Origins and Armor:
MAJOR GROUPS OF THYREOPHORANS
Thyreophora are united by various skeletal attributes, the most obvious of which is:
The derived thyreophorans are the plated Stegosauria (shingled lizards) and the heavily armored Ankylosauria (fused lizards). Both clades are present by the Middle Jurassic. There are several Early Jurassic taxa which fall outside either of the two advanced clades.
BASAL THYREOPHORANS
Lesothosaurus of the Early Jurassic of southern Africa
may be the oldest and most primitive known thyreophoran, based on several recent new analyses. If so, it is united with the later thyreophorans
on various skeletal traits that are outside the scope of this course. Importantly, it would be the only known
unarmored thyreophoran. (But we would expect the oldest ones to have been unarmored.)
The oldest and most primitive definite (i.e., armored) thyreophoran is Scutellosaurus of the Early Jurassic of western North America. It was a 1.5 m long biped (possibly facultative biped) not very dissimilar to other primitive ornithischians like Lesothosaurus or Hexinlusaurus: small herbivores with small hands. The primary distinction of Scutellosaurus is the presence of a great many small osteoderms over the body. These would protect against small-bodied predators, but might not help against the new larger theropods that had begun to appear in the Early Jurassic.
In response, thyreophorans evolved larger size and heavier armor, as seen in Emausaurus and Scelidosaurus (new specimen shown here), both of Europe. The larger body size (3-4 m long) and proportionately larger osteoderms may have been more effective defense against attakcing predators, but forced them onto all fours (at least for Scelidosaurus: in other words, they were obligate quadrupeds.
(Note that the hypothesis shown here is that Scutellosaurus, Emausaurus, and Scelidosaurus were progressive closer to the Stegosauria-Ankylosauria clade (Eurypoda). However, some paleontologists have considered Emausaurus to be a primitive stegosaur, and others that Scelidosaurus was the oldest and most primitive ankylosaur. However, eurypods share a number of transformation not found in these Early Jurassic taxa: these include:
STEGOSAURIA
From a Scelidosaurus-like ancestor, the stegosaurs evolved armor that was less covering and more concentrated. While they
had some small osteoderms in their skin (particularly around the neck and the hips), most of their armor was specialized as:
In general, the stegosaur armament suggests active defense: the dinosaur probably turned in response to attacking predators, trying to keep the tail towards the theropod so that it could use its thagomizer. Damaged thagomizer spikes and theropod bones with thagomizer-generated puncture wounds confirms their use in defense.
Early stegosaurs were only about 2.5-3 m long, but the most derived forms ranged up to 9 m or more. Their narrow snouts suggests that they were rather picky eaters (that is, instead of munching a lot of plants at once, they were selective as to which ones they chomped.) Although they were obligate quadrupeds in terms of locomotion,
Primitive stegosaurs include Middle Jurassic Huayangosaurus and Late Jurassic Chungkingosaurus, Gigantspinosaurus and Tuojiangosaurus (all from China). More derived stegosaurs form the clade Stegosauridae, and include Late Jurassic Dacentrurus and long-necked Miragaia of Europe, Kentrosaurus of eastern Africa, and the Stegosaurinae.
Stegosaurids had dorsoventrally stretched neural arches and disproportionately short forelimbs, giving them an odd profile.
The most derived stegosaurids (Stegosaurinae) lacked shoulder spines (also missing in Tuojiangosaurus), had only plates rather than spikes along the back (again, shared with Tuojiangosaurus), and had alternating rather than parallel plates. This advanced group includes the two Late Jurassic western North American genera (Hesperosaurus and famous Stegosaurus [also known from Portugal]), and Early Cretaceous Wuerhosaurus of China, the last of the stegosaurs. (Some stegosaur fragments are known from Europe about the same age as Wuerhosaurus, but are not distinctive enough to place within the stegosaur phylogeny).
Stegosaurs are relatively common in Middle and Late Jurassic formations (especially so in China), are present but rare in some European assemblages, and vanish before the end of the Early Cretaceous. Claims of later stegosaurs have so far turned out to be either mis-dated or misidentified.
