Dinosaurs were a group of reptiles that dominated land for more than 140 million years (more than 160 million years in some parts of the world). They evolved in different shapes and sizes, from the terrifying giant Spinosaurus to the chicken-sized Microraptor, and were able to survive in a variety of ecosystems.
Dinosaurs come in all shapes and sizes. Some walk on four legs and some walk on two legs. Here are some Late Cretaceous dinosaurs from what is now the United States. From back to front: Ankylosaurus, Tyrannosaurus Rex, Triceratops, Simiomimus, Pachycephalosaurus, and the recently named Acheroraptor and Anzu.
One of the reasons for the dinosaurs' success was that their hind legs were straight and perpendicular to their bodies. This allowed them to use less energy to move than other reptiles that had outstretched postures like today's lizards and crocodiles. The dinosaur's weight was also better supported because their legs were located under the body rather than sticking out to the side.
The posture of dinosaurs is upright (as shown on the left), while the postures of lizards and crocodiles are with their limbs extended (as shown on the right).
Many dinosaur species went extinct about 66 million years ago, but one group of dinosaurs is still alive with us today: birds.
1. Other prehistoric reptiles
The dinosaurs were Archosaurs, a large group of reptiles that first appeared about 251 million years ago, near the beginning of the Triassic period. Several other non-dinosaur reptiles are also archosaurs, including pterosaurs (now extinct flying reptiles) and modern crocodiles and their ancestors.
These and many other types of ancient reptiles are often incorrectly called dinosaurs.
Marine reptiles such as ichthyosaurs, plesiosaurs and mosasaurs are not dinosaurs. Neither was Dimetrodon or other reptiles of the same group (formerly called "mammal-like reptiles" and now called synapsids).
None of these other extinct groups had the upright posture that characterized dinosaurs.
Despite its prehistoric appearance, Dimetrodon was not a dinosaur. It is another extinct reptile that lived earlier than the dinosaurs.
2. The main characteristics shared by dinosaurs:
1. They stand upright with their legs perpendicular to their body. This is the main feature that distinguishes dinosaurs from other reptiles.
2. Like other reptiles, they lay eggs.
3. Except for some birds, such as penguins, dinosaurs lived on land, not in the sea.
4. Their skulls have a hole between their eye sockets and nostrils. All archosaurs share this trait.
5. There are also two holes behind the dinosaur’s eye sockets. The large, strong jaw muscles attach directly to the top of the skull through the holes. As a result, the jaw is able to open and clamp with greater force.
animal tags： Dinosaur Ankylosaurus Triceratops Simiformosaurus Pachycephalosaurus Lizard Crocodile
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