The evolution of the amniotic egg gave vertebrates an adaptive advantage. It allowed them to exploit habitats beyond those of their waterbound amphibian ancestors. As a result, the first reptiles rapidly diversified into a variety of distinct families, each with its own peculiarities.
Vertebrate innovations seemed limitless as the Earth entered the Mesozoic Era. Even though the first reptiles were predators, many forms took advantage of the increasingly abundant plants in their world and developed highly specialized jaws, teeth, and chewing mechanisms to capitalize on this new food source. The reptilian backbone became stronger and more flexible, capable of supporting larger and faster species. The skull became lighter but also added important new nooks and crannies for the attachment of increasingly effective jaw muscles. The roof of the mouth developed a separate chamber, connected to the nostrils, to allow an animal to eat and breathe more efficiently at the same time. Legs became stronger and more flexible and were positioned further underneath the body, where they provided a more upright posture and the ability to run without dragging the belly. Some vertebrates developed longer hind limbs than forelimbs, the first step toward bipedalism in later vertebrates, including dinosaurs and mammals.
So numerous were the anatomical adaptations of the early reptiles that they diverged into widely different body plans and habitats on land and in the sea. By the Middle Triassic, the evolutionary stage was set for the rise of many new and distinct species. From the anapsids would arise one of the longest-standing branches of reptiles, the turtles and tortoises. The diapsids would offer up not only the dinosaurs, but also the first flying reptiles, the lizards, the snakes, and the birds. The euryapsid marine reptiles succeeded throughout the Mesozoic, staking their claim as the top predators of the seas. The synapsids, however, took an even more radical route away from their reptilian roots. After the demise of the sailbacked meat eaters and hulking hippopotamus-sized plant eaters of the Late Paleozoic, one branch of synapsids, the cynodonts, found a new formula for success in small size. While living in the shadows of larger and more formidable creatures, the cynodonts developed specializations that led to the appearance of the first mammals. The groundwork was laid for an extraordinary diversification of vertebrates throughout the rest of the Mesozoic Era.
This chapter reviews the rise of the four main groups of amniotic vertebrates that closed the Paleozoic Era and led to the rise of reptiles as the dominant life-form of the Mesozoic Era.
1. A key diagnostic trait used to classify amniotes is a feature of the skull called the temporal fenestrae; these were openings, or "windows," in the skull, just behind the orbits, on the sides of the skull, or temple region.
2. Amniotes can be placed into one of four groups based on skull design: anapsids (no temporal fenestrae); synapsids (one temporal fenestra on each side); diapsids (two temporal fenes-trae on each side); euryapsids (one highly placed temporal fenestra).
3. The earliest reptiles, including Hylonomus and Paleothyris, were anapsids. They were small insectivores that lived among the vegetative debris and tree stumps of the Middle Carboniferous forest. The anapsids also included the pareiasaurs, some of the earliest reptiles that were highly adapted for eating plants.
4. Early, successful synapsids included the sailback predator Dimetrodon and the plant-eating sailbacked Edaphosaurus. Later synapsids included the cynodonts, the ancestors of mammals.
5. Changes to the configuration of bones in the lower jaw of synapsids led to improved hearing and to a new form of flexible jaw joint that improved the way food could be chewed.
6. The diapsids are the most successful radiation of reptile forms. The diapsids are divided into two large groups: The archosaurs include crocodiles, birds, and extinct dinosaurs and pterosaurs. The lepidosaurs include lizards, snakes, two species of Sphenodon—the "living fossil" tuatara—and extinct mosasaurs.
7. The euryapsids included four groups of extinct marine reptiles, the placodonts, the nothosaurs, the ichthyosaurs, and the plesiosaurs.
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