Ceratopia, also spelled Ceratopsia, is composed of those marginocephalians who are closer to Ceratops (or to the much better known Triceratops) than to Pachycephalosaurus.  An important character all ceratopians possess is the rostral bone, which gives the upper jaw the parrot-like beak characteristic of ceratopians. 
       There are several groups of ceratopians, including the basal psittacosaurids and the neoceratopians (psittacosaurids cut out), consisting of the protoceratopids, leptoceratopids, and the ceratopids.  Also, Protoceratopidae and Leptoceratopidae are sometimes broken up and seen as a lineage leading to Ceratopidae, but I'm tentatively retaining them. 
       Ceratopians generally get progressively larger; late ceratopids are elephant-sized.  Horns also are a common feature of ceratopians; all ceratopids and some more basal neoceratopians possess at least prominent bumps over the nose and eyes.  The shelf at the rear of the skull gets much larger, and in some cases becomes walled over.  The holes in the frill, technically called fenestrae, both lightened the heavy frill and provided an attachment for jaw muscles, although this probably only occurred in the front sections of the frill.  The main purpose of the frill was probably display; it was usually not strong enough to be much good in absorbing a predator's bite.  To support the frill, the first three cervicals fuse, as is seen in all ceratopids.
       Ceratopians evolved from bipeds, but become quadrupeds as they progressed.  The position of the forelimbs is still in dispute.  Trackway evidence favors forelimbs that are not sprawling widely, but skeletal information favors forelimbs spread out.  This is a major problem in ceratopids, which lend themselves to lively illustrations of prancing animals, but are not readily mounted with forelimbs that cooperate with this ideal.  It may be that the elbows were gently bent. 
       Although ceratopids are pretty clearly herbivores, basal ceratopians may well have been omnivorous.  It is also fairly well accepted that ceratopians were often gregarious animals, in the sense that they grouped together.  This is shown by bonebeds consisting of large numbers of bones belonging to one species.  Whether or not they herded, in the sense of a complex social order, is (and probably always will be) unknown, but they certainly possessed much of the equipment to behave in such a manner: prominent display surfaces (horns, frills, and in some animals, tails) and what is best interpreted as sexual dimorphism of these features, such as prominent tall frills versus lower frills.  Some of the postcranial material may also show dimorphic features.
       Ceratopian jaws are very powerful, but not very good at grinding, working like giant shears.  Ceratopians probably sliced off short lengths of plants and swallowed them, letting the guts do the rest of the work.  Basal ceratopians like the psittacosaurids and leptoceratopids may have been omnivorous.
       Ceratopia as a whole is one of the best-known dinosaur groups, represented by hundreds of skulls and skeletons.  
    The following "cladogram" is based mostly off of Chinnery, 2004 (description of Prenoceratops in the Journal of Vertebrate Paleontology), with Bagaceratopidae added and some differences at the base:

          |    |--Chaoyangsaurus
          |    `--Xuanhuaceratops
               |    |--Hongshanosaurus
               |    `--Psittacosaurus
                              |   |--Montanoceratops
                              |   `--+--Prenoceratops
                              |        `--+--Leptoceratops
                              |             `--Udanoceratops 
                                   |      `--Bagaceratops
                                   `--+ --Protoceratopidae
                                        |     |--?Graciliceratops
                                        |     `--Protoceratops    

Ceratopia:  An entry that suggested ceratopians in the late Campanian-early Maastrichtian (LK) of France turned out to be a typo for "ceratosaurians". 

Taxon or Taxa: Time\Place: Comments:
Yinlong downsi Xu X., Forster, Clark, and Mo J., 2006 Oxfordian (LJ) of China Yinlong is a basal marginocephalian/ceratopian known from a nearly complete skeleton, its name a reference to the movie "Crouching Tiger, Hidden Dragon" (Yin Long means Hiding Dragon), which was filmed near where it was found.  To picture it, think of a psittacosaur with a somewhat turtle-like skull.  It helps to tie the pachies to ceratopians, and may drag the heterodontosaurids in too.

