1. Dinosaurs were the dominant vertebrate animals of the terrestrial ecosystems for over 160 million years, from the late Triassic period (about 230 million years ago) to the end of the Cretaceous period (65 million years ago). They are divided into a number of subgroups, the largest two being based on the structure of the hip. Saurischians retained the lizard-like hips of their ancestors, and included theropods like Tyrannosaurus and sauropodomorphs like Diplodocus. Ornithischians were herbivores with bird-like hips, and included such groups as armored, horned, bone-headed, and duck-billed dinosaurs. These groups evolved a variety of adaptations and occupied diverse niches as the Earth and its environments changed during the Mesozoic Era, but (with the exception of birds; see below), perished in the Cretaceous–Tertiary extinction event approximately 65 million years ago. Research since the 1970s has changed the scientific and public understanding of dinosaurs, from slow, unintelligent cold-blooded animals, to active animals with elevated metabolisms, often with adaptations for social interactions. One important component of the renewed interest in dinosaurs is their relationship with birds; research indicates that theropod dinosaurs are the most likely ancestors of birds. In fact, most paleontologists regard birds as the only surviving dinosaurs. Crocodilians are the other surviving close relatives of dinosaurs, and both groups are members of the Archosauria, a group of reptiles that first appeared in the very late Permian and became dominant in the mid-Triassic. Since the first dinosaur fossils were recognized in the early nineteenth century, mounted dinosaur skeletons have become major attractions at museums around the world. Dinosaurs have become a part of world culture and remain consistently popular among children and adults. They have been featured in best-selling books and films (notably Jurassic Park), and new discoveries are regularly covered by the media. The term "dinosaur" is sometimes used informally to describe other prehistoric reptiles, such as the pelycosaur Dimetrodon, the winged pterosaurs, and the aquatic ichthyosaurs, plesiosaurs and mosasaurs, although none of these were dinosaurs. In colloquial English "dinosaur" is sometimes used to to describe an obsolete or unsuccessful thing or person, despite the dinosaurs' 160 million-year reign and the global abundance and diversity of their descendants, the birds.