TRICERATOPS – VIDEO

4 years ago by in Ecology, Ecology, Evolution, Evolution, Featured, Systematics and Taxonomy

Triceratops (2)Triceratops(/traɪˈsɛrətɒps/try-serr-ə-tops) a genus of herbivorous ceratopsid dinosaur that lived during the late Maastrichtian stage of the LateCretaceous Period, around 68 to 65.5 million years ago (Mya) in what is now North America. It was one of the last non-avian dinosaur genera to appear before the Cretaceous–Paleogene extinction event. The term Triceratops, which literally means “three-horned face”, is derived from the Greek τρί- (tri-) meaning “three”, κέρας (kéras) meaning “horn”, and ὤψ (ops) meaning “face”.

Bearing a large bony frill and three horns on its large four-legged body, and conjuring similarities with the modern rhinoceros, Triceratops is one of the most recognizable of all dinosaurs and the best known ceratopsid. It shared the landscape with and was preyed upon by the fearsome Tyrannosaurus, though it is less certain that the two did battle in the manner often depicted in traditional museum displays and popular images.

The exact placement of the Triceratops genus within the ceratopsid group has been debated by paleontologists. Two species, T. horridus and T. prorsus, are considered valid although many other species have been named. Recent research suggests that the contemporaneous Torosaurus, a ceratopsid long regarded as a separate genus, represents Triceratops in its mature form, though this is disputed.

Triceratops has been documented by numerous remains collected since the genus was first described in 1889, including at least one complete individual skeleton. Paleontologist John Scannella observed: “It is hard to walk out into the Hell Creek Formation and not stumble upon a Triceratopsweathering out of a hillside.” Forty-seven complete or partial skulls were discovered in just that area during the decade 2000–2010. Specimens representing life stages from hatchling to adult have been found.

The function of the frills and three distinctive facial horns has long inspired debate. Traditionally these have been viewed as defensive weapons against predators. More recent theories, noting the presence of blood vessels in the skull bones of ceratopsids, find it more probable that these features were primarily used in identification, courtship and dominance displays, much like the antlers and horns of modern reindeer, mountain goats, or rhinoceros beetles. The theory finds additional support if Torosaurus represents the mature form of Triceratops, as this would mean the frill also developed holes (fenestrae) as individuals reached maturity, rendering the structure more useful for display than defense.

DESCRIPTION

Individual Triceratops are estimated to have reached about 7.9 to 9.0 m (26.0–29.5 ft) in length, 2.9 to 3.0 m (9.5–9.8 ft) in height, and 6.1–12.0 tonnes (13,000–26,000 lb) in weight. The most distinctive feature is their large skull, among the largest of all land animals. The largest known skull (specimen BYU 12183) is estimated to have been 2.5 metres (8.2 ft) in length when complete, and could reach almost a third of the length of the entire animal. It bore a single horn on the snout, above the nostrils, and a pair of horns approximately 1 m (3 ft) long, with one above each eye. To the rear of the skull was a relatively short, bony frill, adorned with epoccipitals in some specimens. Most other ceratopsids had large fenestrae in their frills, while those of Triceratops were noticeably solid.

The skin of Triceratops was unusual compared to other dinosaurs. Skin impressions from an as-yet undescribed specimen show that some species may have been covered in bristle-like structures, similar to the more primitive ceratopsian Psittacosaurus.

Limbs

Triceratops species possessed a sturdy build, with strong limbs, short hands with three hooves each, and short feet with four hooves each. Although certainly quadrupedal, the posture of these 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 angles from the thorax, in order to better bear the weight of the head. 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 and slightly bowed out, in an intermediate state between fully upright and fully sprawling (as in the modern rhinoceros).

The hands and forearms of Triceratops retained a fairly primitive structure compared to other quadrupedal dinosaurs such as thyreophorans and manysauropods. 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, walked with most of their fingers pointing out and away from the body, the primitive 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 the third and fourth were vestigial and lacked claws or hooves.

CLASSIFICATION

Triceratops is the best known genus of the Ceratopsidae, a family of large North American horned dinosaurs. The exact location of Triceratops among the ceratopsians has been debated over the years. Confusion stemmed mainly from the combination of short, solid frills (similar to that of Centrosaurinae), and the long brow horns (more akin to Ceratopsinae, also known as Chasmosaurinae). In the first overview of horned dinosaurs, R. S. Lull hypothesized 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. Later revisions supported this view, formally describing the first, short-frilled group as Centrosaurinae (including Triceratops), and the second, long-frilled group as Chasmosaurinae.