Chaoyangsauridae: Both genera here are mostly known for having been described informally or mentioned back in the mid 1980s, with somewhat different names.  Both were considered to be either basal pachycephalosaurs, psittacosaurid-like basal ceratopians, or otherwise right around that split.

Taxon or Taxa: Time\Place: Comments:
Chaoyangsaurus youngi Zhao, Cheng, and Xu, 1999 Tithonian (LJ) of China Long known under the informal names "Chaoyoungosaurus" and "Chaoyangosaurus" as an animal possibly more basal than the ceratopian-pachycephalosaurian split, this animal has turned out to be a very basal ceratopian, more primitive than Psittacosaurus.  It is known from a skull, jaws, seven cervicals, a humerus, and a scapula.
Xuanhuaceratops niei Zhao, X., Cheng, Z., Xu., X., and Makovicky, 2006 LJ of China Known as "Xuanhuasaurus" for twenty years, Xuanhuaceratops finally is published and comes out right where it was always said to be.  It is based on four fragmentary specimens.  Remains include two incomplete skulls, verts, a scapulacoracoid, a humerus, and an ischium. 


Taxon or Taxa: Time\Place: Comments:
Hongshanosaurus houi You, Xu, and Wang, 2003 Valanginian-early Barremian (EK) of China Based on the skull and jaws of a juvenile from the Yixian, this basal ceratopian may be closely related to Psittacosaurus.  The skull is a little less rectangular than the parrot-snouted Psittacosaurus.  The skull and jaws of an adult have been referred to it as well, although they may have since been referred to Psittacosaurus lujiatunesis.
Psittacosaurus: Osborn, 1923 P. mongoliensis (type) Osborn, 1923 Barremian-Aptian (EK) of China and Mongolia One of the best-known dinosaurs, Psittacosaurus is a small (less than two meters in length) biped with a distinctive skull profile; it has a prominent parrot-like beak and high nostrils, giving this animal a skull that looks almost squashed (P. mazongshanensis is known to have had an unusually long skull).  Processing of food was accomplished by sliding the jaws back and forth.  
Over 18 species have been named, but not all considered valid; however, it has the most well-supported species of any classic dinosaur.  Two general types are known to exist; one type includes species with prominent cheek "horns," and the other group includes species with more subdued cheeks.  Some have been found with stones in the abdominal region, suggesting it had a gizzard-like organ, like some other dinosaurs.  
It has only four fingers, while ceratopids have five; thus, it's not in the line leading directly to ceratopids, due to its modified hand.  
It was originally classified as an ornithopod (one specimen was named Protiguanodon).  At one time, it was also thought to have something to do with ankylosaurians, due to what was interpreted as small armor studs on the body.  Since then, the rostral bone at the front of the upper jaw has been described for what it is, and this creature classified as a very basal ceratopian.  Well over a hundred specimens are known for this animal; unfortunately, few mounts of it are to be seen in the United States.  Pigeon-sized juvenile remains are known; in one case, 34 juveniles were found with an adult, together in an area with dimensions of 0.5 square meters.  The sheer number in that space suggests some kind of grouping, as simple as a nest or as complex as a family gathering.  
Psittacosaurus is known to have typical dinosaurian scalation; new specimens from the Yixian also indicate that at least some species had unusual bristles along the midline of the proximal third of the tail.  These bristles appear to be about 15 cm long, and may have had a defensive function.
Recently, a new specimen of the opossum-sized EK Chinese triconodont mammal Repenomamus robustus was described, having been found to include partial remains of a juvenile Psittacosaurus in its gut region.  The juvenile had a head-body length of about 14 cm, or about a third of its consumer's head-body length, and the fact that some of its long bones were articulated suggests that it was eaten in chunks.  This find establishes that early mammals ate dinosaurs, although whether this was a case of predation or scavenging can't be established.
P. lujiatunensis Zhou, C.-F., Gao, K.-Q., Fox, R., and Chen, S.-H., 2006   Valanginian-early Barremian (EK) of China
P. major Sereno, Zhao, X.-J, Brown, and Tan, L., 2007  Valanginian-early Barremian (EK) of China
P. mazongshanensis Xu, 1997 Barremian (EK) of China
P. meileyingensis Sereno, Zhao, Cheng, and Rao, 1988 late Aptian or late Barremian (EK) of China
P. neimongoliensis Russell and Zhao, 1996 ?Barremian (EK) of China
P. ordosensis Russell and Zhao, 1996 ?Barremian (EK) of China
P. sattayaraki (?N.D.) Buffetaut and Suteethorn, 1992 Aptian-Albian (EK) of Thailand
P. sinensis Yang, 1958 Aptian-Albian (EK) of China
P. xinjiangensis Sereno and Zhao, 1988 ?Valanginian-Albian (EK) of China