ANKYLOSAURIA
While the stegosaurs evolved active defense, the ankylosaurs (at least at first) seem to have been selected for passive defense: the
ability to stay put and absorb damage from an attack. They were even more extensively armored than Scelidosaurus, and were
characterized by:
Ankylosaur systematics remains contentious. There does seem to be a well-supported clade comprised of Early Cretaceous Sauropelta and Late Cretaceous Edmontonia and Panoplosaurus (all of North America): the Nodosauridae. There is also the well-supported clade Ankylosauridae discussed below. There are a number of other ankylosaurs, however, which are considered by some to be either primitive nodosaurids and ankylosaurids; and/or to lie outside a nodosaurid-ankylosaurid clade; and/or to form their own clade Polacanthidae. The latest published study places such forms as either basal nodosaurids or basal ankylosaurids, but this is likely to change in the very near future! However, the oldest ankylosaur (Middle-Late Jurassic Tianchisaurus of China) was not included in this study and may represent an ankylosaur outside the Nodosauridae-Ankylosauridae clade.
Some primitive ankylosaurs, such as the primitive Early Cretaceous ankylosaurids Minmi of Australia and Liaoningosaurus of China, were fairly small at 2-3 m long.
Gargoyleosaurus of the Late Jurassic of North America and Early Cretaceous Gastonia of North America, Polacanthus and Hylaeosaurus of Europe share flattened laterally-directed osteoderms and a sacral shield of fused osteoderms over the pelvis. These were once thought to form a clade Polacanthidae, but the latest study shows that these 3-5 m long North American and European forms were primitive nodosaurids.
Advanced nodosaurids are united by a large muscle attachment projection on the scapula and often have a tall laterally-directed shoulder spine. In addition to the North American taxa listed above, nodosaurids include many additional Cretaceous North American taxa such as Animantarx, some recently described taxa from the Late Cretaceous of Asia, and Struthiosaurus of Late Cretaceous Europe. They range from about 3-7 m in length. They are known (as shown above) from Europe, North America, and Asia; there are possible nodosaurid bones from other regions (including Antarctopelta from Antarctica), but these may belong to more primitive ankylosaurs.
ANKYLOSAURIDAE
The most specialized clade of ankylosaurs is Ankylosauridae. This clade is transformed relative to other armored dinosaurs in having:
Advanced ankylosaurids are only known from the Late Cretaceous of Asia and western North America at present; similar distributions are known for advanced pachycephalosaurs, ceratopsids, and tyrannosaurids.
EVOLUTIONARY PATTERNS IN THYREOPHORA
Defense:
Probably the most conspicuous aspect of their evolution. Thyreophorans begin with a set of small scutes; develop larger scutes at
the expense of bipedalism (and speed); than split between specialists in active (Stegosauria) vs. passive (Ankylosauria) defenses. Within
the ankylosaurs, though, Ankylosauridae independantly evolves an active defensive tail weapon.
Relative success
Stegosaurs and ankylosaurs, as sister taxa, have their origins at the same time. However, stegosaurs flourish first (in the Middle and Late
Jurassic), only to peter out during the Early Cretaceous and disappear before its end. Ankylosaurs are typically rare in the Jurassic (although
at one location Gargoyleosaurus is very common), but become abundant in the Cretaceous.
Feeding adaptations:
Narrow-snouted basal thyreophorans and stegosaurs differ from broader-snouted ankylosaurs. The latter have a greatly reduced predentary bone,
which may have allowed for more complex motion of the dentaries (for additional munching power) while feeding.
Group behavior:
Thyreophorans are only rarely found in mass death assemblages, and so (unlike some sauropods, ornithopods, and marginocephalians) probably
did not live in large groups.
Display structures:
The spikes, plates, and osteoderms of thyreophorans almost certainly had a defensive function. But many are very broad (particularly
stegosaur plates), and the patterns tend to be specific to each species. They may have served an additional function: as visual displays
of species recognition. There may also have been a sexual display function to them, but at present it is uncertain if the variation we see
in some thyreophorans is from sexual dimorphism or if it is from multiple species of the same genus living together.