In 1949, C. M. Sternberg was the first to question this and favoured instead that Triceratops was more closely related to Arrhinoceratops and Chasmosaurusbased on skull and horn features, making Triceratops a ceratopsine (chasmosaurine of his usage) genus. He was largely ignored, with John Ostrom, and later David Norman both placing Triceratops within Centrosaurinae.

Subsequent discoveries and analyses upheld Sternberg’s view on the position of Triceratops, with Lehman defining both subfamilies in 1990 and diagnosingTriceratops as ceratopsine (chasmosaurine of his usage) on the basis of several morphological features. In fact, it fits well into the ceratopsine subfamily, apart from its one feature of a shortened frill. 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 ceratopsine subfamily.

This classification found additional support in 2010 from John Scannella and Jack Horner at the Museum of the Rockies (Bozeman, Montana). After examining 38 skulls from the Hell Creek formation, Scanella and Horner concluded that the mature form of Triceratops did not even have a shortened frill. The specimens long classified as Triceratops, they argued, represented juvenile and young adult individuals while mature individuals had been incorrectly assigned to a separate genus, Torosaurus. It was already known that the frills of Triceratops grew progressively longer as individuals matured; Scanella and Horner said their findings showed that this growth could culminate in the extended, fenestrated frill associated with Torosaurus. With the name Triceratopstaking historical priority, they announced that references to the genus Torosaurus would be eliminated from Museum of the Rockies exhibits.

triceratopsUse in phylogenetics

In phylogenetic taxonomy, the genus has been used as a reference point in the definition of Dinosauria; Dinosaurs have been designated as all descendants of the most recent common ancestorof Triceratops and Neornithes (i.e. modern birds). Furthermore, the bird-hipped dinosaurs, Ornithischia, have been designated as all dinosaurs with a more recent common ancestor toTriceratops than modern birds.

For many years after its discovery the evolutionary origins of Triceratops remained largely obscure. In 1922 the newly discovered Protoceratops was seen as its ancestor by Henry Fairfield Osborn, but many decades passed before additional findings came to light. Recent years have been fruitful for the discovery of several dinosaurs related to ancestors of TriceratopsZuniceratops, 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. As Triceratops is increasingly shown to be a member of the long-frilled Ceratopsinae subfamily, a likely ancestor may have resembled Chasmosaurus, which thrived some 5 million years earlier.

DISCOVERY AND IDENTIFICATION

The first named specimen now attributed to Triceratops is a pair of brow horns attached to a skull roof, found near Denver, Colorado in the spring of 1887. This specimen was sent to 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. He realized that there were horned dinosaurs by the next year, which saw his publication of the genus Ceratops from fragmentary remains, 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. After reflection, Marsh changed his mind and gave it the generic name Triceratops, accepting his Bison alticornis as another species of Ceratops (it would later be added to Triceratops). 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 Canadian provinces ofSaskatchewan and Alberta.

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.

800px-Tric1SPECIES

Within the first decades after Triceratops was described, various skulls were collected, which varied to a lesser or greater degree from the originalTriceratops, named T. horridus by Marsh (from the Latin horridus; “rough, rugose”, suggesting the roughened texture of those bones belonging to the type specimen, later identified as an aged individual). 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. Discoverers would name these as 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. horridusT. prorsus, and T. brevicornus; the other of T. elatus and T. calicornis. Two species (T. serratus and T. flabellatus) stood apart from these groups. 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. T. horridusT. prorsusT. 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.-elatusT. calicornis was defined by large brow horns and small nasal horn. 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. This pattern was followed until the major studies of the 1980s and 1990s.

With time, the idea that the differing skulls might be representative of individual variation within one (or two) species gained popularity. In 1986, Ostrom and Wellnhofer published a paper in which they proposed that there was only one species, Triceratops horridus. Part of their rationale was that generally there are only one or two species of any large animal in a region (modern examples being the elephant and the giraffe in modern Africa). To their findings, Lehman added the old Lull-Sternberg lineages combined with maturity and sexual dimorphism, suggesting that the T. horridusT. prorsusT. brevicornus lineage was composed of females, the T.calicornisT.elatus lineage was made up of males, and the T. obtususT. hatcheri lineage was of pathologic old males. 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. 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.

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.

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