MAJOR GROUPS OF MARGINOCEPHALIA
The third major clade of ornithischians (after Thyreophora and Ornithopoda) is Marginocephalia ("ridged heads"). There are two main clades of marginocephalians: thick-skulled Pachycephalosauria ("thick headed lizards") and beaked (and later frilled (and eventually horned)) Ceratopsia ("horned faces"). The bipedal pachycephalosaurs and psittacosaurid ceratopsians were once included in Ornithopoda, but are now recognized as closer to the horned dinosaurs (Ceratopsidae) and their more primitive frilled-but-hornless relatives (the rest of Neoceratopsia).
Various postcranial shared derived characters unite Pachycephalosauria and Ceratopsia, but the most distinctive specialization is the one that gives this clade its name:
In the pachycephalosaurid pachycephalosaurs and the ceratopsid ceratopsians there is the shared attribute of extensive bone growth and remodeling of the skull at the fully adult stage: however, this trait is not seen in basal ceratopsians, so it is likely convergently evolved between these two clades.
At present there are no dinosaurs known which are closer to marginocephalians than to the ornithopods, but we can infer that such proto-marginocephalians did exist. When Heterodontosauridae was thought to be a clade of ornithopods, there was the mystery of the missing Late Triassic, Jurassic, and early Early Cretaceous marginocephalians. Now that the oldest known ornithopods are only Middle Jurassic, and we now have Middle-Late Jurassic marginocephalian fossils, there is no big gap in time.
PACHYCEPHALOSAURIA Pachycephalosaurs (aka boneheads aka domeheads aka buttheads aka headbangers...) are specialized by the presence of:
Stenopelix of the Early Cretaceous of Germany is often considered to be a basal member of Pachycephalosauria based on some limb and pelvic characters. Its skull is unknown, so we cannot determine if it had evolved the thickened skull roof of later pachycephalosaurs. Interestingly, it is the only currently known Early Cretaceous pachycephalosaur, and the only member of the clade from outside Asia or western North America. Of note, however, is a recent study that finds it not to be a pachycephalosaur, but instead a basal ceratopsian (sister taxon to Yinlong): if this is upheld with additional analyses, this would remove the only non-Asiamerican, non-Late Cretaceous pachycephalosaur from the record.
The more specialized Pachycephalosauridae are characterized by a tall thickened dome formed by the frontals and parietals. Paleontologists have debated whether these dinosaurs butted heads together in the manner of modern bighorn sheep, or if the domes may have been more for visual display. There does seem to be some sexual dimorphism in the size and development of the dome.
It was once thought that flat-topped specimens (such as those called "Homalocephale" of Mongolia) were primitive pachycephalosaurs. However, recent work strongly suggests that these are simply subadults which had not grown their adult dome. (The Homalocephale specimens seem to be juvenile Prenocephale.) Recent work suggests that the is a clade of primarily Asian forms (including Prenocephale, Tylocephale, and Goyocephale), nested among a radiation of mostly North American taxa (such as Stegoceras, Gravitholus, Sphaerotholus, and the like). There is a clade (or species!) of latest Cretaceous (68-65.6 Ma) western North American large bodied (4-5 m, and thus bigger than humans) long-snouted spike-fringed and spike-nosed forms: Stygimoloch, Pachycephalosaurus, and Dracorex (which is quite like just a subadult of one or the other of the previous two.) In fact, these three may just represent juvenile ("Dracorex"), subadult ("Stygimoloch"), and adults (Pachycephalosaurus: this name has priority) of the same species.
While material of the pachycephalosaurs are rather limited, the ceratopsian fossil record is very good. The last decade has seen an explosion of discoveries in primitive ceratopsians, extending the stratigraphic range of this group and giving us a more complete picture of the transformations from a basal neornithischian to the derived Ceratopsidae.