Psittacosauridae i.s.:

Taxon or Taxa: Time\Place: Comments:
"Psittacosaurus" sibiricus Voronkevich and Averianov vide Leschinskiy, Voronkevich, Maschenko, and Averianov, 2000 Aptian-Albian (EK) of Russia (western Siberia) This recently-described (with some controversy over the source) and very obscure species has some features that seem quite unusual for Psittacosaurus, including a frill that is 15-18% of the skull length, three postorbital horns, a long shallow predentary, and two more presacral vertebrae (23 total) than any other known species of Psittacosaurus.  It is known from at least two complete skeletons, and was described as the largest and most derived species of the genus.  
Photos show an animal that appears to have been the result of a breeding experiment with a basal neoceratopian and a Psittacosaurus, followed by terrible acne or something.  It had the typical psittac jugal horns, but also eyebrow nodes, nodes beside the eyes, and triangles flaring out just behind the nostrils.
Thanks to Frederik Spindler for pressing me about this strange animal, and providing me images of the skull!  
A new description found it to be closest to P. sinensis among psittacosaurids, and included it within Psittacosaurus, but noted numerous differences.

Neoceratopia: Basal neoceratopians are coming out of the woodwork, and it will be very interesting to see how they all sort out.  For much of the 20th century, it was just Protoceratops and a couple of friends, but now there are a lot of good remains from the middle of the Cretaceous through to the end, from North America and especially Asia (and not just Mongolia and China, either).  A few small internal lineages may become apparent, like the recent suggestion of Leptoceratopidae. 
    An unusual feature of all neoceratopians is the fusion of the first three cervicals into a solid unit.  It is believed that this was probably an adaptation to support the increasingly large head.  The skull is perfectly balanced with a ball and socket joint at the end of this complex, giving it great mobility.
    An undescribed basal neoceratopian is known from hips and caudals from the mid-late Albian (EK) of Idaho.

Taxon or Taxa: Time\Place: Comments:
Liaoceratops yanzigouensis Xu, Makovicky, Wang, Norell, and You, 2002 Valanginian-early Barremian (EK) of China A Yixian ceratopian known from two complete skulls (a subadult and a juvenile) equipped with premaxillary teeth and a rudimentary frill, this small (described as "dog-size") animal is one of the most basal known neoceratopians, if not the most basal neoceratopian, more derived than Psittacosaurus but similar. 
Yamaceratops dorngobiensis Makovicky and Norell, 2006 late Early Cretaceous (EK) of Mongolia This basal neoceratopian includes skull remains with a mix of basal and derived (mostly in the cheek and lower jaw) features, and a frill similar to those of Liaoceratops and Leptoceratops, this similarity suggesting that some function outside of display may be also present.
Archaeoceratops oshimai Dong and Azuma, 1997 Barremian (EK) of China Known from a partial skeleton and skull, little has yet been published on this animal.  The Protoceratops-like teeth in the upper jaws contrast with the Psittacosaurus-like teeth in the lower jaws, suggesting this animal is less derived than the protoceratopids.  It was a small, fast animal.

