Ceratopsia

Ceratopsia or Ceratopia (/ˌsɛrəˈtɒpsiə/ or /ˌsɛrəˈtoʊpiə/; Greek: "horned faces") is a group of herbivorous, beaked dinosaurs that thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. The earliest known ceratopsian, Yinlong downsi, lived between 161.2 and 155.7 million years ago.[4] The last ceratopsian species, Triceratops prorsus, became extinct during the Cretaceous–Paleogene extinction event, 66 million years ago.[4]

Early members of the ceratopsian group, such as Psittacosaurus, were small bipedal animals. Later members, including ceratopsids like Centrosaurus and Triceratops, became very large quadrupeds and developed elaborate facial horns and frills extending over the neck. While these frills might have served to protect the vulnerable neck from predators, they may also have been used for display, thermoregulation, the attachment of large neck and chewing muscles or some combination of the above. Ceratopsians ranged in size from 1 meter (3 ft) and 23 kilograms (50 lb) to over 9 meters (30 ft) and 9,100 kg (20,100 lb).

Triceratops is by far the best-known ceratopsian to the general public. It is traditional for ceratopsian genus names to end in "-ceratops", although this is not always the case. One of the first named genera was Ceratops itself, which lent its name to the group, although it is considered a nomen dubium today as its fossil remains have no distinguishing characteristics that are not also found in other ceratopsians.[5]

Ceratopsians
Temporal range: Late Jurassic – Late Cretaceous, 161–66 Ma
Triceratops front view
Triceratops skeleton, Smithsonian Museum of Natural History
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Ornithischia
Clade: Marginocephalia
Suborder: Ceratopsia
Marsh, 1890
Subgroups

Anatomy

Centrosaurus
Centrosaurus, with large nasal horn, exaggerated epoccipitals, and bony processes over the front of the frill. Museum of Victoria.

Ceratopsians are easily recognized by features of the skull. On the tip of a ceratopsian upper jaw is the rostral bone, an edentulous (toothless) ossification, unique to ceratopsians. Othniel Charles Marsh recognized and named this bone, which acts as a mirror image of the predentary bone on the lower jaw. This ossification evolved to morphologically aid the mastication of plant matter.[6] Along with the predentary bone, which forms the tip of the lower jaw in all ornithischians, the rostral forms a superficially parrot-like beak. Also, the jugal bones below the eye are prominent, flaring out sideways to make the skull appear somewhat triangular when viewed from above. This triangular appearance is accentuated in later ceratopsians by the rearwards extension of the parietal and squamosal bones of the skull roof, to form the neck frill.[7][8]

The epoccipital is a distinctive bone found lining the frills of ceratopsians. The name is a misnomer, as they are not associated with the occipital bone.[9] Epoccipitals begin as separate bones that fuse during the animal's growth to either the squamosal or parietal bones that make up the base of the frill. These bones were ornamental instead of functional, and may have helped differentiate species. Epoccipitals probably were present in all known ceratopsids with the possible exception of Zuniceratops.[10] They appear to have been broadly different between short-frilled ceratopsids (centrosaurines) and long-frilled ceratopsids (chasmosaurines), being elliptical with constricted bases in the former group, and triangular with wide bases in the latter group. Within these broad definitions, different species would have somewhat different shapes and numbers. In centrosaurines especially, like Centrosaurus, Pachyrhinosaurus, and Styracosaurus, these bones become long and spike- or hook-like.[11] A well-known example is the coarse sawtooth fringe of broad triangular epoccipitals on the frill of Triceratops. When regarding the ossification's morphogenetic traits, it can be described as dermal. The term epoccipital was coined by famous paleontologist Othniel Charles Marsh in 1889.[9][12]

