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Paleontology (continued) + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com age from Bavaria, Germany. These animals apparently evolved from small, feathered theropod dinosaurs, which were relatives of the large predatory tyrannosaurids. In terms of plant life, the Triassic was an age of ferns and early conifers, but the succeeding Jurassic Period is sometimes called the "Age of Cycads," a group of related gymnosperms with short, stocky trunks and palmate leaves. Later, midway through the Cretaceous Period, a very important new group of seed-bearing plants—the angiosperms or flowering plants— appeared (Fig. 8); their key breakthrough, double fertilization, led to accelerated reproduction. These plants coevolved with new groups of pollinating insects, including butterflies and moths. Dinosaurs continued to flourish during the Cretaceous and included the largest terrestrial carnivores of all time, such as Tyrannosaurus, which may have weighed 6 tons (5500 kg). In response to these predators, herbivorous dinosaurs evolved varied defenses, including armor in ankylosaurs and horns in the ceratopsians, such as Triceratops, the youngest known dinosaur. Dinosaurs, pterosaurs, and many marine animals (for example, swimming reptiles and ammonoids) became extinct at the end of the Cretaceous Period. A great debacle in the form of an asteroid impact occurred at this time, as evidenced by en- richment of iridium and shocked quartz in Cretaceous-Tertiary boundary clays about 65 million years ago, and this contributed to the demise of these organisms. The demise of dinosaurs in terrestrial ecosystems may have opened ecological space and cleared the way for an extremely rapid adaptive radiation of mammals during the Cenozoic Era, often called the "Age of Mammals." However, this era could equally be called the "Age of Angiosperms," the "Age of Frogs," the "Age of Snakes," or the "Age of Songbirds," as all of these groups also evolved and radiated during the Cenozoic. The evolution of C-4 plants, with their more efficient photosyn- thetic pathways, including the constantly growing grasses, was closely tied to the rise of ungulates, that is, hoofed, grazing mammals, during the mid-Cenozoic. These included cloven- hoofed, even-toed ungulates (pigs, camels, and cattle), as well as rhinos and horses, a group of odd-toed ungulates, whose evolution of long legs, reduced side toes, and abrasion-resis- tant high-crowned teeth is well documented. Finally, about 6 million years ago, the hominid family evolved in Africa. Fossils document the early members of the genus Australopithecus, which gave rise to the larger brained genus Homo about 2 million years ago. Subsequently, Homo sapiens evolved about 200,000–100,000 years ago also in Africa, and then spread across the Old World and eventually across the Ber- ing land bridge into North America. This immigration may have ultimately led to a wave of extinction, especially of large mam- mals such as mammoths and mastodons, perhaps as a result of overhunting by the one species (humans) whose activities continue to alter the terrestrial ecosystem in many ways.

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