The Science of Discworld Revised Edition

dog-sized beasties.
    Their therapsid descendants would be the mammals’ ancestors, little guys called morganucodontids, which we talk about later. These were small beasties, because most of the action by then was dinosaur. For 150 million years, between 215 and 65 million years ago, any land animal more than three feet (1m) long was a dinosaur.
    Among these mammal-like reptiles in the forests were a few active predators the size of kangaroos and wallabies. These unprepossessing creatures were the earliest dinosaurs. You wouldn’t have marked them out as having a great future; they were just part of the land fauna that came out of the dark wet Carboniferous forests and began to make a living in the drier Permian era. If you had walked about in those forests you would probably have seen some of these beasts; they were rather slow and stupid, and perhaps they would have attacked you in a rather leisurely, crocodilian way. They weren’t as bright as crocodiles, though, nor as quick.
    At least as important as the dinosaurs was something far less filmic: soil. By then, soil had developed much of its complex ecosystem: at least fifty interacting species of bacteria, several very different kinds of fungi, insects, worms, protozoans during the Carboniferous. Soil had become a great basis for land plant growth. These forests were not starved, like the modern rainforests with their six inches of soil and no net oxygen production. Oh, no. The coal that we burn for heat and power was deposited in the Carboniferous, and every ton of coal that was deposited by the forests released more than twice its weight of oxygen into the air. Just as each ton of coal we burn now uses that much oxygen to make carbon dioxide again.
    In those days, plants grew fast, like modern herbs, but many of them grew big, as well. These plants didn’t have woody trunks, they were like great tree-ferns, and the creatures that fed on them made a big input into the terrestrial ecology. Until grass really took over, well after the demise of the dinosaurs, and made savannah and pampas the vegetable basis of the great herds of mammalian herbivores, this thick-soiled forest was the nutritional basis of all land fauna. And the ancient land fauna diversified in these forests, at their edges and in the swamps that they so often became.
    There were several early monitor-lizard-like carnivores in the late Carboniferous and early Permian, and their relatives came down to us as the lizards and the snakes. But to get a better idea of what these early reptiles really looked like, and behaved like, you should look at the tuatara, a genuine ‘living fossil’ from the islands off New Zealand’s coast. You can probably find it in your local zoo. It’s slow, it’s stupid, and any modern iguana or monitor can run rings round it (partly because it’s adapted to colder climes); but it’s a warning not to use any geckos or pythons or goannas you have known to inform yourself about early Permian lizards.
    Slow it may have been, but this lineage became extremely diverse. Its adaptive radiation, its explosive evolutionary diversification, simply swamped the other reptiles, turtles and mammal-like creatures. The early reptiles produced several kinds of marine lizards, of which plesiosaurs and ichthyosaurs were biggest and the most famous. However, another reptile lineage had slid back into the seas in the early Permian, creatures called mesosaurs that were related to turtles and probably lived on plankton that they sieved from the water, like many whales do now. The plesiosaurs, particularly some rather nasty short-necked crocodile-like ones called pliosaurs, were worthy opponents of the big sharks, and probably fed on mesosaurs. But the most successful marine reptiles, as fully adapted to marine life as whales and dolphins are now, were the ichthyosaurs. They flourished long before the famous dinosaurs on land, reaching their peak of size in the Triassic, as far in the past of the tyrannosaur as he was in our past. A length of 30 feet (10m) was common, and occasionally they reached 45 feet (15m).
    They were trumped in the seas by a later branch of the lizard lineage, the so-called fish-lizards or mosasaurs, which took over the seas just when the big brontosaurs and allosaurs were taking over the land. Some of them were only a foot (30cm) long, a few stretched to forty feet (12m). But all those films you’ve seen showing ichthyosaurs in the seas, tyrannosaurs on land are