History of study

Ceratopsidae-BW-002
Ceratopsidae is the most advanced group of Ceratopsians

The first ceratopsian remains known to science were discovered during the U.S. Geological and Geographical Survey of the Territories led by the American geologist F.V. Hayden. Teeth discovered during an 1855 expedition to Montana were first assigned to hadrosaurids and included within the genus Trachodon. It was not until the early 20th century that some of these were recognized as ceratopsian teeth.[13] During another of Hayden's expeditions in 1872, Fielding Bradford Meek found several giant bones protruding from a hillside in southwestern Wyoming. He alerted paleontologist Edward Drinker Cope, who led a dig to recover the partial skeleton. Cope recognized the remains as a dinosaur, but noted that even though the fossil lacked a skull, it was different from any type of dinosaur then known. He named the new species Agathaumas sylvestris, meaning "marvellous forest-dweller".[14] Soon after, Cope named two more dinosaurs that would eventually come to be recognized as ceratopsids: Polyonax and Monoclonius. Monoclonius was notable for the number of disassociated remains found, including the first evidence of ceratopsid horns and frills. Several Monoclonius fossils were found by Cope, assisted by Charles Hazelius Sternberg, in summer 1876 near the Judith River in Chouteau County, Montana. Since the ceratopsians had not been recognised yet as a distinctive group, Cope was uncertain about much of the fossil material, not recognizing the nasal horn core, nor the brow horns, as part of a fossil horn. The frill bone was interpreted as a part of the breastbone.[15]

In 1888 and 1889, Othniel Charles Marsh described the first well preserved horned dinosaurs, Ceratops and Triceratops. In 1890 Marsh classified them together in the family Ceratopsidae and the order Ceratopsia. This prompted Cope to reexamine his own specimens and to realize that Triceratops, Monoclonius, and Agathaumas all represented a single group of similar dinosaurs, which he named Agathaumidae in 1891. Cope redescribed Monoclonius as a horned dinosaur, with a large nasal horn and two smaller horns over the eyes, and a large frill.

Classification

Ceratopsia was coined by Othniel Charles Marsh in 1890 to include dinosaurs possessing certain characteristic features, including horns, a rostral bone, teeth with two roots, fused neck vertebrae, and a forward-oriented pubis. Marsh considered the group distinct enough to warrant its own suborder within Ornithischia.[16] The name is derived from the Greek κέρας/kéras meaning 'horn' and ὄψῐς/ópsis meaning 'appearance, view' and by extension 'face'. As early as the 1960s, it was noted that the name Ceratopsia is actually incorrect linguistically and that it should be Ceratopia.[17] However, this spelling, while technically correct, has been used only rarely in the scientific literature, and the vast majority of paleontologists continue to use Ceratopsia. As the ICZN does not govern taxa above the level of superfamily, this is unlikely to change.

Taxonomy

Psittacosaurus mongoliensis skeleton
An early ceratopsian: Psittacosaurus
Montanoceratops skeleton
Montanoceratops, a leptoceratopsid
Protoceratops-skeleton
A typical protoceratopsid: Protoceratops skeleton at the Wyoming Dinosaur Center
Styracosaurus skeleton AMNH5372
Styracosaurus, a centrosaurine ceratopsid
Triceratops-skull-Zachi-Evenor-002
Triceratops, one of the largest ceratopsians (a chasmosaurinae ceratopsid). It had solid frill and long horns.

Following Marsh, Ceratopsia has usually been classified as a suborder within the order Ornithischia. While ranked taxonomy has largely fallen out of favor among dinosaur paleontologists, some researchers have continued to employ such a classification, though sources have differed on what its rank should be. Most who still employ the use of ranks have retained its traditional ranking of suborder,[18] though some have reduced to the level of infraorder.[19]

Possible ceratopsians from the Southern Hemisphere include the Australian Serendipaceratops, known from an ulna, and Notoceratops from Argentina is known from a single toothless jaw (which has been lost).[20] Craspedodon from the Late Cretaceous (Santonian) of Belgium may also be a ceratopsian, specifically a neoceratopsian closer to ceratopsoidea than protoceratopsidae.[21] Possible leptoceratopsid remains have also been described from the early Campanian of Sweden.[22]

Phylogeny

Ceratopsian skulls
Ceratopsid skulls at the Natural History Museum of Utah

Paleontologists today agree on the overall structure of the ceratopsian family tree, although there are differences on individual taxa. There have been several cladistic studies performed on basal ceratopsians since 2000. None have used every taxon listed above and many of the differences between the studies are still unresolved.

