Dinosaur Primer

The Ornithischians, or "bird hipped" dinosaurs, arose in the Jurassic Period, and reached their highest diversity in the Cretaceous Period. Because the Ornithischians arose after the Saurischians, and because of many other characteristics they have, paleontologists believe that the Saurischian dinosaurs gave rise to the Ornithischian dinosaurs. It is important to remember that birds did not arise from the "bird hipped" dinosaurs, but instead came from the "lizard hipped" dinosaurs.

The Ornithischians are further divided into two groups of bipedal herbivores, the ornithopods, such as Iguanodon and the hadrosaurs, such as Corythyosaurus, and two groups of quadrupedal herbivores, the ceratopsians, such as Triceratops, and the stegosaurs, who were succeeded by the ankylosaurs. The Ornithischians solved the hip problem by rearranging their pelvic bones in order to allow more freedom of movement while walking.

Let's look at the two different pelvic structures. The bone on the left is the ischium, the bone on the right is the pubis, and the bone on top is the ilium, the three join together to form the acetabulum, the cup-like structure in which the femur fits.

The Saurischian dinosaurs kept the hip structure of the reptiles they evolved from, and this structure can still be seen today in modern crocodiles. The Ornithischian dinosaurs instead moved the bones around, allowing the leg muscles to attach at different sites, giving them more freedom of movement.

In my next article, I will be talking about Archaeopteryx and other early birds.

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