Discovery and identification[edit]

See also: Timeline of ceratopsian research

Illustration of specimen YPM 1871E, the horn cores that were erroneously attributed to Bison alticornis, the first named specimen of Triceratops

The first named specimen now attributed to Triceratops is a pair of brow horns attached to a skull roof, found by George Lyman Cannon near Denver, Colorado in the spring of 1887.[1] This specimen was sent to American paleontologist Othniel Charles Marsh, who believed that the formation from which it came dated from the Pliocene, and that the bones belonged to a particularly large and unusual bison, which he named Bison alticornis.[1][2] He realized that there were horned dinosaurs by the next year, which saw his publication of the genus Ceratops from fragmentary remains,[3] but he still believed B. alticornis to be a Pliocene mammal. It took a third and much more complete skull to change his mind. The specimen, collected in 1888 by John Bell Hatcher from the Lance Formation of Wyoming, was initially described as another species of Ceratops.[4] Cowboy Edmund B. Wilson had been startled by the sight of a monstrous skull poking out of the side of a ravine. He tried to recover it by throwing a lasso around one of the horns. When it broke off, the skull tumbling to the bottom of the cleft, Wilson brought the horn to his boss, the rancher and avid fossil collector Charles Arthur Guernsey, who happened to show it to Hatcher. Marsh subsequently ordered Hatcher to locate and salvage the skull.[5] This is the holotype YPM 1820. It was first named Ceratops horridus. When further preparation uncovered the third, nose, horn, Marsh changed his mind and gave the piece the generic name Triceratops, "three horn face", accepting his Bison alticornis as another species of Ceratops[6] (it would later be added to Triceratops[7]). The sturdy nature of the animal's skull has ensured that many examples have been preserved as fossils, allowing variations between species and individuals to be studied. Triceratops remains have subsequently been found in the American states of Montana and South Dakota (in addition to Colorado and Wyoming), and in the provinces of Saskatchewan and Alberta, Canada.

An earlier specimen, also recovered from the Lance Formation, was named Agathaumas sylvestris by Edward Drinker Cope in 1872. Originally identified as a hadrosaur, this specimen consists only of post-cranial remains and is only provisionally considered an example of Triceratops.[8]

Species[edit]

Type specimen YPM 1820 of the type species, T. horridus

After Triceratops was described, between 1889 and 1891 Hatcher with great effort collected another thirty-one of its skulls. The first species had been named Triceratops horridus by Marsh. Its specific name was derived from the Latin horridus; "rough, rugose", perhaps suggesting the roughened texture of those bones belonging to the type specimen, later identified as an aged individual. The additional skulls varied to a lesser or greater degree from the original Triceratops specimen. This variation is unsurprising, given that Triceratops skulls are large three-dimensional objects, coming from individuals of different ages and both sexes, and which were subjected to different amounts and directions of pressure during fossilization.[5]

However, not a single one of these skulls was referred to T. horridus by Marsh who instead named eight further species and eventually even a new genus Sterrholophus. In 1889, he named two species. Triceratops flabellatus, the "fan-shaped", was based on skull YPM 1821. Triceratops galeus, "the helmeted one", was exceptionally based on a specimen not found by Hatcher, USNM 2410, a horn and frill excavated by George Homans Eldridge in Colorado in the Laramie Formation. In December 1889, Marsh published the first illustration ever of a Triceratops skull, that of T. flabellatus. In January 1890, two additional species were added. Triceratops serratus, "the serrated one", was based on skull YPM 1823, and Triceratops prorsus, "pointing forward", on specimen YPM 1822, a nose horn. In May 1890, Triceratops sulcatus was added, named "the one with troughs" because of grooves on the horns, based on the fragmentary skull USNM 4276. On this occasion, Marsh reported a natural brain cast, that he considered remarkably small. In February 1891, Marsh published additional images, for the first time concluding that the animal walked on all fours: initially he had assumed it was a bipedal form, brought to the brink of extinction by its overly heavy skull. A full skeletal restoration followed in April, together with one of Brontosaurus in the same publication, the first for Mesozoic dinosaurs ever drawn. The same year he renamed T. flabellatus into the separate genus Sterrholophus, the "stiffly crested". In September, he named Triceratops elatus, "the raised one", based on specimen YPM 1201, an enormous skull with an upwards pointing low nose horn found on 23 October 1890 by Hatcher. Marsh named his last Triceratops species in 1898, when part of his collection was transferred from Yale to the Smithsonian Institution. Triceratops calicornis, "the chalice horned", was based on specimen USNM 4928, a skull with a strange depression on the rear of the horn base. Triceratops obtusus, the "stumpy one", was based on specimen USNM 4720 with an obtuse nose horn.

After Marsh's death, Hatcher attempted to revise the material but fell ill writing the study, never to recover. Richard Swann Lull completed his monograph. Lull and subsequent researchers would disagree as to the number of separate species (listed below), and came up with several phylogenetic schemes for how they were related to each other.

In the first attempt to understand the many species, Lull found two groups, although he did not say how he distinguished them: one composed of T. horridus, T. prorsus, and T. brevicornus; the other of T. elatus and T. calicornis. Two species (T. serratus and T. flabellatus) stood apart from these groups.[7] By 1933, and his revision of the landmark 1907 Hatcher-Marsh-Lull monograph of all known ceratopsians, he retained his two groups and two unaffiliated species, with a third lineage of T. obtusus and T. hatcheri that was characterized by a very small nasal horn.[9] T. horridus-T. prorsus-T. brevicornus was now thought to be the most conservative lineage, with an increase in skull size and a decrease in nasal horn size, and T. elatus-T. calicornis was defined by large brow horns and small nasal horn.[9][10] C. M. Sternberg made one modification, adding T. eurycephalus and suggesting that it linked the second and third lineages closer together than they were to the T. horridus lineage.[11] This pattern was followed until the major studies of the 1980s and 1990s.

1896 skeletal restoration of T. prorsus by O. C. Marsh, based on the holotype skull YPM 1822 and referred elements

With time, the idea that the differing skulls might be representative of individual variation within one (or two) species gained popularity. In 1986, John Ostrom and Peter Wellnhofer published a paper in which they proposed that there was only one species, Triceratops horridus.[12] Part of their rationale was that generally there are only one or two species of any large animal in a region. To their findings, Lehman added the old Lull-Sternberg lineages combined with maturity and sexual dimorphism, suggesting that the T. horridus-T. prorsus-T. brevicornus lineage was composed of females, the T. calicornis-T. elatus lineage was made up of males, and the T. obtusus-T. hatcheri lineage was of pathologic old males.[13] His reasoning was that males had taller, more erect horns and larger skulls, and females had smaller skulls with shorter, forward-facing horns.

These findings were contested a few years later by Catherine Forster, who reanalyzed Triceratops material more comprehensively and concluded that the remains fell into two species, T. horridus and T. prorsus, although the distinctive skull of T. ("Nedoceratops") hatcheri differed enough to warrant a separate genus.[14] She found that T. horridus and several other species belonged together, and T. prorsus and T. brevicornus stood alone, and since there were many more specimens in the first group, she suggested that this meant the two groups were two species. It is still possible to interpret the differences as representing a single species with sexual dimorphism.[5][15]

In 2009, John Scannella and Denver Fowler supported the separation of T. prorsus and T. horridus, and noted that the two species are also separated stratigraphically within the Hell Creek Formation, indicating that they did not live together at the same time.[16]

Valid species[edit]

T. prorsus, Carnegie Museum of Natural History

The skull (AMNH 5116) of this T. horridus composite specimen was formerly assigned to T. elatus

T. horridus (Marsh, 1889) Marsh, 1889 (originally Ceratops) (type species)

T. prorsus Marsh, 1890

Synonyms and doubtful species[edit]

Some of the following species are synonyms, as indicated in parentheses ("=T. horridus" or "=T. prorsus"). All the others are considered nomina dubia ("dubious names") because they are based on remains too poor or incomplete to be distinguished from pre-existing Triceratops species.