In clade-based phylogenetic taxonomy, Ceratopsia is often defined to include all marginocephalians more closely related to Triceratops than to Pachycephalosaurus.[23] Under this definition, the most basal known ceratopsians are Yinlong, from the Late Jurassic Period, along with Chaoyangsaurus and the family Psittacosauridae, from the Early Cretaceous Period, all of which were discovered in northern China or Mongolia. The rostral bone and flared jugals are already present in all of these forms, indicating that even earlier ceratopsians remain to be discovered.

The clade Neoceratopsia includes all ceratopsians more derived than psittacosaurids. Another subset of neoceratopsians is called Coronosauria, which currently includes all ceratopsians more derived than Auroraceratops. Coronosaurs show the first development of the neck frill and the fusion of the first several neck vertebrae to support the increasingly heavy head. Within Coronosauria, three groups are generally recognized, although the membership of these groups varies somewhat from study to study and some animals may not fit in any of them. One group can be called Protoceratopsidae and includes Protoceratops and its closest relatives, all Asian. Another group, Leptoceratopsidae, includes mostly North American animals that are more closely related to Leptoceratops. The Ceratopsoidea includes animals like Zuniceratops, which are more closely related to the family Ceratopsidae. This last family includes Triceratops and all the large North American ceratopsians and is further divided into the subfamilies Centrosaurinae and Ceratopsinae (also known as Chasmosaurinae).

Farke phylogeny

Andrew Farke and his colleagues in 2014 published a description of a new neoceratopsian, Aquilops americanus, through the peer-reviewed science journal PLOS ONE. They analysed their taxa as well as most other primitive ceratopsians to get a consensus cladogram. They created their own data matrix and through it found that many groups of ceratopsians could be supported, and that Aquilops was a basal neoceratopsian that could potentially be a protoceratopsid, leptoceratopsid, or ceratopsid, although any one of these groups would have a large ghost lineage with Aquilops.[24]

Cerapoda

Hypsilophodon

Marginocephalia

Stegoceras

Ceratopsia

Yinlong

Chaoyangsaurus

Xuanhuaceratops

Psittacosaurus

P. mongoliensis

P. sinensis

Neoceratopsia

Liaoceratops

Ajkaceratops

Aquilops

Auroraceratops

Yamaceratops

Helioceratops

Archaeoceratops

Koreaceratops

Leptoceratopsidae

Asiaceratops

Cerasinops

Montanoceratops

Prenoceratops

Udanoceratops

Leptoceratops

Zhuchengceratops

Unescoceratops

Gryphoceratops

Coronosauria

Graciliceratops

Protoceratopsidae

Protoceratops Protoceratops andrewsi - IMG 0691 white background.jpg

Bagaceratops

Ceratopsoidea

Zuniceratops

Turanoceratops

Ceratopsidae

Chasmosaurus ChasmosaurusROM white background.JPG

Triceratops LA-Triceratops mount-2.jpg

Diabloceratops

Centrosaurus

Their study also found an equal consensus cladogram finding Ajkaceratops not as a neoceratopsian but a protoceratopsid. Nothing else about the cladograms changed.[24]

Xu/Makovicky/Chinnery Phylogeny

Xu Xing of the Chinese Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing, along with Peter Makovicky, formerly of the American Museum of Natural History (AMNH) in New York City and others, published a cladistic analysis in the 2002 description of Liaoceratops.[25] This analysis is very similar to one published by Makovicky in 2001.[26] Makovicky, who currently works at the Field Museum of Natural History in Chicago, also included this analysis in his 2002 doctoral thesis. Xu and other colleagues added Yinlong to this analysis in 2006.[27]

Brenda Chinnery, formerly of the Museum of the Rockies in Bozeman, Montana, independently described Prenoceratops in 2005 and published a new phylogeny.[28] In 2006, Makovicky and Mark Norell of the AMNH incorporated Chinnery's analysis into their own and also added Yamaceratops, although they were not able to include Yinlong.[29] The cladogram presented below is a combination of Xu, Makovicky, and their colleagues' most recent work.