Ceratopsians are united by some important cranial specializations, including:
The oldest and most primitive known ceratopsian is Yinlong from the latest Middle Jurassic (or earliest Late Jurassic) of China. Its skull shows the large size of the temporal (jaw muscle) region, indicating that this dinosaur had begun to evolve the powerful bite that characterizes ceratopsians. Other early forms are Late Jurassic Chinese Chaoyangsaurus and Xuanhuaceratops.
The next oldest ceratopsians are the Psittacosauridae ("parrot lizards") of Early Cretaceous Asia. These are actually some of the most common dinosaur fossils of Early Cretaceous Asia: in particular, the several species of Psittacosaurus. These include Psittacosaurus babies found with an adult, and many complete skeletons including one with scales and apparently some form of hollow quill sticking out of its tail.
(Note: we do not yet know the homology of the quills of Psittacosaurus. If (and it remains an "if") they represent homologs to the fuzz of the heterodontosaurid Tianyulong, they may indicate that ornithischians in general had some expression of quill or fuzz structures ancestrally.)
Some have considered the psittacosaurids to be facultative quadrupeds, but a recent study showed that their hands could not be placed in a position to support weight. Thus, they retained the ancestral bipedal condition.
The remaining ceratopsians form the clade Neoceratopsia. This group is further modified from the ceratopsian condition by the presence of a frill: itself just an elaboration of the ridge of typical marginocephalians. In earlier forms the frill is short, and its expansion was probably primarily for increased muscle attachment of the supratemporal muscles.
A number of primitive Asian Early Cretaceous neoceratopsians have recently been described: Liaoceratops, Archaeoceratops, Auroraceratops, and Yamaceratops. These forms were still bipedal.
The Late Cretaceous neoceratopsians, however, seem to have been quadrupedal because of increasing relative skull size. Among the Late Cretaceous groups is transcontinental (Asia and North America) Leptoceratopsidae, including North American Prenoceratops, Montanoceratops, and Leptoceratops, and Asian Udanoceratops. Another is the transcontinental (Asia and European) Bagaceratopsidae, including Mongolian Bagaceratops and Chinese Magnirostris, and Hungarian Ajkaceratops.
More advanced are the Coronosauria. Coronosaurs have an enlarged frill and enlarged skull, and consequently were obligate quadrupeds. The frill is enlarged but not thickened, so most of the expansion was not for muscle attachment. It may have served for visual display.
One branch of the coronosaurs was the Asian clade Protoceratopsidae, best known from the genus Protoceratops: literally hundreds of skeletons (including hatchlings and embryos), eggs, and nests are known from this Mongolian dinosaur.
The remaining coronosaurs are the Ceratopsoidea. More primitive ceratopsians were generally small: 1-3 m, and no heavier than a sheep or pig at most (Udanoceratops being an exception). The ceratopsoids were all larger: cow-to-elephant sized. Ceratopsoids had postorbital horns sticking from the eyebrow over the eyes. Cow-sized Zuniceratops of western North America and Asian Turanoceratops and Asiaceratops represent primitive ceratopsoids. The remaining forms are the Ceratopsidae proper.
The horns of ceratopsoids may have had both a defensive function (against predators) and a display function against other ceratopsids. These displays may have been species recognition, sexual, or dominance displays; and could have been merely visual or may have involved direct combat.
True Ceratopsidae is the most speciose and specialized branch of Marginocephalia. This clade is restricted to the last twenty million years or so of the Late Cretaceous. (Maybe. Turanoceratops from about 92 Ma of Uzbekistan may be a true ceratopsid.) Up until 2009 it seemed clear that all members of Ceratopsidae proper were from western North America, but (questionably) Turanoceratops and (unquestionably) the centrosaurine Sinoceratops document the presence of this clade in Asia. Yet in that restricted time and space they underwent a major radiation. We have complete skeletons of several species; complete skulls of even more; growth series; skin impressions; and trackways.