T. albertensis C. M. Sternberg, 1949

T. alticornis (Marsh 1887) Hatcher, Marsh, and Lull, 1907 [originally Bison alticornis, Marsh 1887, and Ceratops alticornis, Marsh 1888]

T. brevicornus Hatcher, 1905 (=T. prorsus)

T. calicornis Marsh, 1898 (=T. horridus)

T. elatus Marsh, 1891 (=T. horridus)

T. eurycephalus Schlaikjer, 1935

T. flabellatus Marsh, 1889 (= Sterrholophus Marsh, 1891) (=T. horridus)

T. galeus Marsh, 1889

T. hatcheri (Hatcher & Lull 1905) Lull, 1933 (contentious; see Nedoceratops below)

T. ingens Marsh vide Lull, 1915

T. maximus Brown, 1933

T. mortuarius (Cope, 1874) Kuhn, 1936 (nomen dubium; originally Polyonax mortuarius)

T. obtusus Marsh, 1898 (=T. horridus)

T. serratus Marsh, 1890 (=T. horridus)

T. sulcatus Marsh, 1890

T. sylvestris (Cope, 1872) Olshevsky, 2000 (nomen dubium; originally Agathaumas sylvestris)

Description[edit]

Size[edit]

Size comparison with T. horridus in blue and T. prorsus in red

Individual Triceratops are estimated to have reached about 7.9 to 9 metres (26 to 30 ft) in length, 2.9 to 3.0 m (9.5 to 9.8 ft) in height,[17][18] and 6.1 to 12.0 tonnes (13,400 to 26,500 lb) in weight.[19] The most distinctive feature is their large skull, among the largest of all land animals. The largest-known skull (specimen MWC 7584, formerly BYU 12183) is estimated to have been 2.5 m (8.2 ft) in length when complete,[20] and could reach almost a third of the length of the entire animal.[21] A specimen of T. horridus named Kelsey measured 7.3 m (24 ft) long with a 1.98 m (6.5 ft) skull, stood about 2.3 m (7.5 ft) tall, and was estimated by the Black Hills institute to weight nearly 6 t (5.9 long tons; 6.6 short tons).[22] A Triceratops 8 m (26.2 ft) long has been estimated by Gregory S. Paul to have massed 9 t (8.9 long tons; 9.9 short tons).[23]

Skull[edit]

Skull of Triceratops horridus from the Houston Museum of Natural Science

Like all chasmosaurines, Triceratops had a large skull relative to its body size. The front of the head was equipped with a large beak in front of the teeth. The core of the top beak was formed by a special rostral bone.[24] Behind it, the premaxillae bones were located, embayed from behind by very large circular nostrils. In chasmosaurines, the premaxillae met on their midline in a complex bone plate, the rear edge of which was reinforced by the "narial strut". From the base of this strut, a triangular process jutted out into the nostril. Triceratops differs from most relatives in that this process was hollowed out on the outer side.[24] Behind the toothless premaxilla, the maxilla bore thirty-six to forty tooth positions, in which three to five teeth per position were vertically stacked. The teeth were closely appressed, forming a "dental battery" curving to the inside. The skull bore a single horn on the snout, above the nostrils. In Triceratops, the nose horn is sometimes recognisable as a separate ossification, the epinasal.[24]

The skull also featured a pair of "brow" or supraorbital horns approximately 1 m (3.3 ft) long, with one above each eye.[25][26] The jugal bones pointed to below at the rear sides of the skull and were capped by separate epijugals. With Triceratops these were not particularly large and sometimes touched the quadratojugals. The bones of the skull roof were fused. By a folding of the frontal bones, a "double" skull roof was created.[24] In Triceratops, some specimens show a fontanelle, an opening in the upper roof layer.[24] The cavity between the layers invaded the bone cores of the brow horns.[24]

At the rear of the skull, the outer squamosal bones and the inner parietal bones grew into a relatively short, bony frill, adorned with epoccipitals in young specimens. These were low triangular processes on the frill edge, representing separate skin ossifications or osteoderms. Typically, with Triceratops specimens there are two epoccipitals present on each parietal bone, with an additional central process on their border. Each squamosal bone had five processes. Most other ceratopsids had large parietal fenestrae, openings, in their frills, but those of Triceratops were noticeably solid,[27] unless the genus Torosaurus represents mature Triceratops individuals. Under the frill, at the rear of the skull, a huge occipital condyle, up to 106 millimetres in diameter, connected the head to the neck.[24]

The lower jaws were elongated and met at their tips in a shared epidentary bone, the core of the toothless lower beak. In the dentary bone, the tooth battery curved to the outside to meet the battery of the upper jaw. At the rear of the lower jaw, the articular bone was exceptionally wide, matching the general width of the jaw joint.[24] T. horridus can be distinguished from T. prorsus by having a shallower snout.[23]

Postcranial skeleton[edit]

Life reconstruction of a subadult Triceratops horridus

Chasmosaurines showed little variation in their postcranial skeleton.[24] The skeleton of Triceratops is markedly robust. Both Triceratops species possessed a sturdy build, with strong limbs, short hands with three hooves each, and short feet with four hooves each.[28] The vertebral column consisted of ten neck, twelve back, ten sacral and about forty-five tail vertebrae. The front neck vertebrae were fused into a syncervical. Traditionally, this was assumed to have incorporated the first three vertebrae which would imply that the frontmost atlas was very large and sported a neural spine. Later interpretations revived an old hypothesis by John Bell Hatcher that at the very front a vestige of the real atlas can be observed, the syncervical then consisting of four vertebrae. The vertebral count mentioned, is adjusted to this view. In Triceratops the neural spines of the neck are constant in height, instead of gradually sloping upwards. Another peculiarity is that the neck ribs only begin to lengthen with the ninth cervical vertebra.[24]

The rather short and high vertebrae of the back were in its middle region reinforced by ossified tendons running along the tops of the neural arches. The straight sacrum was long and adult individuals show a fusion of all sacral vertebrae. In Triceratops the first four and last two sacrals had transverse processes, connecting the vertebral column to the pelvis, that were fused at their distal ends. Sacrals seven and eight had longer processes, causing the sacrum to have an oval profile in top view. On top of the sacrum a neural plate was present formed by a fusion of the neural spines of the second through fifth vertebrae. Triceratops had a large pelvis with a long ilium. The ischium was curved downwards. The foot was short with four functional toes. The phalangeal formula of the foot is 2-3-4-5-0.[24]

Closeup of pelvis of Triceratops specimen at the Oxford University Museum of Natural History

Although certainly quadrupedal, the posture of horned dinosaurs has long been the subject of some debate. Originally, it was believed that the front legs of the animal had to be sprawling at a considerable angle from the thorax in order to better bear the weight of the head.[5] This stance can be seen in paintings by Charles Knight and Rudolph Zallinger. Ichnological evidence in the form of trackways from horned dinosaurs and recent reconstructions of skeletons (both physical and digital) seem to show that Triceratops and other ceratopsids maintained an upright stance during normal locomotion, with the elbows flexed to behind and slightly bowed out, in an intermediate state between fully upright and fully sprawling, comparable to the modern rhinoceros.[28][29][30][31]

The hands and forearms of Triceratops retained a fairly primitive structure compared to other quadrupedal dinosaurs such as thyreophorans and many sauropods. In those two groups, the forelimbs of quadrupedal species were usually rotated so that the hands faced forward with palms backward ("pronated") as the animals walked. Triceratops, like other ceratopsians and the related quadrupedal ornithopods, together forming the Cerapoda, walked with most of their fingers pointing out and away from the body, the original condition for dinosaurs, also retained by bipedal forms like the theropods. In Triceratops, the weight of the body was carried by only the first three fingers of the hand, while digits 4 and 5 were vestigial and lacked claws or hooves.[28] The phalangeal formula of the hand is 2-3-4-3-1, meaning that the first or innermost finger of the forelimb has two bones, the next has three, etc.[32]

Classification[edit]

