Dinosaur Primer

Many different species of dinosaurs walked the Earth during the Mesozoic Era. One of the things that paleontologists like to do, is to put the dinosaurs into groups where they share similar characteristics. The reason they do this is because we would like to know how the dinosaurs are all related. Since none of us were around 248 million years ago, when the first dinosaurs were appearing, we do not know for certain how they are related, but we can guess at how the different species are related by looking at similarities in their bones.

By examining these shared characteristics, and by looking at where in the fossil record we find different species, we can determine how dinosaurs are related. From this information, paleontologists have divided the dinosaurs into two main groups, called the Saurischians and the Ornithischians.

The main characteristic that paleontologists use to divide the dinosaurs into these two groups is the shape of their hip bones. There are many other characteristics, but this is the most noticeable.

In my last article ("What do you mean, It's not a dinosaur?"), I talked about how an animal must have an upright stance before it can properly be called a dinosaur. The dinosaurs evolved from reptiles that had a sprawling gait, which made it very difficult for them to breathe while running. The dinosaurs solved this breathing problem by developing an upright stance, but when they did this, they had to do something with the structure of their hips in order to be able to walk.

The dinosaurs did this in two different ways. The first group of dinosaurs to appear was the group that we call the Saurischians, or "lizard hipped" dinosaurs. These dinosaurs first appeared in the Triassic Period and reached their highest diversity in the Cretaceous Period. The Saurischians can be further divided into the Theropods and the Sauropods.

The Theropods ("beast foot") were bipedal carnivores (meat eaters). Paleontologists further divide them into the smaller coelurosaurs, such as Ornithomimus, and the larger carnosaurs, such as Tyrannosaurus. Archaeopteryx, an early bird, resembles the coelurosaurs, and paleontologists believe that birds arose from the Theropod dinosaurs. As the Theropods were habitually bipedal animals, their hips were tipped in such a way that they were able to move quickly, and not have the problems with breathing that the sprawling lizards would have.

The Sauropods ("lizard foot") were quadrupedal herbivores (plant eaters), such as Apatosaurus. The Sauropods were very heavy animals, and probably did not move quickly, so their legs never moved in a wide arc, and they did not have any problems with using the old lizard hip structure.

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