Ceratopsids are characterized by the following specializations:
Ceratopsids include some definite herd dwellers. Ceratopsids are rivalled (and in some formations exceed) only by hadrosaurids in their abundance in the Late Cretaceous of western North America.
There were two major subclades of Ceratopsidae: Centrosaurinae and Chasmosaurinae (The latter group is sometimes called "Ceratopsinae" to some paleontologists. However, it is no longer certain that the fragmentary Ceratops is definitely closer to the chasmosaurines than the centrosaurines, so in this course we will use "Chasmosaurinae".). It was once easy to distinguish these two groups based on their horn patterns: in centrosaurines the nasal horn was large and the postorbitals reduced or lost; in chamsosaurines the nasal horn was small and the postorbital horns were (with one or two exceptions) long.
Unfortunately, the discovery of Zuniceratops and the basal centrosaurines such as Albertaceratops and Diabloceratops has muddled that.
Under our current understanding, long postorbital horns are characteristic for Ceratopsoidea as a whole, so the fact that Chasmosaurinae retain them is simply a primitive feature.
Instead, we know recognize Centrosaurinae by the presence of:
Diabloceratops and Albertaceratops are among the most primitive centrosaurines. Additional forms include the closely related Centrosaurus and spike-frilled Styracosaurus, which share a powerful nasal horn; the aforementioned Asian centrosaurine Sinoceratops; hook-horned Einiosaurus and sword-horned Rubeosaurus; and a great radiation of lump-nosed species (the Pachyrostra), including the genera Achelousaurus; and several species of Pachyrhinosaurus, last and largest centrosaurine.
Chasmosaurinae is characterized by:
Chasmosaurinae includes the newly described basal form Medusaceratops (found in the same bonebed as the centrosaurine Albertaceratops); the various species of Chasmosaurus; Agujaceratops and its newly (2010) described relative Mojoceratops; Pentaceratops; Anchiceratops, Arrhinoceratops, and their enormously-horned Mexican relative Coahuilaceratops; and the latest Cretaceous giants Titanoceratops, Ojoceratops, Eotriceratops, Torosaurus, Triceratops, Nedoceratops (formerly "Diceratops"), and little Tatankaceratops. Some authors consider all of these separate taxa, with the older Titanoceratops, Ojoceratops, and Eotriceratops close to the ancestry of the others. Alternatively, "Tatankaceratops" and "Nedoceratops" may be the juvenile and subadult (respectively) of Triceratops and/or Torosaurus. Furthermore, it has been proposed that "Torosaurus" itself may simply be the fully adult form of Triceratops, so it may be that instead of four genera (Tatankaceratops, Nedoceratops, Triceratops, Torosaurus) in the latest Cretaceous of western North America, there was simply Triceratops.
Ceratopsids were a major component of the large bodied herbivore fauna in western North America (and probably the wetter parts of Asia) until the very end of the Cretaceous.
EVOLUTIONARY PATTERNS IN MARGINOCEPHALIA
Feeding adaptation transformations:
Pachycephalosaurs, basal ceratopsians, and basal neoceratopsians were bipedal; increased skull size forced advanced neoceratopsians onto all fours, such that coronosaurs were obligate quadrupeds.
Most pachycephalosaurs, basal ceratopsians, and basal neoceratopsians were small (all in the 1-3 m range, smaller than humans); increased size in pachycephalosaurs only occurs at the very end of the Late Cretaceous. Advanced neoceratopsians show an increase earlier, culminating in the major size increases at the base of Ceratopsoidea, at the base of Ceratopsidae, and independantly in Centrosaurinae and Chasmosaurinae. For most of its history, Ceratopsia consisted of only small dinosaurs.
Social behavior in Marginocephalia:
Both pachycephalosaurs and ceratopsians seem to have used their heads in within-species interactions. In particular, visual display structures (marginal osteoderms, domes, frills, horns, etc.) and possible combat features (thickened skulls, horns) are present in both clades, and some of these only appear at fully adult sizes. We'll examine these more in the third section of the course.
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