Specimen nicknamed "Lane", the most complete specimen known

Triceratops is the best-known genus of the Ceratopsidae, a family of large, mostly North American horned dinosaurs. The exact location of Triceratops among the ceratopsids has been debated over the years. Confusion stemmed mainly from the combination of a short, solid frill (similar to that of Centrosaurinae), with long brow horns (more akin to Chasmosaurinae).[33] In the first overview of horned dinosaurs, R. S. Lull hypothesized the existence of two lineages, one of Monoclonius and Centrosaurus leading to Triceratops, the other with Ceratops and Torosaurus, making Triceratops a centrosaurine as the group is understood today.[7] Later revisions supported this view, Lawrence Lambe in 1915 formally describing the first, short-frilled group as the Centrosaurinae (including Triceratops), and the second, long-frilled group as the Chasmosaurinae.[9][34]

In 1949, Charles M. Sternberg was the first to question this position, proposing instead that Triceratops was more closely related to Arrhinoceratops and Chasmosaurus based on skull and horn features, making Triceratops a chasmosaurine ("ceratopsine" in his usage) genus.[11] He was largely ignored, with John Ostrom,[35] and later David Norman placing Triceratops within the Centrosaurinae.[36]

First mounted T. horridus skeleton (the holotype of T. "obtusus"), nicknamed "Hatcher", Smithsonian Museum

Yoshi's Trike, a juvenile specimen with 115 cm horn cores, on display in the Museum of the Rockies in Montana, USA

Subsequent discoveries and analyses, however, proved the correctness of Sternberg's view on the position of Triceratops, with Thomas Lehman defining both subfamilies in 1990 and diagnosing Triceratops as "ceratopsine" on the basis of several morphological features. Apart from the one feature of a shortened frill, Triceratops shares no derived traits with centrosaurines.[13] Further research by Peter Dodson, including a 1990 cladistic analysis and a 1993 study using RFTRA (resistant-fit theta-rho analysis), a morphometric technique which systematically measures similarities in skull shape, reinforces Triceratops' placement in the chasmosaurines.[37][38]

The cladogram below follows Longrich (2014), who named a new species of Pentaceratops, and included nearly all species of chasmosaurine.[39]

Chasmosaurinae

 

Mercuriceratops

 

 

 

Judiceratops

 

 

 

 

Chasmosaurus

 

 

Mojoceratops

 

 

 

 

Agujaceratops

 

 

 

 

Pentaceratops aquilonius

 

 

Williams Fork chasmosaur

 

 

 

Pentaceratops sternbergii

 

 

Utahceratops

 

 

 

 

 

Kosmoceratops

 

 

 

 

Anchiceratops

 

 

Almond Formation chasmosaur

 

 

 

 

 

Bravoceratops

 

 

Coahuilaceratops

 

 

 

 

Arrhinoceratops

 

Triceratopsini

 

Titanoceratops

 

 

 

Torosaurus

 

 

Triceratops

 

 

 

 

 

 

 

 

 

 

 

 

Skull of specimen DMNH 48617 from the Laramie Formation of eastern Colorado. Based on the age of the formation, it may be the oldest Triceratops known.

For many years after its discovery, the deeper evolutionary origins of Triceratops and its close relatives remained largely obscure. In 1922, the newly discovered Protoceratops was seen as its ancestor by Henry Fairfield Osborn,[5] but many decades passed before additional findings came to light. Recent years have been fruitful for the discovery of several antecedents of Triceratops. Zuniceratops, the earliest-known ceratopsian with brow horns, was described in the late 1990s, and Yinlong, the first known Jurassic ceratopsian, in 2005.

These new finds have been vital in illustrating the origins of horned dinosaurs in general, suggesting an Asian origin in the Jurassic, and the appearance of truly horned ceratopsians by the beginning of the late Cretaceous in North America.[24]

In phylogenetic taxonomy, the genus Triceratops has been used as a reference point in the definition of Dinosauria; dinosaurs have been designated as all descendants of the most recent common ancestor of Triceratops and Neornithes (i.e. modern birds).[40] Furthermore, the bird-hipped dinosaurs, Ornithischia, have been defined as those dinosaurs more closely related to Triceratops than to modern birds.[41]

Paleobiology[edit]

A Triceratops mounted next to a Tyrannosaurus at the Los Angeles Natural History Museum

Although Triceratops are commonly portrayed as herding animals, there is currently little evidence that they lived in herds. While several other genera of horned dinosaurs are known from bonebeds preserving bones from two to hundreds or thousands of individuals, to date there is only one documented bonebed dominated by Triceratops bones: a site in southeastern Montana with the remains of three juveniles. It may be significant that only juveniles were present.[42] In 2012, a group of three Triceratops in relatively complete condition, each of varying sizes from a full-grown adult to a small juvenile, were found in Wyoming, near Newcastle. The remains are currently under excavation by paleontologist Peter Larson and a team from the Black Hills Institute. It is believed that the animals were traveling as a family unit, but it remains unknown if the group consists of a mated pair and their offspring, or two females and a juvenile they were caring for. The remains also show signs of predation or scavenging from Tyrannosaurus, particularly on the largest specimen, with the bones of the front limbs showing breakage and puncture wounds from Tyrannosaurus teeth.[43] New evidence presented in 2020 by Illies and Fowler et al., suggests that Triceratops lived in small groups of perhaps between five to ten individuals based on fossils collected in the previous decade.[44]

For many years, Triceratops finds were known only from solitary individuals.[42] These remains are very common; for example, Bruce Erickson, a paleontologist of the Science Museum of Minnesota, has reported having seen 200 specimens of T. prorsus in the Hell Creek Formation of Montana.[45] Similarly, Barnum Brown claimed to have seen over 500 skulls in the field.[5]:79 Because Triceratops teeth, horn fragments, frill fragments, and other skull fragments are such abundant fossils in the Lancian faunal stage of the late Maastrichtian (late Cretaceous, 66 mya) Period of western North America, it is regarded as among the dominant herbivores of the time, if not the most dominant herbivore. In 1986, Robert Bakker estimated it as making up five sixths of the large dinosaur fauna at the end of the Cretaceous.[46] Unlike most animals, skull fossils are far more common than postcranial bones for Triceratops, suggesting that the skull had an unusually high preservation potential.[47]

Dentition and diet[edit]

Close up of the jaws and teeth

Triceratops were herbivorous, and because of their low head, their primary food was probably low growth, although they may have been able to knock down taller plants with their horns, beak, and bulk.[24][48] The jaws were tipped with a deep, narrow beak, believed to have been better at grasping and plucking than biting.[35]

Triceratops teeth were arranged in groups called batteries, of 36 to 40 tooth columns in each side of each jaw, with 3 to 5 stacked teeth per column, depending on the size of the animal.[24] This gives a range of 432 to 800 teeth, of which only a fraction were in use at any given time (tooth replacement was continuous throughout the life of the animal).[24] They functioned by shearing in a vertical to near-vertical orientation.[24] The great size and numerous teeth of Triceratops suggests that they ate large volumes of fibrous plant material, with some researchers suggesting palms and cycads,[49][50] and others suggesting ferns, which then grew in prairies.[51]

Functions of the horns and frill[edit]

Front view of skull with a prominent epoccipital fringe, Houston Museum of Natural Science

There has been much speculation over the functions of Triceratops' head adornments. The two main theories have revolved around use in combat and in courtship display, with the latter now thought to be the most likely primary function.[24]

Early on, Lull postulated that the frills may have served as anchor points for the jaw muscles to aid chewing by allowing increased size and thus power for the muscles.[52] This has been put forward by other authors over the years, but later studies do not find evidence of large muscle attachments on the frill bones.[53]

Triceratops were long thought to have used their horns and frills in combat with predators such as Tyrannosaurus, the idea being discussed first by Charles H. Sternberg in 1917 and 70 years later by Robert Bakker.[46][54] There is evidence that Tyrannosaurus did have aggressive head-on encounters with Triceratops, based on partially healed tyrannosaur tooth marks on a Triceratops brow horn and squamosal; the bitten horn is also broken, with new bone growth after the break. Which animal was the aggressor is not known.[55] Since the Triceratops wounds healed, it is most likely that the Triceratops survived the encounter. Tyrannosaurus is also known to have fed on Triceratops, as shown by a heavily tooth-scored Triceratops ilium and sacrum.[56]

