In order to follow the evolution of the other major group of reptiles, the diapsids, we must return to the Triassic. Early diapsids had evolved into small or medium-sized carnivores, but had not been able to compete successfully with mammallike reptiles. However, following the end-Permian extinction event, they radiated significantly and during the Triassic replaced synapsids by active competition in almost all ecological niches. This successful group of diapsids is called archosaurs. It may be that they were able to outcompete the mammallike reptiles because they developed a range of solutions to Carrier's constraint. Many evolved a bipedal stance, and from that an erect gait that permitted them to move and breathe at the same time. Evidence suggests that primitive archosaurs may have been at least partially warm blooded. Endothermy requires a lot of energy, and it reduces the number of predators able to live on a given number of prey animals. The predator to prey ratios of Triassic ecosystems are intermediate between modern warm- and cold-blooded predators.
Most archosaurs became extinct in the late Triassic, and their ecological niches were filled by their descendents, the dinosaurs. At the same time the first flying vertebrates appeared, the pterosaurs. Dinosaurs evolved from small, carnivorous archosaurs and by the end of the Triassic they had radiated into three main groups: the theropods, which are primitive in some ways, the sauropods, and the bird-hipped dinosaurs or ornithischians (Fig. 11.12).
The Mesozoic is often called the "Age of the dinosaurs", but it might better, though less catchily, be called the "Age of the diapsids". Marine reptiles in the ocean, pterosaurs in the air, and dinosaurs on the land dominated the middle and upper parts of all ecosystems, representing the top predators and often the lesser predators, omnivores, and herbivores as well. More has been written and is popularly known about these groups than about any other fossils. It is easy to forget their general rarity, and the major uncertainties that still exist about their life habits and appearance.
Pterosaurs ranged in size from a wingspan of a few centimeters to over 15 m. They were active flyers, and had narrow, membranous wings attached to a modified fourth finger and probably to their thighs. The aerodynamics of their wings suggests that they were predominantly gliders and soarers, but rarer species were adapted to other flying habits. They were covered in hair, and their highly energetic lifestyle means that they must have been warm blooded. Most pterosaurs appear
to have been coastal or marine predators, although this may well be an artifact of preservation, with these being the most likely environments in which to preserve a skeleton. In common with dinosaurs, they became extinct at the end of the Cretaceous, though the physiologically similar birds survived.
Marine reptiles appeared in the Triassic, and radiated through much of the Mesozoic. Some groups that took to the water are familiar today, such as turtles and crocodiles. Others are extinct, and the most important of these are the ichthyosaurs and sauropterygians, including the plesiosaurs and pliosaurs. The origin of the ichthyosaurs is unknown; they are a highly evolved, or derived, family. They had large eyes and sharp, conical teeth, and well-preserved specimens often contain fish scales or belemnite hooks in their stomachs. Rare specimens have been preserved in the act of giving birth, showing that they bore live young. Sauropterygians evolved from the line leading to modern reptiles and snakes in the Permian. They evolved into marine predators that swam using their highly modified limb paddles. Some may have been specialized bottom feeders, while others were predatory on smaller marine reptiles. Along with pterosaurs and dinosaurs, most marine reptiles became extinct at the end of the Cretaceous.
The most famous Mesozoic diapsids are the dinosaurs. Theropod dinosaurs appeared in the late Triassic and quickly evolved to a large size, with species exceeding 9 m in length by the early Jurassic. All theropods were predators, and the group includes the Cretaceous dinosaur, Tyrannosaurus rex. Theropods evolved into birds during the Jurassic.
Ornithischians, or bird-hipped dinosaurs, also evolved from theropods during the Jurassic. They were all herbivores, and included armored forms such as Triceratops and the stegosaurs, as well as the highly specialized iguanodonts and duck-billed dinosaurs. These dinosaurs had highly modified teeth and jaws that allowed them to chew food thoroughly before it was swallowed, significantly shortening digestion times. Their name derives from a modification of the pelvic region, which is shown in Fig. 11.13.
(a) Lizard-hipped pelvis (b) Bird-hipped pelvis
(a) Lizard-hipped pelvis (b) Bird-hipped pelvis
Sauropods also evolved from the theropod lineage and share with them the more primitive lizard-hipped pelvic pattern. They were vegetarians like the bird-hipped dinosaurs, but evolved a different strategy for acquiring and processing the tough vegetation of the Mesozoic. These are the group of dinosaurs that evolved to extremely large size, often in excess of 20 m in length and 50 tonnes in weight.
Dinosaurs are found worldwide, including from high latitudes, where it would have been dark in winter for several months. Even in the warm global conditions of the Mesozoic, this would have necessitated a warm-blooded habit. In addition, dinosaurs were active predators and herd dwellers, and the predator: prey ratios of dinosaurs are consistent with endothermy. Evidence of long-term protection of the eggs and young, and complicated adaptations for display and communication, suggest that dinosaurs also evolved complicated social patterns in a wide variety of different environments.
The reason for the extinction of the major diapsid groups at the end of the Cretaceous is unknown. A catastrophic meteorite impact, probably onto the Yucatan region of Mexico, may have been the cause of this mass extinction. However, there is also evidence for vast igneous eruptions in India close to the Cretaceous-Tertiary boundary that might have been responsible for significant climatic change. Most puzzling is why dinosaurs, marine reptiles, and pterosaurs perished when birds and mammals survived.
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