Evolution Of ReptilesEvolution of Reptiles: It's impossible to look at reptiles for long without wondering about the relationships between the reptile groups. Once we start down this path it is inevitable that we will compare their bodies. The part that tells us the clearest story is their skulls.
Ancient Amphibian AncestorsAll reptiles and higher vertebrates evolved from early land going amphibians called Labyrinthodonts. There's a picture of the fossilized skeleton of one of these creatures to the right. An example of such a creature is Eryops. This is known because early reptiles had many similarities with Labyrinthodont amphibians, more so than with any other group of animals. There were similarities in the skulls, teeth, vertebrae, limbs and more. Early reptiles were little more than watertight Labyrinthodont amphibians. A picture of the classic Labyrinthodont amphibian skull is shown below from three angles:
The reptiles that arose from these early creatures have been grouped according to differences in the skull. The Labyrinthodonts had no openings in the outer skull save for the nostrils and eyes. How Were The Reptiles Different?We know what a reptile looks like, but how exactly do we define a reptile when all we are looking at is a small pile of 300 million year old fossil bones? There are two main elements. These consist of differences in the hands and feet and the presence of certain reptile characteristics in the skull. When both these differences are present, we can be fairly certain that the fossil in question is that of a reptile and not just an advanced amphibian. Evolution Of Four Groups Of ReptilesThere were four groups of reptiles that evolved from this primitive condition, and they are grouped according to the number and location of extra holes (called apsids) in the back part of the skull. We can see this in picture form below:
These four variations on the basic skull theme again show us the mechanics of evolution. There was one initial type and from that four variations arose. In the early days of the separation of these groups the members were probably fairly similar but they soon developed into radically different groups including crocodiles, dinosaurs, sea reptiles and mammals. Again, a picture is the best way to see this. Note that the following has no time scale. The animals in grey are extinct, the colored ones are still going. Note that all euryapsid reptiles are now extinct.
This is a simplified view though. A more comprehensive view of the development of reptiles looks like this:
Evolution Of The Four Main GroupsNow we'll look at the four main groups of early reptiles one at a time. Click the skull type of your choice to read more about those reptiles:     The Anapsid GroupThe Anapsid reptiles were among the first serious contenders for vertebrate dominance of the land. There were many groups of these animals, as seen in the Anapsid family tree. Among these animals were some of the following groups: Millerettidae: Initially known from one small lizard-like animal, recent work has shown these creatures to be a diverse group showing many signs of evolutionaty adaptation to their environments. The Synapsid GroupThe mammal-like reptiles, called Synapsids, appeared at the very beginning of reptile history. Some members of this group very quickly developed some features that are still present in many of today's mammals. The Euryapsid GroupEuryapsid reptiles were once thought to be a separate branch of creatures on a par with the other three main groups. They are now though to be an extinct branch of Diapsid reptiles. This group included the Ichthyosaurs and Plesiosaurs, as well as a bizarre collection of creatures called Placodonts. The Diapsid GroupThese are the big winners of the reptile world. With the possible exception of turtles, all living reptiles are members of the Diapsid group. This group also produced those wonders of the past, the dinosaurs.
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