The Tyrannosaurus Rex is a theropod dinosaur existed in the Upper Cretaceus period. It's one of the last non-avian dinosaurs and the name expresses all the strength this animal had: its name is actually the mixture of two Greek word and maenad exactly lizard's tyrant.
The T.Rex is one of the biggest theropods known, with a maximum length of 13 meters and 5 meters maximum height at the withers. They didn't have a standard weight since it changed from young or adult dinosaurs but we can presume that an adult could be around 6 or 7 tons, which is a lot more than an african elephant.
Initially paleontologists thought that this animal was almost a tripod, which meant that was almost erected on the hind legs and that it used the big tail as a third support. Osborn, the first man who ever classified this enormous dinosaur used this teary and in 1915 presented the first completed T.Rex's skeleton rebuilt in this strange position. In the '70, however, it became clear that it was impossible for this gigantic animal to maintain this posture and finally understood that the Tyrannosaurus kept their body parallel to the ground and used the tail to balance the body in order to sustain the big skull.
The discovery of non-adult T.Rex gave paleontologist the chance to study is growth, the estimated life and the evolution during the centuries. The smallest one ever found is known as Theropod Jordan that had an approximate weight of 30 kg, not much more than a turkey, and was probably 2 years old when it died.
Sue, one of the most famous Tyrannosaurus Rex, died probably at 28, weighed more or less 6,700 kg and was 13 meters long. As we know, Sue is probably the oldest T.Rex and supposedly was able to reach the maximum size for a female. Obviously the experts found bigger bones, but since the skeletons are damaged or incomplete it's impossible to validate the suppositions made.In 2000 Horner discovered a new skeleton that is supposedly the 10% bigger that Sue, so that its estimated length and weight are 14 meters and almost 9 tons.
The skull of this theropod is massive, more than 1,5 meters long and very different from the skulls of the other theropods. The back of the head was very large while the front area narrowed near the nostrils; as a consequence, this animal had a optimal front view that was surely superior to the other species. Certain bones, as the nasal ones, were merged together in order to have more resistant bones; this animal had pneumatic bones that were really useful to reduce the body weight and give a better flexibility. This predator had supposedly 30 teeth on the upper arch and 28 on the lower one that were probably long between 10 and 30 cm.
The forelimbs were 1 meter long and very small if compared to the rest of the body, but not atrophied as the Carnotaurus' ones; the forelimbs had two prehensile finger ending with impressive claws. Since this limbs have the sign of great muscles, we can suppose hay had a certain utility like holding its prey down. Someone suggested the animal used them to get up off the ground but we have no proofs.
The biggest question paleontologists still have on this dinosaur is: skin or feathers? Ww can't really know how a T.Rex looked like, and, since several ancestors were feathered scientists are likely to think that the baby Tyrannosaurus had feathers; however, we don't have any trace of a baby's skin so it's impossible to know for sure. We are sure instead that adults had skin, or more probably shedding, as we can suppose from the traces found. We can't exclude that at the beginning this gigantic animal was feathered and, while becoming bigger, he changed in order to became more competitive.
Between '80 and '90 scientists tried to differentiate males from females thinking that the bigger one were the female and the lighter ones the males ( the smaller size was an advantage in an era where they had to hunt for food constantly ). However, since it's still it's still impossible to determinate the sex ( the only T.Rex we know for sure was a female is the MOR 1125 ) we can't really say there was sexual dimorphism in this species and, maybe, the bigger one were only the older ones.
Even if people usually imagine this dinosaur as a great killer, many hypothesis have been formulated on its diet. The first one is that this animal was an active predator which means that, since it had big claws and an uncommon agility, was able to hunt and kill dinosaurs like the Hadrosaurids, that were usually 9 meters long ( quite smaller than the T.Rex). The second idea depicted the theropod as a scavenger that used its massive body to scare smaller predators and steal their prey; this idea clearly shocked people who always imagined this big species as a deadly one.
The third and more likely hypothesis shows this animal as an occasional killer that used its extremely developed sense of smell to locate the prey and used to ambush the victim. This way, the T.Rex did not have to run after the prey for a long time, which was impossible because of its weight ( it's supposed it was able to reach 40 km/h but for a very short time), and had mostly to bite them only one time to kill them. We can say that the third theory roughly combines the ideas of the other ones, stating that T.Rex ate carcasses, which can include also the ones of its own species, and that if it didn't find an easy way to eat was able to hunt smartly.
We can finally imagine Tyrannosaurus Rex as an extremely territorial animal since we found skeletons that showed T.Rex's bites, but it was probably able to live well with a mate or a pack.
As a conclusion, a small list of the most famous skeletons discovered:
1- Sue was named after the paleontologist that discovered it in 1990 in South Dakota. It's 85% complete and until 2001, the bigger one ever found. From recent studies we discovered that Sue probably reached complete adulthood at 19 and died at 28, which makes her the oldest T.Rex found.
2- In 1987 Stan Sacrison found another theropod i South Dakota and it took 30,000 hours to present to public a 65% complete skeleton named Stan.
3- In 2000, Jack Horner discovered five skeletons in Montana. One of these, nicknamed C.Rex is probably the biggest T.Rex ever found, supposedly 10% bigger than Sue.