What is a reptile?

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David L Rowe (UK)

This is a short introduction to what is a reptile – an issue that is a lot more complex that it might seem. To understand what a Reptile is one first needs to understand the cladistic (which is a way of classifying life forms) method and phylogenetic systematics (that is, a way of establishing the relationship of life forms) used by palaeontologists.

Skull structure is the most important factor to be considered. The term ‘Reptile’ refers to the Class Reptilia, which includes the ectothermic (cold-blooded) turtles, snakes, lizards, crocodilians, tuataras and dinosaurs, and the endothermic (warm-blooded) birds, which are also referred as non-avian dinosaurs. However, the way in which body temperature is maintained (cold versus warm-blooded) is not a factor in the classification of Reptiles. The main diagnostic physical characteristic of a Reptile that separates them from other animals is that they are Diapsids (Fig. 1).

A Diapsid has a pair of openings in the skull – temporal fenestra – behind the eyes. Turtles and a few extinct Reptiles are an exception and have no temporal fenestra. They are classified as Anapsids.

Fig. 1. Diapsid skull.

Birds are descended from or are dinosaurs (depending on your view) with which they have shared derived traits including being Diapsids. Therefore, dinosaurs and birds are classified as Reptiles. However, birds are certainly quite different from other living Reptilia. The traits that modern birds possess started with the first Diapsid, the tiny lizard-like Petrolacosaurus, about 350 million years ago. Modern bird’s traits were acquired over many million years of evolution and are very different from those of their primitive ancestors. In the Jurassic, many looked more like reptiles, such as Archaeopteryx, meaning “ancient wing” (Fig. 2).

Fig. 2. Archaeopteryx.

Some Reptiles evolved towards the Mammalian form of the Synapsid skull structure (F1g. 3.) that has one temporal fenestra. Permian wildlife was succeeded by a very different cast of animals during the ensuing Triassic period. The primitive mammal-like reptiles – the Pelycosauria – became extinct, but they left behind a variety of advanced mammal-like reptiles, collectively termed the Therapsida. Therapsids were the transitional Synapsid between Reptiles and Mammals. Their lower jaw, however, was reptilian-like and was made of several bones, unlike a true Mammal, where the lower jaw has a single bone, the mandible.

Fig. 3. Synapsid skull.

During much of the Triassic, therapsids were abundant and diverse. About 280 million years ago, the first animal that evolved into a mammal-like reptile was Tetraceratops, a Therapsid (Fig. 4).

Fig. 4. Tetraceratops.

An imposing carnivorous beast, Cynognathus (Fig. 5), evolved about 220 million years ago. Cynognathus was a predaceous animal, as is indicated by its sharp teeth and large fangs. Unlike most of its related predecessors, Cynognathus and some other therapsids were able to assume a more nearly upright posture at times, in sharp contrast to the sprawling mode of the pelycosaurs and the labyrinthodonts. Cynognathus, however, was much more like a mammal in its anatomy than other Therapsids. In fact, it was the first mammal-like Therapsid to have a Mammalian type lower jaw configuration and, in all probability, was warm-blooded and bore its offspring live. It is thought to be the link to all living Mammals.

Fig. 5. Cynognathus.

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