Dinosaurs: Their Lives, Their Deaths and Their Evolution!

by Dr. Bob Gardner
Department of Mathematics
Department of Physics and Astronomy
Institute of Mathematical and Physical Sciences
East Tennessee State University

The Evolutionary Relationships of Dinosaurs

Dinosaurs are classified into two large groups: lizard-hipped dinosaurs, called Saurischia, and bird-hipped dinosaurs, called Ornithischia.
The Saurischians are further divided into two categories: the Theropods and the Sauropodomorphs.

The Theropods include all of the meat-eating (or carnivorous) dinosaurs. Theropods have long, powerful hind limbs ending in sharply clawed feet. They have small front limbs, large eyes and long jaws lined with dagger-like teeth. Some of the best known Theropods are the carnosaurs: Carnotaurus, and Tyrannosaurus.

Another group of Theropods are the ceratosaurs which includes Coelophysis, Dilophosaurus, and Ceratosaurus.
Other examples of Theropods are Ornithomimus, Troodon, and Deinonychus.
As we will discuss later, birds evolved from Theropod dinosaurs.

The Sauropodomorphs were all plant-eaters (that is herbivores). This group includes the true giants of the Mesozoic Era. Some of the famous sauropods are Diplodicus, Apatosaurus (or Brontosaurus), Brachiosaurus, and Camarasaurus.

The bird-hipped Ornithischians all have a horn covered beak on their lower jaws. This device was used in tearing vegetation and shows that all Ornithischians were herbivores. The Ornithischians are a varied category which is divided into 5 major groups: ornithopods, ceratopsians, pachycephalosaurs, stegosaurs, and ankylosaurs.
Ornithopods were small to medium-sized and ran on their hind legs most of the time. Examples are Heterodontosaurus, Dryosaurus, Iguanodon and the hadrosaurs.
The ceratopians had horns and frills. Examples are Protoceratops, Triceratops, and Styracosaurus.
The Pachycephalosaurs had domed reinforced heads presumably used as battering devices.
The stegasaurs had spines and plates which served for display and heat absorption and radiation.
The ankylosaurs were fully covered with thick boney plates for protection against the predatory theropods.

Here is a simplified version of the evolutionary relationships of some of the dinosaurs. 66 million years ago, all dinosaurs went extinct - all but one group. This group of theropod dinosaurs lives on today. We commonly call these living dinosaurs... birds.

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