceratopsian.