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20.12.2015 18:26 - Largest prehistoric animals Vol.1 Vertebrates part3 Dinosaurs ch.1 Theropods-Tyrannosaurids,the family of the famous Tyrannosaurus rex
Автор: valentint Категория: Забавление   
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Последна промяна: 05.12.2016 23:00

Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian theropod dinosaurs which comprises two subfamilies containing up to eleven genera, including the eponymous Tyrannosaurus. The exact number of genera is controversial, with some experts recognizing as few as three. All of these animals lived near the end of the Cretaceous Period and their fossils have been found only in North America and Asia.
Although descended from smaller ancestors, tyrannosaurids were almost always the largest predators in their respective ecosystems, putting them at the apex of the food chain. The largest species was Tyrannosaurus rex, one of the largest known land predators, which measured up to 12.3 metres (40 ft) in length and up to 6,500 kilograms (7.2 short tons) in weight.Tyrannosaurids were bipedal carnivores with massive skulls filled with large teeth. Despite their large size, their legs were long and proportioned for fast movement. In contrast, their arms were very small, bearing only two functional digits.
Unlike most other groups of dinosaurs, very complete remains have been discovered for most known tyrannosaurids. This has allowed a variety of research into their biology. Scientific studies have focused on their ontogeny, biomechanics and ecology, among other subjects. Soft tissue, both fossilized and intact, has been reported from one specimen of Tyrannosaurus rex.
The tyrannosaurids were all large animals, with all species capable of weighing at least 1 metric ton.A single specimen of Alioramus of an individual estimated at between 5 and 6 metres(16 and 20 ft) long has been discovered,although it is considered by some experts to be a juvenile.Albertosaurus, Gorgosaurus and Daspletosaurus all measured between 8 and 10 metres (26 and 33 ft) long,while Tarbosaurus reached lengths of 12 metres (39 ft) from snout to tail. The massive Tyrannosaurus reached 12.3 metres (40 ft) in the largest specimen.
Tyrannosaurid skull anatomy is well understood as complete skulls are known for all genera but Alioramus, which is known only from partial skull remains.Tyrannosaurus, Tarbosaurus, and Daspletosaurus had skulls which exceeded 1 metre (3 ft 3 in) in length.The largest discovered Tyrannosaurus skull measures over 1.5 metres (4 ft 11 in) long.Adult tyrannosaurids had tall, massive skulls, with many bones fused and reinforced for strength. At the same time, hollow chambers within many skull bones and large openings (fenestrae) between those bones helped to reduce skull weight. Many features of tyrannosaurid skulls were also found in their immediate ancestors, including tall premaxillae and fused nasal bones.
Tyrannosaurid skulls had many unique characteristics, including fused parietal bones with a prominent sagittal crest, which ran longitudinally along the sagittal suture and separated the two supratemporal fenestrae on the skull roof. Behind these fenestrae, tyrannosaurids had a characteristically tall nuchal crest, which also arose from the parietals but ran along a transverse plane rather than longitudinally. The nuchal crest was especially well-developed in Tyrannosaurus, Tarbosaurus and Alioramus. Albertosaurus, Daspletosaurus and Gorgosaurus had tall crests in front of the eyes on the lacrimal bones, while Tarbosaurus and Tyrannosaurus had extremely thickened postorbital bones forming crescent-shaped crests behind the eyes. Alioramus had a row of six bony crests on top of its snout, arising from the nasal bones; lower crests have been reported on some specimens of Daspletosaurus and Tarbosaurus, as well as the more basal tyrannosauroid Appalachiosaurus.
The skull was perched at the end of a thick, S-shaped neck, and a long, heavy tail acted as a counterweight to balance out the head and torso, with the center of mass over the hips. Tyrannosaurids are known for their proportionately very small two-fingered forelimbs, although remnants of a vestigial third digit are sometimes found.Tarbosaurus had the shortest forelimbs compared to its body size, while Daspletosaurus had the longest.
Tyrannosaurids walked exclusively on their hindlimbs, so their leg bones were massive. In contrast to the forelimbs, the hindlimbs were longer compared to body size than almost any other theropods. Juveniles and even some smaller adults, like more basal tyrannosauroids, had longer tibiae than femora, a characteristic of fast-running dinosaurs like ornithomimids. Larger adults had leg proportions characteristic of slower-moving animals, but not to the extent seen in other large theropods like abelisaurids or carnosaurs. The third metatarsals of tyrannosaurids were pinched between the second and fourth metatarsals, forming a structure known as the arctometatarsus.
It is unclear when the arctometatarsus first evolved; it was not present in the earliest tyrannosauroids like Dilong,but was found in the later Appalachiosaurus.This structure also characterized troodontids, ornithomimids and caenagnathids,but its absence in the earliest tyrannosauroids indicates that it was acquired by convergent evolution.
Tyrannosaurids, like their tyrannosauroid ancestors, were heterodont, with premaxillary teeth D-shaped in cross section and smaller than the rest. Unlike earlier tyrannosauroids and most other theropods, the maxillary and mandibular teeth of mature tyrannosaurids are not blade-like but extremely thickened and often circular in cross-section.Tooth counts tend to be consistent within species, and larger species tend to have lower tooth counts than smaller ones. For example, Alioramus had 76 to 78 teeth in its jaws, while Tyrannosaurus had between 54 and 60.

William Abler observed in 2001 that Albertosaurus tooth serrations resemble a crack in the tooth ending in a round void called an ampulla.Tyrannosaurid teeth were used as holdfasts for pulling meat off a body, so when a tyrannosaur would have pulled back on a piece of meat, the tension could cause a purely crack-like serration to spread through the tooth.However, the presence of the ampulla would have distributed these forces over a larger surface area, and lessened the risk of damage to the tooth under strain.The presence of incisions ending in voids has parallels in human engineering. Guitar makers use incisions ending in voids to, as Abler describes, "impart alternating regions of flexibility and rigidity" to the wood they work with.The use of a drill to create an "ampulla" of sorts and prevent the propagation of cracks through material is also used to protect airplane surfaces.Abler demonstrated that a plexiglass bar with incisions called "kerfs" and drilled holes was more than 25% stronger than one with only regularly placed incisions.Unlike tyrannosaurs, ancient predators like phytosaurs and Dimetrodon had no adaptations to prevent the crack-like serrations of their teeth from spreading when subjected to the forces of feeding.
While earlier tyrannosauroids are found on all three northern continents, tyrannosaurid fossils are known only from North America and Asia. Sometimes fragmentary remains uncovered in the Southern Hemisphere have been reported as "Southern Hemisphere tyrannosaurids," although these seem to have been misidentified abelisaurid fossils.The exact time and place of origin of the family remain unknown due to the poor fossil record in the middle part of the Cretaceous on both continents, although the earliest confirmed tyrannosaurids lived in the early Campanian stage in western North America.
Tyrannosaurid remains have never been recovered from eastern North America, while more basal tyrannosauroids, like Dryptosaurus and Appalachiosaurus, persisted there until the end of the Cretaceous, indicating that tyrannosaurids must have evolved in or dispersed into western North America after the continent was divided in half by the Western Interior Seaway in the middle of the Cretaceous.Tyrannosaurid fossils have been found in Alaska, which may have provided a route for dispersal between North America and Asia.Alioramus and Tarbosaurus are found to be related in one cladistic analysis, forming a unique Asian branch of the family.This was later disproven with the discovery of Qianzhousaurus and the description of the tyrannosaur family Alioramini. Tyrannosaurid teeth from a large species of unknown variety were discovered in the Nagasaki Peninsula by researchers from the Fukui Prefectural Dinosaur Museum, further expanding the range of the group. The teeth were estimated to be 81 million years old (Campanian Age).
Of the two subfamilies, tyrannosaurines appear to have been more widespread. Albertosaurines are unknown in Asia, which was home to the tyrannosaurines such as Tarbosaurus and Zhuchengtyrannus, and Qianzhousaurus and Alioramus of the Alioramini. Both subfamilies of Tyrannosaurinae and Albertosaurinae were present in the Campanian and early Maastrichtian stages of North America, with tyrannosaurines like Daspletosaurus ranging throughout the Western Interior, while the albertosaurines Albertosaurus and Gorgosaurus are currently known only from the northwestern part of the continent.
By the late Maastrichtian, albertosaurines appear to have gone extinct, while the tyrannosaurine Tyrannosaurus roamed from Saskatchewan to Texas. This pattern is mirrored in other North American dinosaur taxa. During the Campanian and early Maastrichtian, lambeosaurine hadrosaurs and centrosaurine ceratopsians are common in the northwest, while hadrosaurines and chasmosaurines were more common to the south. By the end of the Cretaceous, centrosaurines are unknown and lambeosaurines are rare, while hadrosaurines and chasmosaurines were common throughout the Western Interior.A study published in the journal Scientific Reports on February 2, 2016 by Steve Brusatte, Thomas Carr et al. indicates that during the later Maastrichtian, Tyrannosaurus itself might have been partially responsible for the extinction of the other tyrannosaurids in most of western North America. The study indicates that Tyrannosaurus might have been an immigrant from Asia as opposed to having evolved in North America (possibly a descendent of the closely related Tarbosaurus) that supplanted and outcompeted other tyrannosaurids. This theory is further supported by the fact that few to no other types of tyrannosaurid are found within Tyrannosaurus" known range.
Paleontologist Gregory Erickson and colleagues have studied the growth and life history of tyrannosaurids. Analysis of bone histology can determine the age of a specimen when it died. Growth rates can be examined when the age of various individuals are plotted against their size on a graph. Erickson has shown that after a long time as juveniles, tyrannosaurs underwent tremendous growth spurts for about four years midway through their lives. After the rapid growth phase ended with sexual maturity, growth slowed down considerably in adult animals. A tyrannosaurid growth curve is S-shaped, with the maximum growth rate of individuals around 14 years of age.
The smallest known Tyrannosaurus rex individual (LACM 28471, the "Jordan theropod") is estimated to have weighed only 29.9 kilograms (66 lb) at only 2 years old, while the largest, such as FMNH PR2081 ("Sue") most likely weighed over 5,400 kg (11,900 lb), estimated to have been 28 years old, an age which may have been close to the maximum for the species.T. rex juveniles remained under 1,800 kg (4,000 lb) until approximately 14 years of age, when body size began to increase dramatically. During this rapid growth phase, a young T. rex would gain an average of 600 kg (1,300 lb) a year for the next four years. This slowed after 16 years, and at 18 years of age, the curve plateaus again, indicating that growth slowed dramatically.For example, only 600 kg (1,300 lb) separated the 28-year-old "Sue" from a 22-year-old Canadian specimen (RTMP 81.12.1).This sudden change in growth rate may indicate physical maturity, a hypothesis which is supported by the discovery of medullary tissue in the femur of an 18-year-old T. rex from Montana (MOR 1125, also known as "B-rex"). Medullary tissue is found only in female birds during ovulation, indicating that "B-rex" was of reproductive age.
Other tyrannosaurids exhibit extremely similar growth curves, although with lower growth rates corresponding to their lower adult sizes.Compared to albertosaurines, Daspletosaurus showed a faster growth rate during the rapid growth period due to its higher adult weight. The maximum growth rate in Daspletosaurus was 180 kilograms (400 lb) per year, based on a mass estimate of 1,800 kg (4,000 lb) in adults. Other authors have suggested higher adult weights for Daspletosaurus; this would change the magnitude of the growth rate but not the overall pattern.The youngest known Albertosaurus is a two-year-old discovered in the Dry Island bonebed, which would have weighed about 50 kg (110 lb) and measured slightly more than 2 metres (6 ft 7 in) in length. The 10-metre (33 ft) specimen from the same quarry is the oldest and largest known, at 28 years of age. The fastest growth rate is estimated to be around 12–16 years, reaching 122 kilograms (269 lb) per year, based on an adult 1,300 kilograms (2,900 lb) which is about five times slower than for T.-rex. For Gorgosaurus the calculated maximum growth rate is about 110 kilograms (240 lb) during the rapid growth phase, which is comparable to that of Albertosaurus.

Tyrannosaurus rex
Tyrannosaurus rex, whose largest specimens known estimated at 13.2 metres (43 ft) and 12.3 metres (40 ft) in length respectively. Tyrannosaurus ("tyrant lizard", from the Ancient Greek tyrannos (τύραννος), "tyrant", and sauros (σαρος), "lizard")) is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex (rex meaning "king" in Latin), commonly abbreviated to T. rex, is one of the most well-represented of the large theropods. Tyrannosaurus lived throughout what is now western North America, which then was an island continent named Laramidia. Tyrannosaurus had a much wider range than other tyrannosaurids. Fossils are found in a variety of rock formations dating to the Maastrichtian age of the upper Cretaceous Period, 68 to 66 million years ago. It was among the last non-avian dinosaurs to exist before the Cretaceous–Paleogene extinction event.


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Автор: valentint
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