Examples of periosteal reactive bone in selected specimens of Triceratops

In addition to combat with predators using horns, Triceratops are popularly shown engaging each other in combat with horns locked. While studies show that such activity would be feasible, if unlike that of present-day horned animals,[57] there is disagreement about whether they did so. Although pitting, holes, lesions, and other damage on Triceratops skulls (and the skulls of other ceratopsids) are often attributed to horn damage in combat, a 2006 study finds no evidence for horn thrust injuries causing these forms of damage (for example, there is no evidence of infection or healing). Instead, non-pathological bone resorption, or unknown bone diseases, are suggested as causes.[58] A newer study compared incidence rates of skull lesions and periosteal reaction in Triceratops and Centrosaurus and showed that these were consistent with Triceratops using its horns in combat and the frill being adapted as a protective structure, while lower pathology rates in Centrosaurus may indicate visual rather than physical use of cranial ornamentation, or a form of combat focused on the body rather than the head.[59] The frequency of injury was found to be 14% in Triceratops.[60] The researchers also concluded that the damage found on the specimens in the study was often too localized to be caused by bone disease.[61] Histological examination reveals that the frill of Triceratops is composed of fibrolamellar bone[62] which contains fibroblasts that play a critical role in wound healing, and are capable of rapidly depositing bone during remodeling.[63][64]

Juvenile and adult skulls—the juvenile skull is about the size of an adult human head

One skull was found with a hole in the jugal bone, apparently a puncture wound sustained while the animal was alive, as indicated by signs of healing. The hole has a diameter close to that of the distal end of a Triceratops horn. This, and other apparent healed wounds in the skulls of ceratopsians, has been cited as evidence of non-fatal intraspecific competition in these dinosaurs.[65][66]

The large frill also may have helped to increase body area to regulate body temperature.[67] A similar theory has been proposed regarding the plates of Stegosaurus,[68] although this use alone would not account for the bizarre and extravagant variation seen in different members of the Ceratopsidae, which would rather support the sexual display theory.[24]

The theory that frills functioned as a sexual display was first proposed by Davitashvili in 1961 and has gained increasing acceptance since.[13][53][69] Evidence that visual display was important, either in courtship or other social behavior, can be seen in the horned dinosaurs differing markedly in their adornments, making each species highly distinctive. Also, modern living creatures with such displays of horns and adornments use them similarly.[70] A 2006 study of the smallest Triceratops skull, ascertained to be a juvenile, shows the frill and horns developed at a very early age, predating sexual development and thus probably important for visual communication and species recognition in general.[71] The use of the exaggerated structures to enable dinosaurs to recognize their own species has been questioned, as no such function exists for such structures in modern species.[72]

Growth and ontogeny[edit]

Skull growth series

In 2006, the first extensive ontogenetic study of Triceratops was published in the journal Proceedings of the Royal Society. The study, by John R. Horner and Mark Goodwin, found that individuals of Triceratops could be divided into four general ontogenetic groups, babies, juveniles, subadults, and adults. With a total number of 28 skulls studied, the youngest was only 38 cm (15 in) long. Ten of the 28 skulls could be placed in order in a growth series with one representing each age. Each of the four growth stages were found to have identifying features. Multiple ontogenetic trends were discovered, including the size reduction of the epoccipitals, development and reorientation of postorbital horns, and hollowing out of the horns.[73]

Torosaurus as growth stage of Triceratops[edit]

Main article: Torosaurus

Torosaurus is a ceratopsid genus first identified from a pair of skulls in 1891, two years after the identification of Triceratops. The genus Torosaurus resembles Triceratops in geological age, distribution, anatomy and size and it has been recognised as a close relative.[74] Its distinguishing features are an elongated skull and the presence of two fenestrae, or holes, in the frill. Paleontologists investigating dinosaur ontogeny (growth and development of individuals over the life span) in the Hell Creek Formation, Montana, US, have recently presented evidence that the two represent a single genus.

A, Triceratops prorsus holotype YPM 1822 and B, Torosaurus latus ANSP 15192

John Scannella, in a paper presented in Bristol, UK at the conference of the Society of Vertebrate Paleontology (25 September 2009) reclassified Torosaurus as especially mature Triceratops individuals, perhaps representing a single sex. Jack Horner, Scannella's mentor at Bozeman Campus, Montana State University, noted that ceratopsian skulls consist of metaplastic bone. A characteristic of metaplastic bone is that it lengthens and shortens over time, extending and resorbing to form new shapes. Significant variety is seen even in those skulls already identified as Triceratops, Horner said, "where the horn orientation is backwards in juveniles and forward in adults". Approximately 50% of all subadult Triceratops skulls have two thin areas in the frill that correspond with the placement of "holes" in Torosaurus skulls, suggesting that holes developed to offset the weight that would otherwise have been added as maturing Triceratops individuals grew longer frills.[75] A paper describing these findings in detail was published in July 2010 by Scannella and Horner. It formally argues that Torosaurus and the similar contemporary Nedoceratops are synonymous with Triceratops.[20]

The assertion ignited debate. Andrew Farke had in 2006 stressed that, apart from the frill, no systematic differences could be found between Torosaurus and Triceratops.[74] He nevertheless disputed Scannella's conclusion by arguing in 2011 that the proposed morphological changes required to "age" a Triceratops into a Torosaurus would be without precedent among ceratopsids. Such changes would include the growth of additional epoccipitals, reversion of bone texture from an adult to immature type and back to adult again, and growth of frill holes at a later stage than usual.[76] A study by Nicholas Longrich and Daniel Field analyzed 35 specimens of both Triceratops and Torosaurus. The authors concluded that Triceratops individuals too old to be considered immature forms are represented in the fossil record, as are Torosaurus individuals too young to be considered fully mature adults. The synonymy of Triceratops and Torosaurus cannot be supported, they said, without more convincing intermediate forms than Scannella and Horner initially produced. Scannella's Triceratops specimen with a hole on its frill, they argued, could represent a diseased or malformed individual rather than a transitional stage between an immature Triceratops and mature Torosaurus form.[77][78]

Other genera as growth stages of Triceratops[edit]

Main article: Nedoceratops

Comparisons between the skulls of Triceratops and Nedoceratops

Opinion has varied on the validity of a separate genus for Nedoceratops. John Scannella and Jack Horner regarded it as an intermediate growth stage between Triceratops and Torosaurus.[20][79] Andrew Farke, in his 2011 redescription of the only known skull, concluded that it was an aged individual of its own valid taxon, Nedoceratops hatcheri.[76] Nicholas Longrich and Daniel Fields also did not consider it a transition between Torosaurus and Triceratops, suggesting that the frill holes were pathological.[78]

As described above, John Scannella had argued in 2010 that Nedoceratops should be considered a synonym of Triceratops.[20] Andrew Farke (2011) maintained that it represents a valid distinct genus.[76] Nick Longrich agreed with Scannella about Nedoceratops and made a further suggestion: that the recently described Ojoceratops was likewise a synonym. The fossils, he argued, are indistinguishable from the T. horridus specimens that were previously attributed to the defunct species T. serratus.

Longrich observed that another newly described genus, Tatankaceratops, displayed a strange mix of characteristics already found in adult and juvenile Triceratops. Rather than representing a distinct genus, Tatankaceratops could as easily represent a dwarf Triceratops or a Triceratops individual with a developmental disorder that caused it to stop growing prematurely.[80]

Paleoecology[edit]

Pie chart of the time averaged census for large-bodied dinosaurs from the entire Hell Creek Formation in the study area

Triceratops lived during the Late Cretaceous of North America, its fossils coming from the Evanston Formation, Scollard Formation, Laramie Formation, Lance Formation, Denver Formation, and Hell Creek Formation.[81] These fossil formations date back to the time of the Cretaceous–Paleogene extinction event, which has been dated to 66 ± 0.07 million years ago.[82] Many animals and plants have been found in these formations, but mostly from the Lance Formation and Hell Creek Formation.[81] Triceratops was one of the last ceratopsian genera to appear before the end of the Mesozoic. The related Torosaurus, and the more distantly related diminutive Leptoceratops, were also present, though their remains have been rarely encountered.[5]