Ceratopsia 

 Yinlong

 void 
 void 

 Chaoyangsaurus

 IVPP V12722 (Xuanhuaceratops)

 void 

 Psittacosauridae

 Neoceratopsia 

 Liaoceratops

 void 

 Yamaceratops

 void 

 Archaeoceratops

 Coronosauria 
 Leptoceratopsidae 

 Montanoceratops

 void 

 Udanoceratops

 Leptoceratops

 Prenoceratops

 void 

 Graciliceratops

 void 
 Protoceratopsidae 

 Bagaceratops

 Protoceratops Protoceratops andrewsi - IMG 0691 white background.jpg

 void 

 Zuniceratops

 Ceratopsidae LA-Triceratops mount-2.jpg

Chaoyangsaurus is recovered in a more basal position than Psittacosauridae, although Chinnery's original analysis finds it within Neoceratopsia. Protoceratopsidae is considered to be the sister group of Ceratopsoidea. The fragmentary Asiaceratops was included in these studies and is found to have a variable position, either as a basal neoceratopsian or as a leptoceratopsid, most likely due to the amount of missing information. Removal of Asiaceratops stabilizes the entire cladogram.

Makovicky's latest analysis includes IVPP V12722 ("Xuanhuasaurus"), a Late Jurassic ceratopsian from China that, at the time, was awaiting publication, but has since been published as Xuanhuaceratops. Kulceratops and Turanoceratops are considered nomina dubia in this study. Makovicky believes Lamaceratops, Magnirostris, and Platyceratops to be junior synonyms of Bagaceratops, and Bainoceratops to be synonymous with Protoceratops.

You/Dodson Phylogeny

You Hailu of Beijing's Chinese Academy of Geological Sciences, was a co-author with Xu and Makovicky in 2002 but, in 2003, he and Peter Dodson from the University of Pennsylvania published a separate analysis.[30] The two presented this analysis again in 2004.[7] In 2005, You and three others, including Dodson, published on Auroraceratops and inserted this new dinosaur into their phylogeny.[31]

Ceratopsia 

 Psittacosauridae

 Neoceratopsia 

 Chaoyangsaurus

 void 

 Liaoceratops

 void 

 Archaeoceratops

 void 

 Auroraceratops

 Coronosauria 
 Protoceratopsidae 

 Bagaceratops

 Protoceratops Protoceratops andrewsi - IMG 0691 white background.jpg

 void 
 Leptoceratopsidae 

 Montanoceratops

 Leptoceratops

 Ceratopsidae LA-Triceratops mount-2.jpg

In contrast to the previous analysis, You and Dodson find Chaoyangsaurus to be the most basal neoceratopsian, more derived than Psittacosaurus, while Leptoceratopsidae, not Protoceratopsidae, is recovered as the sister group of Ceratopsidae. This study includes Auroraceratops, but lacks seven taxa found in Xu and Makovicky's work, so it is unclear how comparable the two studies are. Asiaceratops and Turanoceratops are each considered nomina dubia and not included. Along with Dong Zhiming, You described Magnirostris in 2003, but to date has not included it in any of his cladograms.[32]

Paleobiology

Biogeography

Ceratopsian map
Ceratopsian fossil discoveries. The presence of Jurassic ceratopsians only in Asia indicates an Asian origin for the group, while the more derived ceratopsids occur only in North America save for one Asian species. Questionable remains are indicated with question marks.

Ceratopsia appears to have originated in Asia, as all of the earliest members are found there. Fragmentary remains, including teeth, which appear to be neoceratopsian, are found in North America from the Albian stage (112 to 100 million years ago), indicating that the group had dispersed across what is now the Bering Strait by the middle of the Cretaceous Period.[33] Almost all leptoceratopsids are North American, aside from Udanoceratops, which may represent a separate dispersal event, back into Asia. Ceratopsids and their immediate ancestors, such as Zuniceratops, were unknown outside of western North America, and were presumed endemic to that continent.[7][28] The traditional view that ceratopsoids originated in North America was called into question by the 2009 discovery of better specimens of the dubious Asian form Turanoceratops, which confirmed it as a ceratopsid. It is unknown whether this indicates ceratopsids actually originated in Asia, or if the Turanoceratops immigrated from North America.[34]