But even at the dawn of the 21st Century, Heterodontosauridae was still generally considered as sharing a more recent common ancestor with the "hypsilophodonts" and iguanodontians than with any other group of dinosaur: thus, the heterodontosaurids were thought to be the oldest branch of Ornithopoda. More recently, however, heterodontosaurids have been recognized as splitting from other ornithischians at a very basal divergence, and thus are no closer to ornithopods than to marginocephalians or to thyreophorans. So there are at present no recognzied Late Triassic or Early Jurassic ornithopods.
Even more recently, Middle Jurassic taxa (such as Chinese Agilisaurus and Hexinlusaurus and Late Jurassic western North American Othnielosaurus (all of which were considered to be primitive hypsilophodont-grade ornithopods in many cladistic analyses) now seem to be basal members of Neornithischia. This latter clade includes marginocephalians, true ornithopods, and all taxa closer to them than to Thyreophora. Among various other cranial (and pelvic) characters, neornithischians share asymmetrical teeth with enamel on only one side, allowing them to be ever-sharpening.
(The recognition of the non-ornithopod status of these Jurassic forms comes in great part with the discovery of Eocursor, Stormbergia, and other Late Triassic/Early Jurassic ornithischians. The data from these fossils are helping to sort out the relationships of the bird-hipped dinosaurs. Expect additional changes and refinements in the near future.)
(A note on the name "Ornithopoda": advanced iguanodontians do indeed have three-toed feet something like birds, as seen in these tracks. But basal ornithopods have four forward-facing toes, and no ornithopod seems to have the backwards-facing digit I of birds. In fact, it is kind of a lousy name for the clade, but rather late in the game to change it...)
MAJOR GROUPS OF ORNITHOPODS
So what an ornithopod, then? Ornithopoda contains the clade Iguanodontia and a series of primitive forms traditionally
called "hypsilophodonts". (Note that Agilisaurus and Othnieliosaurus were also included among the "hypsilophodonts" until recently.)
Some paleontologists consider the hypsilophodonts to be a clade, but most recent work demonstrates that they are a paraphyletic series in
relation to Iguanodontia.
The removal of Heterodontosauridae and the basal neornithischians from Ornithopoda means that the shared derived characters that unite true Ornithopoda are a bit muddled at present. However, here is a list of some that seem to apply:
Ornithopods:
"HYPSILOPHODONTS" The primitive ornithopods do not seem to form a single clade, but instead are either more or less closely related to the Iguanodontia. The first discovered and best known of this range of ornithopods is little 2 m long Hypsilophodon of Early Cretaceous Europe (and possibly North America). Most of these early forms have the same general grade of organization: bipedal dinosaurs 1-2 m long with small triangular heads. They seem adapted for browsing on low vegetation and for running as a defense. The Orodrominae Oryctodromeus of the Early Cretaceous of western North America) is known to have dug burrows, and recently described burrows from Early Cretaceous Australia might also be from "hypsilophodonts".
Relationships among the "hypsilophodonts" are not yet well resolved, but getting better. The most recent (2013) study puts Hypsilophodon proper as closer to a diverse clade, the Thescelosauridae. Thescelosaurids includes the aforementioned burrowing orodromines (including Early Cretaceous Oryctodromeus and Zephyrosaurus and Late Cretaceous Orodromeus) and a clade (Thescelosaruinae) that contains primitive forms once called "changchunsaurs" or "jeholosaurs" from the late Early to early Late Cretaceous of eastern Asia (including Changchunsaurus, Haya, and Jeholosaurus) and larger derived taxa of the Late Cretaceous of North America, including Parksosaurus and long-snouted Thescelosaurus.
The Early Cretaceous small ornithopods of Australasia, including ultra-long tailed Leaellynasaura, are of uncertain position. Some studies suggest one or more these might actually be basal ornithischians rather than ornithopods; others ally these taxa with the Late Cretaceous Argentine ornithopods including Gasparinisaura and Talenkauen in a clade closer to Iguanodon than to Thescelosaurus (and thus by definition be primitive iguanodontians). Analyses to sort out these relationships are ongoing.
One puzzle: all the currently known "hypsilophodonts" are from the Cretaceous, but their phylogenetic position predicts that representatives of this clade should have been around in the Middle and Late Jurassic.