Theropods from these formations include genera of tyrannosaurids, ornithomimids, troodontids,[81] avialans,[83] caenagnathids,[84] and dromaeosaurids. Acheroraptor and Dakotaraptor are dromaeosaurids from the Hell Creek Formation. Indeterminate dromaeosaurs are known from other fossil formations. Common teeth previously referred to Dromaeosaurus and Saurornitholestes later were considered to be Acheroraptor.[85] The tyrannosaurids from the formation are Nanotyrannus and Tyrannosaurus, although the former might be a junior synonym of the latter. Among ornithomimids are the genera Struthiomimus as well as Ornithomimus;[81] an undescribed animal named "Orcomimus" could be from the formation.[86] Troodontids are only represented by Pectinodon and Paronychodon in the Hell Creek Formation; with a possible species of Troodon from the Lance Formation. One species of coelurosaur is known from Hell Creek and similar formations by a single species, Richardoestesia. Only three oviraptorosaurs are from the Hell Creek Formation, Anzu, Leptorhynchos[84] and a giant species of caenagnathid, very similar to Gigantoraptor, from South Dakota. However, only fossilized foot prints were discovered.[87] The avialans known from the formation are Avisaurus,[81] multiple species of Brodavis,[88] and several other species of hesperornithoforms, as well as several species of true birds including Cimolopteryx.[83]

Triceratops and other animals of the Hell Creek Formation

Ornithischians are abundant in the Scollard Lance, Laramie, Lance, Denver, and Hell Creek Formation. The main groups of ornithischians are ankylosaurians, ornithopods, ceratopsians, and pachycephalosaurians. Three ankylosaurians are known, Ankylosaurus, Denversaurus, and possibly a species of Edmontonia or an undescribed genus. Multiple genera of ceratopsians are known from the formation other than Triceratops, the leptoceratopsid Leptoceratops, and the chasmosaurine ceratopsids Torosaurus,[81] Nedoceratops and Tatankaceratops.[89] Ornithopods are common in the Hell Creek Formation, and are known from several species of the ornithopod Thescelosaurus and the hadrosaurids Edmontosaurus,[81][90] and a possible species of Parasaurolophus. Several pachycephalosaurians have been found in the Hell Creek Formation and in similar formations. Among them are the derived pachycephalosaurids Stygimoloch,[81] Dracorex,[91] Pachycephalosaurus,[81] Sphaerotholus, and an undescribed specimen from North Dakota. The first two might be junior synonyms of Pachycephalosaurus.

Mammals are plentiful in the Hell Creek Formation. Groups represented include multituberculates, metatherians, and eutherians. The multituberculates represented include Paracimexomys,[92] the cimolomyids Paressonodon,[93] Meniscoessus, Essonodon, Cimolomys, Cimolodon, and Cimexomys; and the neoplagiaulacids Mesodma, and Neoplagiaulax. The alphadontids Alphadon, Protalphodon, and Turgidodon, pediomyids Pediomys,[92] Protolambda, and Leptalestes,[94] the stagodontid Didelphodon,[92] the deltatheridiid Nanocuris, the herpetotheriid Nortedelphys,[93] and the glasbiid Glasbius all represent metatherians of the Hell Creek Formation. A few eutherians are known, being represented by Alostera,[92] Protungulatum,[94] the cimolestids Cimolestes and Batodon, the gypsonictopsid Gypsonictops, and the possible nyctitheriid Paranyctoides.[92]...

"Dinosaurs are a diverse group of reptiles[note 1] of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago, although the exact origin and timing of the evolution of dinosaurs is the subject of active research. They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event 201.3 million years ago; their dominance continued through the Jurassic and Cretaceous periods. The fossil record demonstrates that birds are modern feathered dinosaurs, having evolved from earlier theropods during the Late Jurassic epoch. As such, birds were the only dinosaur lineage to survive the Cretaceous–Paleogene extinction event approximately 66 million years ago. Dinosaurs can therefore be divided into avian dinosaurs, or birds; and non-avian dinosaurs, which are all dinosaurs other than birds.

Etymology

The taxon 'Dinosauria' was formally named in 1842 by paleontologist Sir Richard Owen, who used it to refer to the "distinct tribe or sub-order of Saurian Reptiles" that were then being recognized in England and around the world.[1][2] The term is derived from Ancient Greek δεινός (deinos), meaning 'terrible, potent or fearfully great', and σαῦρος (sauros), meaning 'lizard or reptile'.[1][3] Though the taxonomic name has often been interpreted as a reference to dinosaurs' teeth, claws, and other fearsome characteristics, Owen intended it merely to evoke their size and majesty.[4]

Other prehistoric animals, including pterosaurs, mosasaurs, ichthyosaurs, plesiosaurs, and Dimetrodon, while often popularly conceived of as dinosaurs, are not taxonomically classified as dinosaurs.[5] Pterosaurs are distantly related to dinosaurs, being members of the clade Ornithodira. The other groups mentioned are, like dinosaurs and pterosaurs, members of Sauropsida (the reptile and bird clade), except Dimetrodon (which is a synapsid).

Definition

Triceratops skeleton, Natural History Museum of Los Angeles County

Under phylogenetic nomenclature, dinosaurs are usually defined as the group consisting of the most recent common ancestor (MRCA) of Triceratops and modern birds (Neornithes), and all its descendants.[6] It has also been suggested that Dinosauria be defined with respect to the MRCA of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria.[7] Both definitions result in the same set of animals being defined as dinosaurs: "Dinosauria = Ornithischia + Saurischia", encompassing ankylosaurians (armored herbivorous quadrupeds), stegosaurians (plated herbivorous quadrupeds), ceratopsians (herbivorous quadrupeds with horns and frills), ornithopods (bipedal or quadrupedal herbivores including "duck-bills"), theropods (mostly bipedal carnivores and birds), and sauropodomorphs (mostly large herbivorous quadrupeds with long necks and tails).[8]

Birds are now recognized as being the sole surviving lineage of theropod dinosaurs. In traditional taxonomy, birds were considered a separate class that had evolved from dinosaurs, a distinct superorder. However, a majority of contemporary paleontologists concerned with dinosaurs reject the traditional style of classification in favor of phylogenetic taxonomy; this approach requires that, for a group to be natural, all descendants of members of the group must be included in the group as well. Birds are thus considered to be dinosaurs and dinosaurs are, therefore, not extinct.[9] Birds are classified as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs.[10]

Research by Matthew G. Baron, David B. Norman, and Paul M. Barrett in 2017 suggested a radical revision of dinosaurian systematics. Phylogenetic analysis by Baron et al. recovered the Ornithischia as being closer to the Theropoda than the Sauropodomorpha, as opposed to the traditional union of theropods with sauropodomorphs. They resurrected the clade Ornithoscelida to refer to the group containing Ornithischia and Theropoda. Dinosauria itself was re-defined as the last common ancestor of Triceratops horridus, Passer domesticus and Diplodocus carnegii, and all of its descendants, to ensure that sauropods and kin remain included as dinosaurs.[11][12]

General description

Using one of the above definitions, dinosaurs can be generally described as archosaurs with hind limbs held erect beneath the body.[13] Many prehistoric animal groups are popularly conceived of as dinosaurs, such as ichthyosaurs, mosasaurs, plesiosaurs, pterosaurs, and pelycosaurs (especially Dimetrodon), but are not classified scientifically as dinosaurs, and none had the erect hind limb posture characteristic of true dinosaurs.[14] Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic Era, especially the Jurassic and Cretaceous periods. Other groups of animals were restricted in size and niches; mammals, for example, rarely exceeded the size of a domestic cat, and were generally rodent-sized carnivores of small prey.[15]

Dinosaurs have always been an extremely varied group of animals; according to a 2006 study, over 500 non-avian dinosaur genera have been identified with certainty so far, and the total number of genera preserved in the fossil record has been estimated at around 1850, nearly 75% of which remain to be discovered.[16] An earlier study predicted that about 3,400 dinosaur genera existed, including many that would not have been preserved in the fossil record.[17] By September 17, 2008, 1,047 different species of dinosaurs had been named.[18]