Centrosaurus skin
Skin impression of Centrosaurus

Individual variation

Unlike almost all other dinosaur groups, skulls are the most commonly preserved elements of ceratopsian skeletons and many species are known only from skulls. There is a great deal of variation between and even within ceratopsian species. Complete growth series from embryo to adult are known for Psittacosaurus and Protoceratops, allowing the study of ontogenetic variation in these species.[35][36] Significant sexual dimorphism has been noted in Protoceratops and several ceratopsids.[7][8][37]

Ecological role

Psittacosaurus and Protoceratops are the most common dinosaurs in the different Mongolian sediments where they are found.[7] Triceratops fossils are far and away the most common dinosaur remains found in the latest Cretaceous rocks in the western United States, making up as much as 5/6ths of the large dinosaur fauna in some areas.[38] These facts indicate that some ceratopsians were the dominant herbivores in their environments.

Some species of ceratopsians, especially Centrosaurus and its relatives, appear to have been gregarious, living in herds. This is suggested by bonebed finds with the remains of many individuals of different ages.[8] Like modern migratory herds, they would have had a significant effect on their environment, as well as serving as a major food source for predators.

Although ceratopsians are generally considered herbivorous, a few paleontologists, such as Darren Naish and Mark Witton, have speculated online that at least some ceratopsians may have been opportunistically omnivorous.[39][40]

Posture and locomotion

Most restorations of ceratopsians show them with erect hindlimbs but semi-sprawling forelimbs, which suggest that they were not fast movers. But Paul and Christiansen (2000) argued that at least the later ceratopsians had upright forelimbs and the larger species may have been as fast as rhinos, which can run at up to 56 km or 35 miles per hour.[41]

Daily activity patterns

A nocturnal lifestyle has been suggested for the primitive ceratopsian Protoceratops.[42] However, comparisons between the scleral rings of Protoceratops and Psittacosaurus and modern birds and reptiles indicate that they may have been cathemeral, active throughout the day at short intervals.[43]

Paleopathology

Activity-related bone fractures have been documented in ceratopsians.[44] Periostitis has also been documented in the shoulder blade of a ceratopsian.[45]

References

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Albian

The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous epoch/series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0.9 Ma (million years ago). The Albian is preceded by the Aptian and followed by the Cenomanian.

Aquilops

Aquilops is an early herbivorous ceratopsian dinosaur dating from the Early Cretaceous of North America, approximately 108 million to 104 million years ago. The type species is A. americanus.

Archaeoceratops

Archaeoceratops, meaning "ancient horned face", is a genus of basal neoceratopsian dinosaur from the Early Cretaceous (Aptian stage) of north central China. It appears to have been bipedal and quite small (about 1 meter long) with a comparatively large head. Unlike many later ceratopsians it had no horns, possessing only a small bony frill projecting from the back of its head.

Asiaceratops

Asiaceratops (meaning "Asian horned face") is a genus of herbivorous ceratopsian dinosaur. It lived during the Late Cretaceous. Its fossils have mainly been found in Uzbekistan; a referred species is known from China and Mongolia.

Auroraceratops

Auroraceratops, meaning "dawn horned face", is a genus of bipedal basal neoceratopsian dinosaur, from the Early Cretaceous (Aptian age) of north central China and South Korea. The etymology of the generic name refers to its status as an early ceratopsian and also to Dawn Dodson, wife of Peter Dodson, one of the palaeontologists who described it.

Bainoceratops

Bainoceratops (Bain: mountain, keras: horn, ops: face) is a genus of ceratopsian dinosaur from the late Campanian in the Late Cretaceous. This ceratopsian was first described by Tereschenko and Alifanov in 2003. Its fossils were found in southern Mongolia.

Breviceratops

Breviceratops (meaning "short horn face") was a herbivorous ceratopsian dinosaur from the Late Cretaceous of Mongolia.

Cerasinops

Cerasinops (meaning 'cherry face') was a small ceratopsian dinosaur. It lived during the Campanian of the late Cretaceous Period. Its fossils have been found in Two Medicine Formation, in Montana. The type species of the genus Cerasinops is C. hodgskissi.

