Section 4: Chordate Evolution

Key Ideas What are the key characteristics of chordates? What were the key evolutionary innovations in fish? What characteristics helped amphibians adapt to land? What major evolutionary innovations first appeared in reptiles?

Key Ideas, continued When did birds evolve, and what were the first birds like? When and from what group did mammals evolve?

Characteristics of Chordates A dorsal nerve cord single, hollow cord with nerves attached to it In vertebrates it develops into the spinal cord notochord is a rod-shaped supporting axis that develops along the back of the embryo In most vertebrates, it is present only in the embryo

Characteristics of Chordates, continued pharyngeal pouches develop into gill slits in aquatic chordate embryos In land vertebrates, they develop into different structures in the head and neck parathyroid gland and the inner ear postanal tail tail that extends beyond the anus In some chordates the tail is present only in the embryos

Visual Concept: Parts of a Chordate

Characteristics of Chordates, continued Invertebrate Chordates tunicates filter feeders live in the ocean do not have a nerve cord, notochord, or postanal tail Lancelets fossils have been found in rocks more than 530 million years old Older than any fish species Lancelets are also filter feeders

Visual Concept: Characteristics of Invertebrate Chordates

Characteristics of Chordates, continued The First Vertebrates The first vertebrates appeared about 500 million years ago and were fish that had neither jaws nor paired fins. The earliest fishes, called agnathans, did not have a backbone. The notochord allowed agnathans to swim, which provided a major advantage over other animals.

Evolution of Fishes Jaws and paired fins allowed fish to pursue and grasp prey. Spiny fishes, or acanthodians first appeared about 430 million years ago Spiny fishes had strong jaws with jagged, bony edges that served as teeth Jaws probably evolved from gill arches made of cartilage. Jawed fishes were better able to compete for food than jawless fishes were.

Visual Concept: Origin of Jaws

Evolution of Fishes, continued Spiny fishes internal skeleton composed mainly of cartilage with some bone scales contained small plates of bone placoderms evolved about 410 million years ago jawed fishes that had large heads covered with bony plates Pg 640 Sharks and bony fishes replaced almost all of these early fishes by 350 million years ago.

Visual Concept: Advantage of Paired Fins

Evolution of Fishes, continued first land vertebrates bony fins that functioned like primitive limbs allowed these fishes to move in shallow water. spent little time out of the water transition between fishes and amphibians.

Evolution of Amphibians The first amphibians appeared about 370 million years ago. Three major characteristics Lungs a more-efficient heart strong limbs had to stay close to water in order to keep their skin and eggs moist.

Evolution of Reptiles The first reptiles appeared about 320 million years ago, during a long, dry period in Earth's history. amniotic egg watertight egg that contains a large amount of yolk allowed reptiles to take over land could reproduce away from water. Because reptiles were better suited to dry conditions than amphibians were, reptiles replaced amphibians as the dominant land vertebrate within 50 million years.

Evolution of Reptiles, continued The first dinosaurs appeared about 235 million years ago. Dinosaurs lived through the Triassic Period, the Jurassic Period, and the Cretaceous Period. Dinosaurs became extinct at the end of the Cretaceous Period. Huge asteroid Volcanic eruptions

Evolution of Reptiles, continued Endothermic birds and mammals as well as smaller coldblooded reptiles and amphibians survived. Few large species survived.

Visual Concept: Comparing Endotherms and Ectotherms

Evolution of Birds The first birds appeared about 150 million years ago. The first birds evolved from small, meat-eating dinosaurs and had a skeleton that looked almost exactly like that of a dinosaur. The first fossil found to show the link between dinosaurs and birds was that of Archaeopteryx.

Evolution of Birds, continued Archaeopteryx long reptilian tail, teeth, and arms with fingers and claws solid bones not hollow bones of modern birds feathers on its wings and tail fused collarbone Birds survived and quickly diversified at the end of the Cretaceous Period. On some continents, a brief Age of Birds arose before mammals took over the land.

Evolution of Birds, continued Several features that evolved in birds helped birds fly. Light, hollow bones have reduced birds' weight. Birds have also gained huge flight muscles and a breastbone for the attachment of these muscles.

Evolution of Mammals The first mammals appeared about 220 million years ago. Therapsids extinct mammal-like reptiles brief period of dominance before the dinosaurs Early mammals were tiny and were probably insect-eating tree dwellers that were active at night.

Visual Concept: Therapsids

Evolution of Mammals, continued During the dinosaurs' dominance, mammals were a minor group that changed very little. In the Tertiary Period, many new groups of mammals appeared and many species grew to large sizes and took over the ecological roles once filled by dinosaurs.

Evolution of Mammals, continued Many large mammals, such as giant ground sloths and saber-toothed cats, became extinct during the last ice age (about 2 million to 10,000 years ago). Many of these large mammals disappeared because of changes in the climate, but hunting by early humans also played a role.