IGUANODONTIA
The members of Iguanodontia were transformed from their "hypsilophodont" cousins by a number of features:
All retained some bipedal ability, but many of the iguanodontians were facultative bipeds only, spending a sizeable fraction of time on all fours.
The oldest iguanodontian known is the Middle Jurassic dryosaurid Callovosaurus. Iguanodontians become more common in the Late Jurassic, but really come into their own in the Cretaceous. In many ecosystems the iguanodontians are the most abundant large animals, displacing sauropods and stegosaurs.
Among the diversity of primitive iguanodontians are:
The last two represent the transition from the more primitive iguanodontians to the specialized Styracosterna. Their snouts have become longer and broader-ended with a better developed grinding jaws, while their hands have become better adapted for absorbing weight. These transformations are more fully developed in the hadrosauriforms.
STYRACOSTERNA
This represents the clade comprised of Hadrosauridae and all taxa closer to hadrosaurids than to Camptosaurus. The primitive styracosternans were
once all grouped together as "Iguandontidae", but that family turned out to be paraphyletic with respect to hadrosaurids. In fact, the genus Iguanodon as
it was used up to the 1990s has turned out to be paraphyletic, as mentioned below. Styracosterna is by far the most successful radiation among the ornithischians.
Styracosternans show the following transformations from the ancestral state:
The combination of their great size, ability to walk on their hindlegs or all fours, and powerful beaks with grinding teeth allowed styracosternans to be excellent browsers of both low and high vegetation. At least some seem to have lived in herds.
Styracosternans are known from most of the Cretaceous world, but are most particularly common or diverse in Europe, North America, Asia, and (in the Early Cretaceous) northern Africa. Among the diversity of Early Cretaceous styracosternans are:
One subset of styracosternans (Hadrosauria) in particular shows a seires of transformations including an increase in the number of tooth positions in the jaws and expansion of the snout. These dinosaurs are on the lineage which leads to the duckbilled dinosaurs (Hadrosauridae). Among the precursors and cousins of the hadrosaurids are Early Cretaceous Jinzhousaurus, Equijubus, and Probactrosaurus of China; Early-to-Late Cretaceous Protohadros of western North America; Late Cretaceous Telmatosaurus of Europe and Bactrosaurus of Asia. (These last two, in particular, are sometimes included in an expanded vesion of Hadrosauridae).
HADROSAURIDAE
True Hadrosauridae is the most speciose and specialized branch of the ornithopods. All known members of Hadrosauridae proper are from the
Late Cretaceous. Although known from Europe, South America, and Antarctica, the main diversity of hadrosaurids is in Asia and North America.
The transformations of hadrosaurids relative to their ancestors include:
Hadrosaurids see the fullest expression known of the ornithopod grinding mechanism. As mentioned above, it was once thought that the motion was
relative simple: the side unit would move outwards when the lower jaw was brought up, giving a side-to-side grinding of the teeth during chewing.
This model (proposed during the 1980s) was called "pleurokinesis" (or "side-motion"). Here is a video of a computer animation of this
interpretation:
However, more detailed study using CT scans and more complete computer models show the motion is really a LOT more complex. Pleurokinesis plays
a part in it, but there are other directions as well. No name is given at present for this form of jaw mechanics, but below is a preliminary model
of how it works:
Hadrosaurids include some definite herd dwellers. The entire life cycle of hadrosaurids is preserved: nests, eggs, embryos, hatchlings, juveniles, subadults, and adults. Hadrosaurid footprints and isolated hadrosaurid teeth are among the most common Late Cretaceous fossils of North America. Skin impressions and even mineralized soft tissue are known for duckbills.
The latest on-going phylogenetic analyses show two major subclades of Hadrosauridae: crested Lambeosaurinae and broad-snouted Saurolophinae. The latter group is more traditionally called "Hadrosaurinae", but Hadrosaurus proper (and the very similar Italian Tethyshadros) seems to lie outside the Lambeosaurinae-Saurolophinae clade and so this other, less commonly seen name is used in this course. (However, a note of caution: some preliminary studies suggest that "saurolophines" may be paraphyletic with respect to Lambeosaurinae). Both clades are known from a great number of excellent skeletons.