In 2016, the estimated number of dinosaur species that existed in the Mesozoic was estimated to be 1,543–2,468.[19][20] Some are herbivorous, others carnivorous, including seed-eaters, fish-eaters, insectivores, and omnivores. While dinosaurs were ancestrally bipedal (as are all modern birds), some prehistoric species were quadrupeds, and others, such as Anchisaurus and Iguanodon, could walk just as easily on two or four legs. Cranial modifications like horns and crests are common dinosaurian traits, and some extinct species had bony armor. Although known for large size, many Mesozoic dinosaurs were human-sized or smaller, and modern birds are generally small in size. Dinosaurs today inhabit every continent, and fossils show that they had achieved global distribution by at least the Early Jurassic epoch.[21] Modern birds inhabit most available habitats, from terrestrial to marine, and there is evidence that some non-avian dinosaurs (such as Microraptor) could fly or at least glide, and others, such as spinosaurids, had semiaquatic habits.[22]

Distinguishing anatomical features

While recent discoveries have made it more difficult to present a universally agreed-upon list of dinosaurs' distinguishing features, nearly all dinosaurs discovered so far share certain modifications to the ancestral archosaurian skeleton, or are clear descendants of older dinosaurs showing these modifications. Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical for Dinosauria; the earliest dinosaurs had them and passed them on to their descendants. Such modifications, originating in the most recent common ancestor of a certain taxonomic group, are called the synapomorphies of such a group.[23]

A detailed assessment of archosaur interrelations by Sterling Nesbitt[24] confirmed or found the following twelve unambiguous synapomorphies, some previously known:

in the skull, a supratemporal fossa (excavation) is present in front of the supratemporal fenestra, the main opening in the rear skull roof

epipophyses, obliquely backward-pointing processes on the rear top corners, present in the anterior (front) neck vertebrae behind the atlas and axis, the first two neck vertebrae

apex of deltopectoral crest (a projection on which the deltopectoral muscles attach) located at or more than 30% down the length of the humerus (upper arm bone)

radius, a lower arm bone, shorter than 80% of humerus length

fourth trochanter (projection where the caudofemoralis muscle attaches on the inner rear shaft) on the femur (thigh bone) is a sharp flange

fourth trochanter asymmetrical, with distal, lower, margin forming a steeper angle to the shaft

on the astragalus and calcaneum, upper ankle bones, the proximal articular facet, the top connecting surface, for the fibula occupies less than 30% of the transverse width of the element

exoccipitals (bones at the back of the skull) do not meet along the midline on the floor of the endocranial cavity, the inner space of the braincase

in the pelvis, the proximal articular surfaces of the ischium with the ilium and the pubis are separated by a large concave surface (on the upper side of the ischium a part of the open hip joint is located between the contacts with the pubic bone and the ilium)

cnemial crest on the tibia (protruding part of the top surface of the shinbone) arcs anterolaterally (curves to the front and the outer side)

distinct proximodistally oriented (vertical) ridge present on the posterior face of the distal end of the tibia (the rear surface of the lower end of the shinbone)

concave articular surface for the fibula of the calcaneum (the top surface of the calcaneum, where it touches the fibula, has a hollow profile)

Nesbitt found a number of further potential synapomorphies and discounted a number of synapomorphies previously suggested. Some of these are also present in silesaurids, which Nesbitt recovered as a sister group to Dinosauria, including a large anterior trochanter, metatarsals II and IV of subequal length, reduced contact between ischium and pubis, the presence of a cnemial crest on the tibia and of an ascending process on the astragalus, and many others.[6]

Diagram of a typical diapsid skull

j: jugal bone, po: postorbital bone, p: parietal bone, sq: squamosal bone, q: quadrate bone, qj: quadratojugal bone

A variety of other skeletal features are shared by dinosaurs. However, because they are either common to other groups of archosaurs or were not present in all early dinosaurs, these features are not considered to be synapomorphies. For example, as diapsids, dinosaurs ancestrally had two pairs of Infratemporal fenestrae (openings in the skull behind the eyes), and as members of the diapsid group Archosauria, had additional openings in the snout and lower jaw.[25] Additionally, several characteristics once thought to be synapomorphies are now known to have appeared before dinosaurs, or were absent in the earliest dinosaurs and independently evolved by different dinosaur groups. These include an elongated scapula, or shoulder blade; a sacrum composed of three or more fused vertebrae (three are found in some other archosaurs, but only two are found in Herrerasaurus);[6] and a perforate acetabulum, or hip socket, with a hole at the center of its inside surface (closed in Saturnalia tupiniquim, for example).[26][27] Another difficulty of determining distinctly dinosaurian features is that early dinosaurs and other archosaurs from the Late Triassic epoch are often poorly known and were similar in many ways; these animals have sometimes been misidentified in the literature.[28]

Hip joints and hindlimb postures of: (left to right) typical reptiles (sprawling), dinosaurs and mammals (erect), and rauisuchians (pillar-erect)

Dinosaurs stand with their hind limbs erect in a manner similar to most modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side.[29] This posture is due to the development of a laterally facing recess in the pelvis (usually an open socket) and a corresponding inwardly facing distinct head on the femur.[30] Their erect posture enabled early dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of "sprawling" reptiles.[31] Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs.[32] Some non-dinosaurian archosaurs, including rauisuchians, also had erect limbs but achieved this by a "pillar-erect" configuration of the hip joint, where instead of having a projection from the femur insert on a socket on the hip, the upper pelvic bone was rotated to form an overhanging shelf.[32]

Evolutionary history

Main article: Evolution of dinosaurs

Origins and early evolution

Dinosaurs diverged from their archosaur ancestors during the Middle to Late Triassic epochs, roughly 20 million years after the devastating Permian–Triassic extinction event wiped out an estimated 96% of all marine species and 70% of terrestrial vertebrate species approximately 252 million years ago.[33][34] Radiometric dating of the rock formation that contained fossils from the early dinosaur genus Eoraptor at 231.4 million years old establishes its presence in the fossil record at this time.[35] Paleontologists think that Eoraptor resembles the common ancestor of all dinosaurs;[36] if this is true, its traits suggest that the first dinosaurs were small, bipedal predators.[37] The discovery of primitive, dinosaur-like ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators. Dinosaurs may have appeared as early as 243 million years ago, as evidenced by remains of the genus Nyasasaurus from that period, though known fossils of these animals are too fragmentary to tell if they are dinosaurs or very close dinosaurian relatives.[38] Paleontologist Max C. Langer et al. (2018) determined that Staurikosaurus from the Santa Maria Formation dates to 233.23 million years ago, making it older in geologic age than Eoraptor.[39]

When dinosaurs appeared, they were not the dominant terrestrial animals. The terrestrial habitats were occupied by various types of archosauromorphs and therapsids, like cynodonts and rhynchosaurs. Their main competitors were the pseudosuchia, such as aetosaurs, ornithosuchids and rauisuchians, which were more successful than the dinosaurs.[40] Most of these other animals became extinct in the Triassic, in one of two events. First, at about 215 million years ago, a variety of basal archosauromorphs, including the protorosaurs, became extinct. This was followed by the Triassic–Jurassic extinction event (about 201 million years ago), that saw the end of most of the other groups of early archosaurs, like aetosaurs, ornithosuchids, phytosaurs, and rauisuchians. Rhynchosaurs and dicynodonts survived (at least in some areas) at least as late as early-mid Norian and late Norian or earliest Rhaetian stages, respectively,[41][42] and the exact date of their extinction is uncertain. These losses left behind a land fauna of crocodylomorphs, dinosaurs, mammals, pterosaurians, and turtles.[6] The first few lines of early dinosaurs diversified through the Carnian and Norian stages of the Triassic, possibly by occupying the niches of the groups that became extinct.[8] Also notably, there was a heightened rate of extinction during the Carnian Pluvial Event.[43]