Cerasinops was named and described by Brenda Chinnery and Jack Horner in 2007 from a specimen (MOR 300) almost 80% complete. Cerasinops belonged to the Ceratopsia (the name is Ancient Greek for 'horned face'), a group of herbivorous dinosaurs with parrot-like beaks that throve in North America and Asia during the Cretaceous Period. Within this group, it has been placed as a basal member of Neoceratopia, although the description is variable; at one point, it is explicitly assigned to Leptoceratopsidae, but in others, it is considered a sister taxon to Leptoceratopsidae, or as a neoceratopsian in general.

Ceratops

Ceratops (meaning "horn face") is a dubious genus of herbivorous ceratopsian dinosaur which lived during the Late Cretaceous. Its fossils have been found in the Judith River Formation in Montana. Although poorly known, Ceratops is important in the history of dinosaurs, since it is the type genus for which both the Ceratopsia and the Ceratopsidae have been named. The material is too poor to be confidently referred to better specimens, and Ceratops is thus considered a nomen dubium.

Chaoyangsauridae

Chaoyangsauridae is a family of ceratopsian dinosaurs. They are among the earliest known marginocephalian dinosaurs, with remains dating to about 160 million years ago, during the Late Jurassic period. Members of this group had sharp beaks for snipping off leaves to eat, and a very small frill.

Four dinosaur genera, Chaoyangsaurus, Xuanhuaceratops, Yinlong and Hualianceratops, are usually considered to belong to the Chaoyangsauridae. All four animals are more primitive (or basal) than both Psittacosaurus and neoceratopsians.

Chaoyangsaurus

Chaoyangsaurus ("Chaoyang lizard") was a marginocephalian dinosaur from the Late Jurassic of China (dated to between 150.8 and 145.5 million years ago). Chaoyangsaurus belonged to the Ceratopsia (Greek for "horned faces"), Chaoyangsaurus, like all ceratopsians, was primarily a herbivore.

Dysganus

Dysganus (dis-GANN-us) (meaning "rough enamel") is a dubious genus of ceratopsian dinosaur from the Campanian stage of the Late Cretaceous. Its fossil teeth were discovered by Edward Drinker Cope in the Judith River Formation in Montana.

Helioceratops

Helioceratops is a genus of neoceratopsian dinosaur from the Middle Cretaceous of China. The type species is H. brachygnathus, described in 2009 by a group of paleontologists led by Jin Liyong. Helioceratops was discovered in the Quantou Formation of China's eastern Jilin province and is known mostly from skull fragments. It reached a length of around 1.3 m (4.3 ft) and may have shared its habitat with Changchunsaurus.

Marginocephalia

Marginocephalia (/mär′jə-nō-sə-făl′ē-ən/ Latin: margin-head) is a clade of ornithischian dinosaurs that is characterized by a bony shelf or margin at the back of the skull. These fringes were likely used for display. There are two clades included in Marginocephalia: the thick-skulled Pachycephalosauria and the horned Ceratopsia. All members of Marginocephalia were primarily herbivores. They basally used gastroliths to aid in digestion of tough plant matter until they convergently evolved tooth batteries in Neoceratopsia (or "new Ceratopsia") and Pachycephalosauria. Marginocephalia first evolved in the Jurassic Period and became more common in the Cretaceous. They are basally small facultative quadrupeds while derived members of the group are large obligate quadrupeds. Primitive marginocephalians are found in Asia, but the group migrated upwards into North America.Pachycephalosaurs, or "thick-headed reptiles", have primitive features that include basally small sized bodies, obligate bipedalism, and simple teeth with one row in operation at a time that are replaced as they are worn down. As they evolved, pachycephalosaurs evolved much thicker and advanced skull roofs including dome forms with horn-like ornamentation. Some research suggests these domes were used like helmets for protection while head-butting members in intraspecific combat. Some research suggests their necks were not strong enough to support such an impact. Flat-headed pachycephalosaur speciments have been found in Asia, and there is great controversy on the meaning of these flat heads. Recent research suggests the flat heads could be a juvenile state before developing the dome shape in the adult stage. It could also be evidence of sexual dimorphism with the female being more flat-headed.Ceratopsians, or "horned-faces", differ from pachycephalosaurs in the presence of a rostral bone, or beak. They are also known for having a jugal horn and a thin parietal-squamosal shelf that extends back and up into a frill. This frill could have been used for anchoring jaw muscles, as well as for display. The horns were likely used for establishing dominance, or defending territories. It is also possible they were a factor in sexual display and species recognition. One of the basalmost members of this group is Psittacosaurus, which is one of the most species-rich dinosaur genera from Asia. Ceratopsians later evolved into very large quadrupeds with elaborate facial horns such as Triceratops, Styracosaurus, and Centrosaurus. There was no change in richness of species throughout the Cretaceous before the Cretaceous-Paleogene extinction.

