| Ceratopsians | |
|---|---|
 | |
| Triceratops skeleton,American Museum of Natural History | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Marginocephalia |
| Clade: | †Ceratopsia Marsh, 1890 |
| Type species | |
| †Ceratops montanus Marsh,1888 | |
| Subgroups | |
Ceratopsia orCeratopia (/ˌsɛrəˈtɒpsiə/ or/ˌsɛrəˈtoʊpiə/;Greek: "horned faces") is a group ofherbivorous, beakeddinosaurs that thrived in what are nowNorth America,Asia andEurope, during theCretaceousPeriod, although ancestral forms lived earlier, in theLate Jurassic of Asia. 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 theCretaceous–Paleogene extinction event,66 million years ago.[4]
Triceratops is by far the best-known ceratopsian to the general public. It is traditional for ceratopsiangenus names to end in "-ceratops", although this is not always the case. One of the first named genera wasCeratops itself, which lent its name to the group, although it is considered anomen dubium today as its fossil remains have no distinguishing characteristics that are not also found in other ceratopsians.[5]
Early members of the ceratopsian group, such asPsittacosaurus, were smallbipedal animals. Later members, includingceratopsids likeCentrosaurus andTriceratops, became very largequadrupeds and developed elaborate facialhorns and frills extending over the neck. While these frills might have served to protect the vulnerable neck frompredators, they may also have been used fordisplay,thermoregulation, the attachment of large neck and chewing muscles or some combination of the above. Ceratopsians ranged in size from 1 meter (3.3 feet) and 23 kilograms (51 pounds) to over 9 meters (30 feet) and 9,100 kg (20,100 lb).[citation needed]
Ceratopsians are easily recognized by features of theskull. 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 thepredentary bone on the lower jaw. This ossification evolved to morphologically aid the chewing of plant matter.[6] Along with thepredentary bone, which forms the tip of the lower jaw in allornithischians, the rostral forms a superficiallyparrot-like beak. Also, thejugal 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 theparietal andsquamosal bones of the skull roof, to form the neck frill.[7][8]
The neck frills of ceratopsids are surrounded by theepoccipital bones.[9]: 66 The name is a misnomer, as they are not associated with theoccipital bone.[citation needed] Epoccipitals begin as separate bones thatfuse during the animal's growth to either thesquamosal orparietal bones that make up the base of the frill. These bones were ornamental instead of functional, and may have helped differentiatespecies. Epoccipitals probably were present in all known ceratopsids.[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, likeCentrosaurus,Pachyrhinosaurus, andStyracosaurus, these bones become long and spike- or hook-like.[8] A well-known example is the coarse sawtooth fringe of broad triangular epoccipitals on the frill ofTriceratops. When regarding the ossification's morphogenetic traits, it can be described as dermal. The termepoccipital was coined by paleontologistOthniel Charles Marsh in 1889.[11][12]
The first ceratopsian remains known to science were discovered during the U.S. Geological and Geographical Survey of the Territories led by the AmericangeologistF.V. Hayden. Teeth discovered during an 1855 expedition toMontana were first assigned tohadrosaurids and included within the genusTrachodon. 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 southwesternWyoming. He alertedpaleontologistEdward 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 speciesAgathaumas sylvestris, meaning "marvellous forest-dweller".[14] Soon after, Cope named two more dinosaurs that would eventually come to be recognized as ceratopsids:Polyonax andMonoclonius.Monoclonius was notable for the number of disassociated remains found, including the first evidence of ceratopsid horns and frills. SeveralMonoclonius fossils were found by Cope, assisted byCharles Hazelius Sternberg, in summer 1876 near theJudith River inChouteau 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 andTriceratops. In 1890 Marsh classified them together in the familyCeratopsidae and the order Ceratopsia. This prompted Cope to reexamine his own specimens and to realize thatTriceratops,Monoclonius, andAgathaumas all represented a single group of similar dinosaurs, which he namedAgathaumidae in 1891. Cope redescribedMonoclonius as a horned dinosaur, with a large nasal horn and two smaller horns over the eyes, and a largefrill.
Ceratopsia was coined byOthniel Charles Marsh in 1890 to include dinosaurs possessing certain characteristic features, including horns, arostral bone, teeth with tworoots, fused neckvertebrae, and a forward-orientedpubis. Marsh considered the group distinct enough to warrant its own suborder within Ornithischia.[16] The name is derived from theGreekκέρας/kéras meaning 'horn' andὄψῐς/ópsis meaning 'appearance, view' and by extension 'face'. As early as the 1960s, it was noted that the nameCeratopsia is actually incorrectlinguistically and that it should beCeratopia.[17] However, this spelling, while technically correct, has been used only rarely in the scientific literature, and the vast majority ofpaleontologists continue to use Ceratopsia. As theICZN does not govern taxa above the level ofsuperfamily, this is unlikely to change.
