Mesozoic Era forest with dinosaur

Saurischia and Ornithischia Orders

Dinosaurs are an extinct group of reptiles that dominated the Earth for over 160 million years. They are divided into two major orders based on the structure of their hips - the Saurischia and the Ornithischia.

Two Major Orders of Dinosaurs

Dinosaurs were a diverse group of reptiles that lived during the Mesozoic era, which spanned from approximately 252 to 66 million years ago. One of the most distinguishing features of dinosaurs is their hips, which can be classified into two major orders: Saurischians and Ornithischians. This post will provide an in-depth look at these two major orders, their subdivisions, and the various species that existed within them.

Saurischians and Ornithischians are the two major groups of dinosaurs. The Saurischian dinosaurs are divided into two major groups: Theropods and Sauropods. The Ornithischian dinosaurs are divided into five major groups: Thyreophora, Stegosaurs, Ankylosaurs, Ornithopods, Ceratopsians, and Pachycephalosaurs.

Saurischians Order

Saurischian dinosaurs are characterized by their lizard-like hip structure, with a pubis bone that points forward. There are two major groups of Saurischians:


Theropod dinosaurs were carnivorous, bipedal, and had sharp teeth and claws. They ranged in size from small, feathered dinosaurs like Compsognathus to large predators like Tyrannosaurus rex. Theropods are further divided into subgroups, including Coelurosauria, Allosauria, and Megalosauria.


Sauropod dinosaurs were massive, herbivorous dinosaurs with long necks and tails. They were quadrupedal and had columnar legs to support their weight. Sauropods ranged in size from the small Camarasaurus to the largest animals to ever walk the earth, like Argentinosaurus. Sauropods are further divided into various subgroups, including Titanosauria and Diplodocoidea.

Ornithischians Order

Ornithischian dinosaurs are characterized by their bird-like hip structure, with a pubis bone that points backward. There are five major groups of Ornithischians:


Thyreophora dinosaurs had armor-like plates or spikes on their bodies to protect themselves from predators. This group includes the Stegosaurus and Ankylosaurus. They are further divided into two subgroups: Stegosauria and Ankylosauria.


Stegosaur dinosaurs were characterized by their distinctive rows of plates and spikes on their backs. They were herbivorous and ranged in size from small to large. This group includes Stegosaurus and Kentrosaurus.


Ankylosaur dinosaurs were quadrupedal and had a heavily armored body with bony plates and spikes. They had a club-like tail that they could use to defend themselves. This group includes Ankylosaurus and Euoplocephalus.


Ornithopod dinosaurs were herbivorous and bipedal, with a long tail and a duck-like bill. They ranged in size from small to large and included many different types of dinosaurs like Iguanodon and Hadrosaurus. Ornithopods are further divided into various subgroups, including Hadrosauridae and Iguanodontia.


Ceratopsian dinosaurs were characterized by their distinctive frills and horns on their heads. They were herbivorous and ranged in size from small to large. This group includes Triceratops and Styracosaurus.


Pachycephalosaur dinosaurs were bipedal and had thick, bony skulls. They are thought to have used their heads for head-butting battles with other males during mating season. This group includes Pachycephalosaurus and Stygimoloch.

Each of these groups had unique characteristics and adaptations that helped them survive in their respective environments. Studying the different subdivisions of these dinosaur groups can give us insight into the evolution and diversity of these amazing animals.


From a young age, AJ was constantly seeking out books and documentaries about dinosaurs and spent countless hours poring over their images and stories. Motivated by his desire to share his love for dinosaurs with others, AJ began to research and compile a list of resources to help others learn about these amazing creatures.

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