Evolution and paleobiogeography

Dinosaur evolution after the Triassic follows changes in vegetation and the location of continents. In the Late Triassic and Early Jurassic, the continents were connected as the single landmass Pangaea, and there was a worldwide dinosaur fauna mostly composed of coelophysoid carnivores and early sauropodomorph herbivores.[44] Gymnosperm plants (particularly conifers), a potential food source, radiated in the Late Triassic. Early sauropodomorphs did not have sophisticated mechanisms for processing food in the mouth, and so must have employed other means of breaking down food farther along the digestive tract.[45] The general homogeneity of dinosaurian faunas continued into the Middle and Late Jurassic, where most localities had predators consisting of ceratosaurians, spinosauroids, and carnosaurians, and herbivores consisting of stegosaurian ornithischians and large sauropods. Examples of this include the Morrison Formation of North America and Tendaguru Beds of Tanzania. Dinosaurs in China show some differences, with specialized sinraptorid theropods and unusual, long-necked sauropods like Mamenchisaurus.[44] Ankylosaurians and ornithopods were also becoming more common, but prosauropods had become extinct. Conifers and pteridophytes were the most common plants. Sauropods, like the earlier prosauropods, were not oral processors, but ornithischians were evolving various means of dealing with food in the mouth, including potential cheek-like organs to keep food in the mouth, and jaw motions to grind food.[45] Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from maniraptoran coelurosaurians.[10]

Skeleton of Marasuchus lilloensis, a dinosaur-like ornithodiran

By the Early Cretaceous and the ongoing breakup of Pangaea, dinosaurs were becoming strongly differentiated by landmass. The earliest part of this time saw the spread of ankylosaurians, iguanodontians, and brachiosaurids through Europe, North America, and northern Africa. These were later supplemented or replaced in Africa by large spinosaurid and carcharodontosaurid theropods, and rebbachisaurid and titanosaurian sauropods, also found in South America. In Asia, maniraptoran coelurosaurians like dromaeosaurids, troodontids, and oviraptorosaurians became the common theropods, and ankylosaurids and early ceratopsians like Psittacosaurus became important herbivores. Meanwhile, Australia was home to a fauna of basal ankylosaurians, hypsilophodonts, and iguanodontians.[44] The stegosaurians appear to have gone extinct at some point in the late Early Cretaceous or early Late Cretaceous. A major change in the Early Cretaceous, which would be amplified in the Late Cretaceous, was the evolution of flowering plants. At the same time, several groups of dinosaurian herbivores evolved more sophisticated ways to orally process food. Ceratopsians developed a method of slicing with teeth stacked on each other in batteries, and iguanodontians refined a method of grinding with dental batteries, taken to its extreme in hadrosaurids.[45] Some sauropods also evolved tooth batteries, best exemplified by the rebbachisaurid Nigersaurus.[46]

Full skeleton of an early carnivorous dinosaur, displayed in a glass case in a museum

The early forms Herrerasaurus (large), Eoraptor (small) and a Plateosaurus skull

There were three general dinosaur faunas in the Late Cretaceous. In the northern continents of North America and Asia, the major theropods were tyrannosaurids and various types of smaller maniraptoran theropods, with a predominantly ornithischian herbivore assemblage of hadrosaurids, ceratopsians, ankylosaurids, and pachycephalosaurians. In the southern continents that had made up the now-splitting Gondwana, abelisaurids were the common theropods, and titanosaurian sauropods the common herbivores. Finally, in Europe, dromaeosaurids, rhabdodontid iguanodontians, nodosaurid ankylosaurians, and titanosaurian sauropods were prevalent.[44] Flowering plants were greatly radiating,[45] with the first grasses appearing by the end of the Cretaceous.[47] Grinding hadrosaurids and shearing ceratopsians became extremely diverse across North America and Asia. Theropods were also radiating as herbivores or omnivores, with therizinosaurians and ornithomimosaurians becoming common.[45]

The Cretaceous–Paleogene extinction event, which occurred approximately 66 million years ago at the end of the Cretaceous, caused the extinction of all dinosaur groups except for the neornithine birds. Some other diapsid groups, such as crocodilians, sebecosuchians, turtles, lizards, snakes, sphenodontians, and choristoderans, also survived the event.[48]

The surviving lineages of neornithine birds, including the ancestors of modern ratites, ducks and chickens, and a variety of waterbirds, diversified rapidly at the beginning of the Paleogene period, entering ecological niches left vacant by the extinction of Mesozoic dinosaur groups such as the arboreal enantiornithines, aquatic hesperornithines, and even the larger terrestrial theropods (in the form of Gastornis, eogruiids, bathornithids, ratites, geranoidids, mihirungs, and "terror birds"). It is often cited that mammals out-competed the neornithines for dominance of most terrestrial niches but many of these groups co-existed with rich mammalian faunas for most of the Cenozoic Era.[49] Terror birds and bathornithids occupied carnivorous guilds alongside predatory mammals,[50][51] and ratites are still fairly successful as mid-sized herbivores; eogruiids similarly lasted from the Eocene to Pliocene, only becoming extinct very recently after over 20 million years of co-existence with many mammal groups.[52]

Classification

Main article: Dinosaur classification

Dinosaurs belong to a group known as archosaurs, which also includes modern crocodilians. Within the archosaur group, dinosaurs are differentiated most noticeably by their gait. Dinosaur legs extend directly beneath the body, whereas the legs of lizards and crocodilians sprawl out to either side.[23]

Collectively, dinosaurs as a clade are divided into two primary branches, Saurischia and Ornithischia. Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia, while Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia. Anatomically, these two groups can be distinguished most noticeably by their pelvic structure. Early saurischians—"lizard-hipped", from the Greek sauros (σαῦρος) meaning "lizard" and ischion (ἰσχίον) meaning "hip joint"—retained the hip structure of their ancestors, with a pubis bone directed cranially, or forward.[30] This basic form was modified by rotating the pubis backward to varying degrees in several groups (Herrerasaurus,[53] therizinosauroids,[54] dromaeosaurids,[55] and birds[10]). Saurischia includes the theropods (exclusively bipedal and with a wide variety of diets) and sauropodomorphs (long-necked herbivores which include advanced, quadrupedal groups).[22][56]

By contrast, ornithischians—"bird-hipped", from the Greek ornitheios (ὀρνίθειος) meaning "of a bird" and ischion (ἰσχίον) meaning "hip joint"—had a pelvis that superficially resembled a bird's pelvis: the pubic bone was oriented caudally (rear-pointing). Unlike birds, the ornithischian pubis also usually had an additional forward-pointing process. Ornithischia includes a variety of species that were primarily herbivores. (NB: the terms "lizard hip" and "bird hip" are misnomers – birds evolved from dinosaurs with "lizard hips".)...

"A jigsaw puzzle is a tiling puzzle that requires the assembly of often oddly shaped interlocking and mosaiced pieces. Typically, each individual piece has a portion of a picture; when assembled, the jigsaw puzzle produces a complete picture.

History[edit]

John Spilsbury's "Europe divided into its kingdoms, etc." (1766). He created the jigsaw puzzle for educational purposes, and called them “Dissected Maps”.[2][3]

The engraver and cartographer John Spilsbury, of London, is believed to have produced the first jigsaw puzzle around 1760, using a marquetry saw.[1] Early jigsaws, known as dissections, were produced by mounting maps on sheets of hardwood and cutting along national boundaries, creating a puzzle useful for the teaching of geography.[1] Such "dissected maps" were used to teach the children of King George III and Queen Charlotte by royal governess Lady Charlotte Finch.[4][5]

The name "jigsaw" came to be associated with the puzzle around 1880 when fretsaws became the tool of choice for cutting the shapes. Since fretsaws are distinct from jigsaws, the name appears to be a misnomer.[1] Cardboard jigsaw puzzles appeared during the late 1800s, but were slow to replace the wooden jigsaw due to the manufacturer's belief that cardboard puzzles would be perceived as being of low quality, and the fact that profit margins on wooden jigsaws were larger.[1]

Wooden jigsaw pieces, cut by hand

Jigsaw puzzles soared in popularity during the Great Depression, as they provided a cheap, long-lasting, recyclable form of entertainment.[1][6] It was around this time that jigsaws evolved to become more complex and more appealing to adults.[1] They were also given away in product promotions, and used in advertising, with customers completing an image of the product being promoted.[1][6]

Sales of wooden jigsaw puzzles fell after World War II as improved wages led to price increases, while at the same time improvements in manufacturing processes made paperboard jigsaws more attractive.[6]

According to the Alzheimer Society of Canada, doing jigsaw puzzles is one of many activities that can help keep the brain active and may contribute to reducing the risk of developing Alzheimer's disease.[7]

Modern construction[edit]

Paperboard jigsaw pieces

Most modern jigsaw puzzles are made out of paperboard since they are easier and cheaper to mass-produce than the original wooden models. An enlarged photograph or printed reproduction of a painting or other two-dimensional artwork is glued onto the cardboard before cutting. This board is then fed into a press. The press forces a set of hardened steel blades of the desired shape through the board until it is fully cut. This procedure is similar to making shaped cookies with a cookie cutter. The forces involved, however, are tremendously greater and a typical 1000-piece puzzle requires a press that can generate upwards of 700 tons of force to push the knives of the puzzle die through the board. A puzzle die is a flat board, often made from plywood, which has slots cut or burned in the same shape as the knives that are used. These knives are set into the slots and covered in a compressible material, typically foam rubber, which serves to eject the cut puzzle pieces.