Neoceratopia i.s.:  

Taxon or Taxa: Time\Place: Comments:
Asiaceratops salsopaludalis (N.D.) Nessov, Kaznyshkina, and Cherepanov, 1989 late Albian (EK) or Cenomanian (LK) of Uzbekistan This is a small, indeterminate relative of Microceratops (?or Graciliceratops).  It is known from parts of a skull and a phalanx (either a toe or finger bone).  In recent studies, it tends to bounce around the base of Neoceratopia.
Auroraceratops rugosus You, H., D. Li, Q. Ji, Lamanna, and Dodson, 2005  Valanginian-Albian (EK) of China This basal neoceratopian is based on a skull and lower jaw.  Unusual characters include broad nasals, "fungiform" lacrimals (which means they have a sort of mushroom shape, bulging out), and rugosities on the jugals and lower jaw (hence the species name, I would assume).  It was an approximate contemporary of Archaeoceratops.  The authors found it to be the sister to the Coronosauria, a group which I don't have labeled here due to the recent inflation of basal ceratopians, but which for simplicity's sake you can consider to be the node where Protoceratopidae and Ceratopoidea meet. 
Kulceratops kulensis (N.D.) Nessov, 1995 late Albian (EK) of central Asia This obscure animal is a basal neoceratopian.  It is based on a dentary fragment.
"Microceratops" gobiensis (N.D.) Bohlin, 1953 ?mid-late Campanian (LK) of China As the name suggests, this is a very small animal, less than a meter in length, although much of the remains are juvenile.  Its proportions suggest it was one of the fastest neoceratopians.  Some scientists have suggested that its remains are indeterminate, though; the best material has been referred to Graciliceratops.  Also, the name is preoccupied by a hymenopteran (wasps, bees, and ants; Seyrig, 1952).
Serendipaceratops arthurcclarkei Rich and Vickers-Rich, 2003 EK of Australia This finally gives recognition to the informally-discussed Australian neoceratopian ulna.  The animal may have been similar to Leptoceratops (the gold standard for non-Protoceratops basal neoceratopians)
Turanoceratops tardabilis Nessov, Kaznyshkina, and Cherepanov, 1989 Maastrichtian (LK) of Kazakhstan The remains of this animal may include horn cores, suggesting ceratopid-like horns.  Double-rooted teeth are also known, like ceratopid teeth.


Taxon or Taxa: Time\Place: Comments:
Montanoceratops cerorhynchus Sternberg, 1951 (originally Leptoceratops cerorhynchus Brown and Schlaikjer, 1942) early Maastrichtian (LK) of Montana Displaying a taller nose horn (but see below) than its cousin Protoceratops, Montanoceratops also possesses a fin-tail.  It is known from two specimens, including a partial skull.  The nasal horn thought to distinguish it may actually be a misplaced jugal (outwardly projecting in ceratopians and responsible for the so-called additional "horns" of Pentaceratops).
Prenoceratops pieganensis Chinney, 2004 late middle Campanian (LK) of Montana Prenoceratops is a new basal neoceratopian, known from juvenile bonebed remains covering most of the skeleton (although only the skull has been described to date).  It seems to be close to Leptoceratops, although with a longer frill and a longer, lower skull overall.  It comes from the Two Medicine Formation of Montana.
Leptoceratops gracilis Brown, 1914 late Maastrichtian (LK) of Alberta, Montana, and Wyoming Seemingly one of the most basal neoceratopians, Leptoceratops is also ironically one of the last.  It is known from several good specimens.  Although the type species is Lancian, there is new juvenile bonebed material that is Judithian in age which probably represents a new species (this doesn't seem to be the same material as that referred to Prenoceratops; basal neoceratopians are an understudied part of the North American LK fauna, and Leptoceratops has long been the gold standard for comparison among what is starting to look like a NA radiation of closely-related critters).
Udanoceratops tschizhovi Kurzanov, 1992 late Campanian (LK) of Mongolia Based on a partial skeleton and skull, this is, at over four meters in length, the largest known bipedal neoceratopian.  It has an unusually deep low jaw.  It appears to be closest to Leptoceratops.

Leptoceratopidae i.s.:

Taxon or Taxa: Time\Place: Comments:
?Bainoceratops efremovi Tereschenko and Alifanov, 2003 late Campanian (LK) of Mongolia Based on a vertebral column, Bainoceratops may be affiliated with Leptoceratops and Udanoceratops.

