Dinosaurs were a group of reptiles that ruled the land for over 140 million years (over 160 million years in some parts of the world). They evolved into different shapes and sizes, from the fearsome giant Spinosaurus to the chicken-sized Microraptor, and were able to survive in a variety of ecosystems.
Dinosaurs came in a wide variety of shapes and sizes. Some walked on four legs, while others walked on two. Here are some Late Cretaceous dinosaurs from what is now the United States. From back to front: Ankylosaurus, Tyrannosaurus Rex, Triceratops, Angiosaurus, Pachycephalosaurus, and the recently named Acheroraptor and Anzu.
One of the reasons for the dinosaurs' success was their straight hind legs, perpendicular to their bodies. This allowed them to move with less energy compared to other reptiles like today's lizards and crocodiles, which have an extended limb posture. Because their legs were located under their bodies rather than extending to their sides, the dinosaurs' weight was also better supported.
Dinosaurs are depicted standing upright (as shown in the left image), while lizards and crocodiles are depicted with their limbs outstretched (as shown in the right image).
Many dinosaur species went extinct around 66 million years ago, but a group of living dinosaurs still coexist with us today: birds.
I. Other prehistoric reptiles
The dinosaurs were archosaurs, a large group of reptiles that first appeared approximately 251 million years ago, close to the early Triassic period. Other non-dinosaur reptiles were also archosaurs, including pterosaurs (extinct flying reptiles) and modern crocodiles and their ancestors.
These, and many other types of ancient reptiles, are often mistakenly referred to as dinosaurs.
Marine reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs are not dinosaurs. Neither are Dimetrodons or other reptiles in the same group (formerly known as "mammaloid reptiles," now called synapsids).
None of these other extinct groups had the upright posture characteristic of dinosaurs.
Despite its prehistoric appearance, Heterodontosaurus was not a dinosaur. It was another extinct reptile that lived before dinosaurs.
II. Key characteristics shared by dinosaurs:
1. They stood upright with their legs perpendicular to their bodies. This is the main characteristic that distinguishes dinosaurs from other reptiles.
2. Like other reptiles, they lay eggs.
3. Apart from some birds, such as penguins, dinosaurs lived on land, not in the sea.
4. Their skulls have a hole between the eye sockets and nostrils. All archosaurs have this feature.
5. Dinosaurs also had two holes behind their eye sockets. Large, strong jaw muscles passed through these holes and attached directly to the top of the skull. As a result, the jaws were able to open and close with greater force.