Beginning in the 1930s, jigsaw puzzles were cut using large hydraulic presses which now cost in the hundreds of thousands of dollars. The cuts gave a very snug fit, but the cost limited jigsaw puzzle manufacture only to large corporations. Recent roller press design achieve the same effect, at a lower cost.[citation needed]

New technology has enabled laser-cutting of wooden or acrylic jigsaw puzzles. The advantage of cutting with a laser is that the puzzle can be custom cut into any size, any shape, with any size (or any number) of pieces. Many museums have laser cut acrylic puzzles made of some of their more important pieces of art so that children visiting the museum can see the original piece and then assemble a jigsaw puzzle of the image that is also in the same shape as the piece of art. Acrylic is used because the pieces are very durable, waterproof, and can withstand continued use without the image fading, or the pieces wearing out, or becoming frayed. Also, because the print and cut patterns are computer-based, lost pieces can be manufactured without remaking the entire puzzle.

By the early 1960s, Tower Press was the world's largest maker of jigsaw puzzles, acquired by Waddingtons in 1969.[10] Major jigsaw puzzle manufacturers currently include Ravensburger and Tower Press. Wooden and specialty jigsaw puzzle manufacturers include Artifact Puzzles. In addition to large-scale puzzle manufacturers, numerous puzzle makers work in an artisanal style, handcrafting and handcutting jigsaw puzzles.[11][12][13][14]

Variations[edit]

Jigsaw puzzle software allowing rotation of pieces

A three-dimensional puzzle composed of several two-dimensional puzzles stacked on top of one another

A puzzle without a picture

Jigsaw puzzles come in a variety of sizes. Among those targeted to adults, 300, 500, and 750 piece puzzles are considered "smaller". More sophisticated, but still common, jigsaw puzzles come in sizes of 1,000, 1,500, 2,000, 3,000, 4,000, 5,000, 6,000, 7,500, 8,000, 9,000, 13,200, 18,000, 24,000, 32,000 and 40,000 pieces.

Jigsaw puzzles that are geared towards children may have many fewer pieces, typically much larger. For very young children, a puzzle with as few as 4 to 9 "large"-size pieces (so not a choking hazard) are common. These are usually made of wood or plastic, to maintain durability, and are able to be cleaned without being damaged.

The most common layout for a thousand-piece puzzle is 38 pieces by 27 pieces, for a total count of 1,026 pieces. The majority of 500-piece puzzles are 27 pieces by 19 pieces. A few puzzles are made double-sided, so that they can be solved from either side. This adds a level of complexity, because it cannot be certain that the correct side of the piece is being viewed and assembled with the other pieces.

"Family puzzles" come in 100–550 pieces with three different-sized pieces from large to small. The pieces are placed from large to small, going in one direction or towards the middle of the puzzle. This allows a family of puzzlers of different skill levels and different-sized hands to work on the puzzle at the same time. Companies like Springbok, Cobble Hill, Ravensburger, and Suns Out make this type of specialty puzzle.

There are also three-dimensional jigsaw puzzles. Many of these are made of wood or styrofoam and require the puzzle to be solved in a certain order; some pieces will not fit in if others are already in place. Also common are puzzle boxes: simple three-dimensional jigsaw puzzles with a small drawer or box in the center for storage.

Another type of jigsaw puzzle, which is considered a 3-D puzzle, is a puzzle globe. Like a 2-D puzzle, a globe puzzle is often made of plastic and the assembled pieces form a single layer. But the final form is a three-dimensional shape. Most globe puzzles have designs representing spherical shapes such as the Earth, the Moon, and historical globes of the Earth.

There are also computer versions of jigsaw puzzles, which have the advantages of requiring zero cleanup and no risk of losing any pieces. Many computer-based jigsaw puzzles do not allow pieces to be rotated, so all pieces are displayed in their correct orientation. These puzzles are thus considerably easier than a physical jigsaw puzzle with the same number of pieces. A computer puzzle website can allow users to choose their own puzzle size, cut design, and image, or upload their own images to use as puzzles.[15] An online jigsaw version of Trolleholm Castle in Sweden may be worked and timed for speed of finishing.[16] The New Yorker Magazine subscription website preserves images of the magazine's cover illustrations as jigsaw puzzles, which are timed and offer several levels of difficulty.

In 2016 was introduced a computer version of puzzle globe, the immersive panorama jigsaw, which is based on the use of equirectangular images taken by 360-degree camera. Despite the physical spherical jigsaw, the player, who resides in the perfect center of the globe, assembles triangular-shaped interlocking pieces around him. When complete, this puzzle produces a full-degree panorama all around the player. An example of immersive jigsaw is Sitespot, which also enriches the gaming experience with the scene soundscape and allows pieces to be displayed rotated.

Jigsaw puzzles can vary greatly in price depending on the complexity, number of pieces, and brand. Children's puzzles can be as cheap as around $5.00, while larger puzzles can be closer to $50.00. The most expensive puzzle to date was sold for $27,000 in 2005 at a charitable auction for The Golden Retriever Foundation.[17]

Several word puzzle games use pieces similar to those used in jigsaw puzzles. Examples include Alfa-Lek, Jigsaw Words, Nab-It!, Puzzlage, Typ-Dom, Word Jigsaw, and Yottsugo.[18][citation needed]

Puzzle pieces[edit]

A "whimsy" piece in a wooden jigsaw puzzle

A 3D jigsaw puzzle

Many puzzles are termed "fully interlocking". This means that adjacent pieces are connected in such a way that if one piece is moved horizontally, the other pieces move with it, preserving the connection. Sometimes the connection is tight enough to pick up a solved part by holding one piece.

Some fully interlocking puzzles have pieces all of a similar shape, with rounded tabs out on opposite ends, with corresponding blanks cut into the intervening sides to receive the tabs of adjacent pieces. Other fully interlocking puzzles may have tabs and blanks variously arranged on each piece, but they usually have four sides, and the numbers of tabs and blanks thus add up to four. The uniform-shaped fully interlocking puzzles, sometimes called "Japanese Style", are the most difficult, because the differences in shapes between pieces can be very subtle.[citation needed]

Most jigsaw puzzles are square, rectangular, or round, with edge pieces that have one side that is either straight or smoothly curved to create this shape, plus four corner pieces if the puzzle is square or rectangular. Some jigsaw puzzles have edge pieces that are cut just like all the rest of the interlocking pieces, with no smooth edge, to make them more challenging. Other puzzles are designed so the shape of the whole puzzle forms a figure, such as an animal. The edge pieces may vary more in these cases.

The pieces of spherical jigsaw, like immersive panorama jigsaw, can be triangular shaped, according to the rules of tessellation of the geoid primitive.

The designer Yuu Asaka created “Jigsaw Puzzle 29” which has not four corner pieces but five corner pieces, and is made from pale blue acrylic without a picture. [19] It was awarded the Jury Honorable Mention of 2018 Puzzle Design Competition. [20] But many puzzlers had solved it easily, he created “Jigsaw Puzzle 19” which composed only with corner pieces as revenge. [21] It was made with transparent green acrylic pieces without a picture....

"Random House is an American book publisher and the largest general-interest paperback publisher in the world.[1][2][3] It is part of Penguin Random House, which is owned by German media conglomerate Bertelsmann....