Bagaceratopidae: Bagaceratopidae seemed to appear one day, coming upon the description of three new neoceratopians, two of which were put into said new family, the other a probable member.  While I'm a bit suspicious, especially given that the differences between Bagaceratops, Lamaceratops, and Platyceratops are not all that huge and could be due to age, individual variation, sex, and so on (and may be more worthy of different species instead of genera), I'm willing to put it up and see how future study treats it.  Chinnery, 2004 finds Bagaceratops itself close to but outside of Protoceratopidae, so maybe there's something going on here...

Taxon or Taxa: Time\Place: Comments:
Bagaceratops rozhdestvenskyi Maryanska and Osmolska, 1975 (including Breviceratops kozlowskii Kurzanov, 1990 [originally Protoceratops kozlowskii Maryanska and Osmolska, 1975]) ?mid Campanian (LK) of Mongolia Known from remains of over twenty individuals, Bagaceratops is one of the best known protoceratopids.  Its skull has a small node over the nose.
Now assigned here are the partial juvenile remains first named Protoceratops kozlowskii and later called Breviceratops; the features thought to make it different from Bagaceratops have turned out to be growth-related.

Bagaceratopidae i.s.:

Taxon or Taxa: Time\Place: Comments:
Lamaceratops tereschenkoi Aliafanov, 2003 ?mid Campanian (LK) of Mongolia This animal was described as a bagaceratopid, based on a small skull (about 75% that of Bagaceratops' holotype) with a nasal horn, no flaring jugals, and no premaxillary teeth.
Magnirostris dodsoni You and Dong, 2003 late Campanian (LK) of China Another possible bagaceratopid, this one has an unusually large rostral, giving it a big beak.  It is based on a skull missing the frill, but with nubbins of brow horns, and a well-developed nasal ridge. 
Platyceratops tatarinovi Aliafanov, 2003 ?mid Campanian (LK) of Mongolia Platyceratops is based on a somewhat poorly-preserved skull, originally a paratype of Breviceratops, about a quarter larger than Bagaceratops' holotype, with a short face and a prominent nasal horn.  It also was referred to Bagaceratopidae.

Protoceratopidae: After having come under attack as a paraphyletic family throughout much of the 1990s, it now appears that a small cluster of dinosaurs may make up a true Protoceratopidae.

Taxon or Taxa: Time\Place: Comments:
?Graciliceratops mongoliensis Sereno, 2000 ?Santonian (LK) of Mongolia Graciliceratops is based on possibly juvenile remains (partial articulated skull and skeleton) once referred to "Microceratops".
Protoceratops: Granger and Gregory, 1923 P. andrewsi (type) Granger and Gregory, 1923 late Campanian (LK) of Mongolia and China Known from remains belonging to at least eighty individuals of a wide range of ages and what appears to be both sexes, Protoceratops is one of the most famous dinosaurs.  Skulls interpreted as coming from males have strongly-arching nasal regions and tall frills, while females have less arched nasal regions and lower, smaller frills.  Protoceratops also has a "fin-tail" composed of tall caudal spines.  It had long been considered to be the dinosaur responsible for many nests in Mongolia, but it now appears at least some of the nests and eggs actually belong to an oviraptorid, once accused of seeking out Protoceratops eggs to eat. 
P. hellenikorhinus Lambert, Godefroit, Shang, and Dong, 2001 late Campanian (LK) of China New species P. hellenikorhinus, known from at least nine individuals, was larger than the type, and had a vaguely chasmsosaurine-like frill, larger jugal horns, and double nasal horns (actually more like one nasal horn with the top split into two parallel ridges).


