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6 Animal Evolution (continued) Fish evolution The jawless fish, known as Agnatha, had a sucking-rasping mouth apparatus rather than true jaws. They enjoyed great suc- cess from the Late Cambrian until the end of the Devonian. Most were heavily armored, although a few naked forms are known. They were weak swimmers and lived mostly on the bottom. The modern parasitic lampreys and deep-sea scavenging hagfish are the only surviving descendants of these early fish radiations. Members of a new type of vertebrate, the Gnathostomata, characterized by true jaws and teeth, arose during the Middle to Late Silurian. They constitute the great majority of fishes and all tetrapod vertebrates. The jaws are modified elements of the front parts of the gill apparatus, and the teeth are modified bony scales from the skin of the mouth. With the development of jaws, a whole new set of ecological opportunities was open to the vertebrates. Along with this, new swimming patterns ap- peared, made possible by the origin of paired fins, forerunners of which occur in some agnathans. Four groups of fishes quickly diversified (Fig. 2). Of these, the Placodermi and Acanthodii are extinct. Members of the Placodermi were heavily armored fishes, the dominant marine carnivores of the Silurian and Devonian, rivaled only by the large eurypterid arthropods. Members of the Acanthodii were filter-feeders, mostly of small size. They are possibly related to the dominant groups of modern fishes, that is, the largely car- tilaginous Chondrichthyes (including sharks, rays, and chimae- ras) and the Osteichthyes (the higher bony fishes). These also arose in the Late Silurian, but diversified later. Conquest of land The first land vertebrates, the Amphibia, appeared in the Late Devonian and were derived from an early group of osteichthy- ans called lobe-finned fishes, of which two kinds survive today: (1) the Dipnoi or lungfishes, and (2) the crossopterygian coel- acanth Latimeria. They were lung-breathing fishes that lived in shallow marine waters and in swamps and marshes. The first amphibians fed and reproduced in or near the water. True land vertebrates, the Reptilia, with a modified (amniote) egg that could survive on land, probably arose in the Mississippian. Reptile radiations By the Middle Pennsylvanian, a massive radiation of reptiles was in process. From this radiation, one can trace the line lead- ing to mammals as well as the lineages of the great reptiles that dominated the Mesozoic Era. The most prominent reptiles belong in the Diapsida: dinosaurs, lizards and snakes, and pterosaurs (flying reptiles). The birds, Aves, which diverged from the dinosaur radiation in the Late Triassic or Early Jurassic, are considered to be feathered dinosaurs, and thus members of the Diapsida, whereas older authorities prefer to treat them as a separate class. In addition, there were several Mesozoic radia- tions of marine reptiles, including ichthyosaurs and plesiosaurs. Turtles (Chelonia) first appeared in the Triassic. Mammalian origins The line leading to mammals can be traced to primitive Penn- sylvanian reptiles, that is, the Synapsida, which diversified and spread worldwide during the Permian and Triassic. The first true mammals, based on characteristics of jaw, tooth, and ear structure, arose in the Late Triassic. Derived mammals, marsupi- als (Metatheria) and placentals (Eutheria), are known from the Late Cretaceous, but mammalian radiations began only in the early Cenozoic. By the end of the Eocene, all the major lines of modern mammals had become established. Mammals are easily separated into distinct groups (orders), but their relation- ships are not easy to discover from fossil records because of the explosion of mammalian evolution in the early Cenozoic. Molecular analyses (blood proteins, deoxyribonucleic acid, and ribonucleic acid) of living mammals show that the most primi- tive group of placentals is the edentates (sloths, armadillos, and anteaters). An early large radiation included the rodents, primates (including monkeys, apes, and humans), and bats, possibly all closely related to the insectivores and carnivores. The newest radiations of mammals include the elephants and sea cows, whereas the whales are related to the artiodactyls (cattle and camels). Because of biogeographic isolation, marsu- pials came to flourish in Australia and South America, whereas the placentals diversified widely in Eurasia and North America. Primates and humans Among the early and primitive lines of placental mammals were the Paleocene and Eocene plesiadapoids, from which the primates arose. They were mostly nocturnal, insect- and fruit- eating animals with forwardly directed eyes and a locomotor system well developed for an arboreal life. The living primates consist of prosimians; tarsiers; lemurs; monkeys and anthro- poids; apes; and humans and near relatives, the hominids. During the Oligocene, monkeys diverged into Old World and New World lines. From the former, the apes arose in the late Oligocene, including the gibbons, orangutans, chimpanzees, and gorillas. The human subspecies Homo sapiens sapiens diverged from the apes about 5 million years ago. The closest living relatives among the apes are chimpanzees and gorillas. The earliest known hominids consist of fossils of the genus Australopithe- cus, excavated from rocks formed about 4.2 million years ago in eastern Africa. Several distinct lines of Australopithecus, under + ward ' s science