Dinosaurs

Dinosaurs are an extremelly diverse and very successful group of reptiles. They first appeared in the Middle-Late Triassic, however an exact origin is still uncertain and disputed. With the Tr-J extinction event, dinosaurs became the dominant land animals on Earth, from the beggining of the Jurassic to the end of the Cretaceous. Avialans are the last surviving dinosaurs, as the only lineage known to have sucessfully survived the K-Pg extinction event. Dinosaurs are often divided in two non-taxonomical categories that divide the non-avian dinosaurs from the avian dinosaurs (birds). Dinosaurs are a group of animals that reached many morphological and evolutionary achievements. Today, birds are the most abundant group of land vertebrates in the world, reaching around 10 000 species. Palaeontological discoveries allowed over 1000 species of extinct non-avian dinosaurs to have been described. They inhabited every continent in the world, both in avian and non-avian form. Although initially believed to be no different from other cold-blooded reptiles, the last half-century helped the scientific community understand that all non-avian dinosaurs were actually warm-blooded animals, with high metabolisms and numerous social adaptations. The first dinosaurs were bipedal runners, but their descendants soon specialized the bipedality locomotion and some others also became quadrupeds, or even used both forms of locomotion. They also have a huge variety of diets, with some being carnivores, herbivores or omnivores. Some dinosaurs had elaborate display features, such as horns, crests, bony armor and spikes. Although birds are usually small and lightly-built, extinct dinosaurs could reach to colossal sizes, such as the sauropods, which are the largest and heaviest land animals known to have existed. However, dinosaurs were actually pretty diverse in their size chart with non-avian dinosaurs, such as alvarezsaurs, reaching sizes of 1 meter or less.