Inevitably any mention of the word reptile invokes the thought of the word dinosaur, at least in most Western minds. Given the sometimes high levels of dislike that living reptiles enjoy, dinosaurs and their prehistoric contemporaries are paradoxically popular in both popular culture and entertainment. This permeates not just to the level of films like Jurassic Park but also down to the shelves of the local shopping centre, where people buy cuddly versions of the better known dinosaurs such as Tyrannosaurus Rex or Triceratops. Dinosaurs find their way onto bottle labels and cereal packets, wearing baseball caps and friendly smiles, an interesting sanitisation of the image of the dragon that dwells in many a human psyche.
The fascination is understandable, even if the popular image is often ill-informed. As a class, reptiles have been around on the planet for over 240 million years and completely dominated it during that time for 140 million years. No other group of animals, not even the mammals, has had such a complete and long ascendancy, although obviously we cannot tell how the different classes will fare in the future epochs of this planet.
The received wisdom is that reptiles evolved from the prehistoric amphibians of the Carboniferous and Permian ages. These amphibians (in appearance rather like gigantic salamanders) had in turn evolved from fish and been able to live on dry land, but were still bound to water because of the need to lay their eggs there, even if they did not require moisture to allow breathing through their skins as modern amphibians do. The huge advances the first reptiles made was to have a completely watertight skin and a completely watertight egg structure, in essence freeing them from the need to be in the vicinity of water. Now they could move much further and lay their eggs anywhere, since the reptile egg contained its own fluid for the embryo.
The first reptiles were lizard-like creatures and crocodiles. Interestingly, however, new reptiles began to appear which had features in common with mammals, such as a varied set of teeth, changes in bone structure more akin to that of mammals, and at least in some cases the beginnings of fur. However, at the end of the Permian era there was a huge wave of extinctions that wiped out 90% of the then-living species and left the earth a warmer and more barren place, with several vacant niches for new species. Despite their apparently advanced nature, the mammal-like reptiles were threatened by the dawn of the dinosaurs proper in the following period, the Triassic. These new reptiles were distinguished among other characteristics by walking upright on four or two legs rather than the sprawling gait of the crocodiles, lizard-like and mammal-like reptiles. Radiation and distribution of this new group of animals was astonishly rapid as they supplanted or dominated all other living groups of terrestrial vertebrates throughout the Triassic period. By the beginning of the Jurassic the mammal-like reptiles which had promised so much in terms of advanced body features had all but disappeared, and the remaining mammals or their close relatives grew no bigger than a house cat, most being smaller. The dinosaurs had come to stay.
There followed two more periods, the Jurassic and the Cretaceous, which together with the Triassic constitute the Age of Dinosaurs, zoologically one of the most fantastic periods in Earth's history. Having become the dominant class, reptiles (not just the dinosaurs) diversified and spread out to dominate not just the land but the sea and the sky. The Jurassic period saw the biggest creatures ever to walk the earth, the sauropods (Diplodocus, Brontosaurus et al), some of whom weighed over 100 tonnes, while the seas were filled with reptiles that looked like fish (Ichthyosaurs) as well as gigantic long-necked creatures with paddle-like flippers, the plesiosaurs and their relatives. The skies also had their share of giants in the form of the flying reptiles, pterodactyls, pteranodons and others, some of whom had the wingspan of a small airliner. At about this time the first feathered, bird-like reptiles began to appear that would ultimately produce the lineage of the birds, the new class Avia.
The Cretaceous period saw the final flowering of dinosaur diversity. The overall size of the dinosaurs decreased, partly because the huge sauropods largely disappeared, but many of the Cretaceous creatures featured anatomical innovations such as horns, crests and other cranial fixtures (the ceratopsians and duck-billed dinosaurs) or all-over bony armour (ankylosaurids). The largest ever terrestrial predators, the tyrannosaurids, also appeared during the last millions of years of this period. Finally, and possibly most significantly, smaller but faster and deadly creatures began to appear in the form of dromeosaurids (the deinonychus and velociraptor-type dinosaurs). Apart from their apparent perfection as predators, these creatures seem to show a greater brain size and potential for future mental development, although how far this could have gone is still subject to debate.
By now, however, the hitherto invincible dynasty was running into trouble. Although blame for the extinction of the dinosaurs popularly rests with the asteroid that did strike earth 65 million years ago, the fact is that the earth was already going through a period of change. The continents were beginning to shift, causing significant climate changes, and there was increased volcanic activity with its implications for the atmosphere. By the time of the end of the Cretaceous period, only 18 of the 57 or so dinosaur families remained. Even so it is possible that the dinosaurs and other prehistoric reptiles might have weathered the storm and produced new forms, including more intelligent creatures, to meet their changing environment. What finally sealed their doom appears to have been the asteroid cataclysm that struck the earth and led to the extinction not just of the remaining dinosaurs but also of many other creatures. In particular the extinction of marine invertebrates such as the ammonites would have hit the food chain hard, probably contributing to the disappearance of the mosasaurs and other marine reptiles. Lizards were also hard hit but survived as an order to become the largest modern group of reptiles, followed closely by the snakes which had first appeared in the Cretaceous. Crocodilians and chelonians (turtles and tortoises) also survived and flourished in the Cenozoic era although the later modern periods, in particular the Ice Ages and human activity, struck them hard. After the extinction of the prehistoric reptiles, the field was left open for the survivors, and the dominant niches were filled first by birds (including many fearsome flightless forms) before larger mammals appeared to become the dominant class on earth. Many of these large and spectacular creatures themselves were to die out in the following 55 million years, until one group of animals, the primates, produced one hominid species who came to completely dominate the planet: man.