Taxon or Taxa: Time\Place: Comments:
Zuniceratops christopheri Wolfe and Kirkland, 1998


mid Turonian (LK) of New Mexico Temporarily the last name on the list of valid dinosaur names (now supplanted by Zupaysaurus), Zuniceratops is based on partial remains that suggest it is closest to basal ceratopids, with brow horns and all (although no nasal horn).  The frill is fenestrated, but lacks epoccipitals.  Interestingly, the teeth appear to be single-rooted, unlike the double-rooted teeth of true ceratopids.  A bone thought to be the squamosal has turned out to be a therizinosaurian ischium (Nothronychus).  Remains from several individuals are known.
New bonebed material from at least five individuals provides evidence that the teeth became double-rooted with age, and the brow horns also grew substantially with age.

Ceratopidae i.s.: Ceratopids are distinguished from the more basal neoceratopians by their enlarged frills and full-blown horn cores.  They are also much larger and are in no way bipedal.
    A couple of decades ago some researchers suggested that ceratopids and their frilled ancestors had their frills completely encased in flesh and muscle.  This suggestion led to some rather odd illustrations.  Since frills tend to have distinctive ornamented margins, it is more logical to suppose the frills were not encased in muscle.  For example, why would these animals have ornamented margins to their frills if they were just going to be covered?  If they truly were encased, all ceratopids should have roughly the same form of frill margins.
    Three new ceratopid skulls are known from the lower Campanian (LK) of Utah.

Taxon or Taxa: Time\Place: Comments:
Agauthamas sylvestris (N.D.) Cope, 1872 (?Torosaurus or Triceratops) late Maastrichtian (LK) of Wyoming Based on a section of the hip and back, this historically significant animal (first named ceratopid) is just a large indeterminate ceratopid, probably either Triceratops or Torosaurus.
Avaceratops lammersi Dodson, 1986 (?Ceratops) late middle Campanian (LK) of Montana Avaceratops is known from a partial subadult skeleton, with a skull of a small adult possibly belonging to it.  In defense of Avaceratops as a valid taxon, it appears to have a solid frill and pronounced brow horns, and certain features suggesting it is a basal ceratopid, like more pointed unguals.  It may be the same as Ceratops, or allied to Triceratops.  Where it belongs is not certain; it could be basal to the ceratopine-centrosaurine split.
(N.D.) Cope, 1876
D. encaustatus (N.D.) (type) Cope, 1876 late middle Campanian (LK) of Montana These names are all based on typical ceratopid teeth, with split roots (with some hadrosaurid teeth mixed in for D. encaustus), but beyond that can give little useful information.
D. bicarinatus (N.D.) Cope, 1876
D. haydenianus (N.D.) Cope, 1876
"Monoclonius": "M." fissus (N.D.) Cope, 1876 late middle Campanian (LK) of Montana This is just ceratopid junk.
"M." recurvicornis (N.D.) Cope, 1889 This animal is based on remains that appear to come from a ceratopine, not a centrosaurine like Monoclonius.
"Triceratops" maximus (N.D.) Brown, 1933 late Maastrichtian (LK) of Montana The remains assigned here can only be classified as that of a large ceratopid.

Centrosaurinae and Ceratopinae:  These two subgroups are based on horn arrangement and relative extent of the squamosal.  The two squamosal bones make up a side of the frill each, flanking the parietal bone which makes up the bulk of the frill (and usually encloses two fenestrae, one on either side of the frill midline).  In centrosaurines, the squamosals are short and never reach the end of the frill.  In ceratopines, the squamosals are long and reach the end.  Both groups have bones called epoccipitals, which often line the edges of the frill and give it a jagged look.
    These bony projections were probably only for show.  Centrosaurines also have a flange of bone pointing to the front of the nasal fenestra from the rear, and have nasal horns which are more prominent than brow horns.  Ceratopines have a flange in the nasal fenestra pointing back from the front, and more prominent brow horns.  Both groups have very similar postcrania, and the best way to separate taxa is to use adult skulls. One very interesting possibility is that, if they were social animals, perhaps the presence of dominance hierarchies repressed full expression of these features for most individuals, as happens in modern social animals.
    Some ceratopids have squamosal fenestrae, which have puzzled researchers for a long time.  It may be that the ceratopine versions are natural, while the centrosaurine versions are lesions, remnants of puncture wounds (like, say, from another centrosaurine's nasal horn). 

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