Lambeosaurines are characterized by a hollow crest covering the nasal passage. These crests, which vary between species, may have had both a visual and sound display function. Baby lambeosaurines lacked this structure.
CT scans allow for the pathways of these passages to be studied in greater detail:
Differences in crest size and shapes within some populations may reflect sexual and/or ontogenetic varations.
Among the better known lambeosaurines are Nipponosaurus, Olorotitan, and Charonosaurus of Asia and Parasaurolophus, Corythosaurus, Hypacrosaurus, Velafrons, Lambeosaurus, and GIGANTIC Magnapaulia of North America.
Saurolophines differ from their relatives by greatly flared snouts and greatly expanded nares. Some hadrosaurines had (relatively) short snouts: North American Gryposaurus and Brachylophosaurus, for instance. Others had longer snouts: North American Maiasaura and Prosaurolophus and transcontinental (Asia and North American) Saurolophus. The extreme development of the duckbill can be found in the the edmontosaurs, a group containing Edmontosaurus proper and species sometimes considered separate genera (Anatosaurus and Anatotitan), but sometimes all considered Edmontosaurus. (There is recent work to show that there are only two species, each with different growth stages: personally, I am fine using the old name Anatosaurus for the geologically-younger annectens.) Babies of even the long-snouted saurolophines had relatively short faces.
Both saurolophines and lambeosaurine produced giants of greater than 13 m in length. These represent the largest animals other than sauropods that have ever lived on land, and the heaviest bipeds in Earth's history.
Microwear analysis of the teeth of hadrosaurids is consistent with their complex chewing patterns. In at least the broad-billed edmontosaurs there is a great degree of scratching on the teeth, suggesting that they were primarily low browsers of tough vegetation ("grazers"). (Given the wide snouts of Edmontosaurus, Anatosaurus, and so forth, this makes a lot of sense.) Studies have not yet been published to see if most hadrosaurids were primarily low browsers, or if some of them might have been mostly high browsers. Given the diversity of bill shapes and snout lengths (and the diversity of species overall), there was probably a number of different diets among the hadrosaurids.
EVOLUTIONARY PATTERNS IN ORNITHOPODA
Feeding adaptation transformations:
Locomotory changes:
Social behavior in Ornithopoda:
Ornithopods (in particular iguanodontians (in particular hadrosaurids (in particular lambeosaurines))) have abundant evidence for
socially-related adaptations, including: herding; visual (and possibly aural) displays; species recognition structures; possible sexual dimorphism. We will discuss these
more fully in the third section of the course.
Heterochrony, size, and ornithopod history:
In general, peramorphosis seems to play an important role in ornithopod evolution. Hatchling iguanodontians tend to resemble adult "hypsilophodonts",
while hatchling hadrosaurids tend resemble young primitive iguanodontians, and young hadrosaurids tend to resemble the immediate outgroups of Hadrosauridae.
Basal ornithopods were small (comparable to basal members of other ornithischian groups). But at the base of Iguanodontia and the base of Styracosterna there are major size increases. Additionally, various different styracosternan lineages independantly achieved very large (>12 m) size.
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.
Marginocephalians:
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.
CERATOPSIA
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 together (but the specimen was artificially modified to put an adult with them!), 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. However, that is true of the ADULTS; recent work shows that hatchlings were quadrupedal, but becoming bipedal as juveniles and adults (not unlike certain common modern intelligent primates...).
NEOCERATOPSIA
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.
CERATOPSIDAE
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 (and several others) have 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:
Avaceratops, , Diabloceratops, and Albertaceratops are among the most primitive centrosaurines. Additional forms include the closely related Centrosaurus, Spinops, and Coronoceratops plus sword-horned Rubeosaurus and spike-frilled Styracosaurus, all of which share a powerful nasal horn; the aforementioned Asian centrosaurine Sinoceratops and its North American relative Xenoceratops; hook-horned Einiosaurus and ; 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.
(If you have a spare hour, you can see the following debate as to whether Torosaurus is an adult 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:
Locomotory changes:
Pachycephalosaurs, basal ceratopsians, and basal neoceratopsians were bipedal; increased skull size forced advanced neoceratopsians onto all
fours, such that coronosaurs were obligate quadrupeds.
Size trends:
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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