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]
Inclade-basedphylogenetic taxonomy,Ceratopsia is officially defined in thePhyloCode as "the largest clade containingCeratops montanus andTriceratops horridus, but notPachycephalosaurus wyomingensis.[20] Under this definition, the mostbasal known ceratopsians are the familyChaoyangsauridae and the well known genusPsittacosaurus, from the EarlyCretaceous Period, all of which were discovered in northernChina orMongolia. The rostral bone and flared jugals are already present in all of these forms, indicating that even earlier ceratopsians remain to be discovered.[citation needed]
ThecladeNeoceratopsia is defined as "the largest clade containingTriceratops horridus, but notChaoyangsaurus youngi andPsittacosaurus mongoliensis".[20] By this definition, only the members of Chaoyangosauridae andPsittacosaurus are excluded from Neoceratopsia, while all more derived ceratopsians are part of this clade. A slightly less inclusive group isEuceratopsia, named and defined by Daniel Madzia and colleagues in 2021 as "the smallest clade containingLeptoceratops gracilis,Protoceratops andrewsi, andTriceratops horridus".[20] This clade includes the familyLeptoceratopsidae and all more derived ceratopsians. Leptoceratopsids are a mostly North American group of mostly small bodied and quadrupedal ceratopsians. Another subset of neoceratopsians is calledCoronosauria, which is "the smallest clade containingProtoceratops andrewsi andTriceratops horridus".[20] Coronosaurs show the first development of the neck frill and the fusion of the first several neckvertebrae to support the increasingly heavy head. Within Coronosauria, two groups are generally recognized. One group can be calledProtoceratopsidae and includesProtoceratops and its closest relatives, all Asian. The other group,Ceratopsoidea, includes the familyCeratopsidae and closely related animals likeZuniceratops. This clade is defined as "the largest clade containingCeratops montanus andTriceratops horridus, but notProtoceratops andrewsi".[20] Ceratopsidae itself includesTriceratops and all the large North American ceratopsians and is further divided into the subfamiliesCentrosaurinae andChasmosaurinae.[citation needed]
All previously published neoceratopsian phylogenetic analyses were incorporated into the analysis of Eric M. Morschhauser and colleagues in2019, along with all previously published diagnostic species excluding the incomplete juvenileArchaeoceratops yujingziensis and the problematic generaBainoceratops,Lamaceratops,Platyceratops andGobiceratops that are very closely related to and potentially synonymous withBagaceratops. While there were many unresolved areas of the strict consensus, including all of Leptoceratopsidae, a single most parsimonious tree was found that was most consistent with the relative ages of the taxa included, which is shown below.[21]
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 fromembryo to adult are known forPsittacosaurus andProtoceratops, allowing the study ofontogenetic variation in these species.[22][23]
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 asrhinos, which can run at up to 56 km or 35 miles per hour.[24]
Anocturnal lifestyle has been suggested for the primitive ceratopsianProtoceratops.[25] However, comparisons between thescleral rings ofProtoceratops andPsittacosaurus and modern birds and reptiles indicate that they may have beencathemeral, active throughout the day at short intervals.[26]
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 theAlbian stage (112 to 100 million years ago), indicating that the group haddispersed across what is now theBering Strait by the middle of the Cretaceous Period.[27] Almost all leptoceratopsids are North American, aside fromUdanoceratops, which may represent a separate dispersal event, back into Asia. Ceratopsids and their immediate ancestors, such asZuniceratops, were unknown outside of western North America, and were presumedendemic 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 formTuranoceratops, which may it as a ceratopsid. It is unknown whether this would indicates ceratopsids actually originated in Asia, or if theTuranoceratops immigrated from North America.[29]
Possible ceratopsians from the Southern Hemisphere include theAustralianSerendipaceratops, known from anulna, andNotoceratops fromArgentina is known from a single toothless jaw (which has been lost).[30]Craspedodon from theLate Cretaceous (Santonian) ofBelgium may also be a ceratopsian, specifically a neoceratopsian closer to ceratopsoidea than protoceratopsidae.[31] Possible leptoceratopsid remains have also been described from the earlyCampanian ofSweden.[32]
Psittacosaurus andProtoceratops are the most common dinosaurs in the different Mongoliansediments where they are found.[7]Triceratopsfossils 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 dinosaurfauna in some areas.[33] These facts indicate that some ceratopsians were the dominant herbivores in their environments.
Some species of ceratopsians, especiallyCentrosaurus and its relatives, appear to have been gregarious, living inherds. This is suggested bybonebed 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.
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