Micropachycephalosaurus

Micropachycephalosaurus (meaning "small thick-headed lizard") is a monotypic genus of ceratopsian dinosaur. It lived in China during the Late Cretaceous period. The skeleton of the single specimen was found on a cliff southwest of Laiyang, Shandong Province. It was a bipedal and herbivorous dinosaur.

Neornithischia

Neornithischia ("new ornithischians") is a clade of the dinosaur order Ornithischia. They are the sister group of the Thyreophora within the clade Genasauria. Neornithischians are united by having a thicker layer of asymmetrical enamel on the inside of their lower teeth. The teeth wore unevenly with chewing and developed sharp ridges that allowed neornithischians to break down tougher plant food than other dinosaurs. Neornithischians include a variety of basal forms historically known as "hypsilophodonts", including the Parksosauridae; in addition, there are derived forms classified in the groups Marginocephalia and Ornithopoda. The former includes clades Pachycephalosauria and Ceratopsia, while the latter typically includes Hypsilophodon and the more derived Iguanodontia.

Pachycephalosauria

Pachycephalosauria (; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. Genera include Pachycephalosaurus, Stegoceras, and Prenocephale. With the exception of two species, most pachycephalosaurs lived during the Late Cretaceous Period, dating between about 85.8 and 65.5 million years ago. They are exclusive to the Northern Hemisphere, all of them being found in North America and Asia. They were all bipedal, herbivorous/omnivorous animals with thick skulls. Skulls can be domed, flat, or wedge-shaped depending on the species, and are all heavily ossified. The domes were often surrounded by nodes and/or spikes. Partial skeletons have been found of several pachycephalosaur species, but to date no complete skeletons have been discovered. Often isolated skull fragments are the only bones that are found.Candidates for the earliest known pachycephalosaur include Ferganocephale adenticulatum from Middle Jurassic Period strata of Kyrgyzstan and Stenopelix valdensis from Early Cretaceous strata of Germany, although R.M. Sullivan has doubted that either of these species are pachycephalosaurs.[2] In 2017, a phylogenetic analysis conducted by Han and colleagues identified Stenopelix as a member of the Ceratopsia.

Prenoceratops

Prenoceratops, (meaning 'bent or prone-horned face' and derived from Greek prene-/πρηνη- meaning 'bent forwards' or 'prone', cerat-/κερατ- meaning 'horn' and -ops/ωψ meaning 'face') is a genus of ceratopsian dinosaur from the Late Cretaceous Period. Its fossils have been found in the upper Two Medicine Formation in the present-day U.S. state of Montana, in Campanian age rock layers that have been dated to 74.3 million years ago.

Protoceratopsidae

Protoceratopsidae is a family within the group Ceratopsia. The name Protoceratopsidae is derived from Greek for "first horned face". Protoceratopsids have so far been found in the Late Cretaceous, dating to between about 99.6 and 70.6 million years ago. Although Ceratopsians have been found all over the world, protoceratopsids are only known from Asia with most specimens found in China and the Nemegt Basin in Mongolia. As Ceratopsians, protoceratopsids were herbivorous, with constantly replacing tooth batteries made for slicing through plants and a hooked beak for grabbing them. Protoceratopsids were relatively small, between 1-2.5 m in length from head to tail. Their bony frill and horns were much smaller than more derived members of Ceratopsia. Protoceratopsids were likely slow runners and tended to move at a walk or a trot. Their legs may have been straight, creating an upright posture, but there are some theories that they were splayed out to the side, contributing to their slowness. There is evidence that Protoceratops formed groups. Specimens of juveniles and young adults are often found in groups, although adults tend to be solitary. The nature of these groups is not completely known, though herds of young likely formed for protection from predators, and adults are believed to have come together for communal nesting.

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