Adaptations of Vertebrates • Vertebrates are chordates with a backbone. • Even though many vertebrates appear different, Introduction to Vertebrates they are very much alike internally, indicating that in the distant past they had a common ancestor. • Present-day differences in vertebrates reflect their different evolutionary paths. © Lisa Michalek Adaptations of Vertebrates Adaptations of Vertebrates • The backbone provides support for and protects a dorsal nerve cord. • Vertebrates are chordates with a backbone. • It also provides • The individual segments that make up the backbone are called vertebrae. a site for muscle attachment. • In most vertebrates, the backbone • These functions completely replaces allowed vertebrates the notochord found to grow larger than in invertebrate chordates. their invertebrate ancestors. • In addition to a backbone, vertebrates have a bony skull that encases and protects their brain. Adaptations of Vertebrates Adaptations of Vertebrates • The tissues of vertebrates compose organs. • Vertebrates share a number of other characteristics, including segmentation, bilateral symmetry, and two pairs of jointed appendages, such as limbs or • The organs compose organ systems. fins. • Vertebrate organ systems tend to be more complex than the organ systems • They exhibit cephalization and have complex brains and sense organs. found in invertebrates. • All vertebrates have a true coelom and a closed circulatory system with a chambered heart. The First Vertebrates The First Vertebrates Unlike most of the fishes that we are now familiar with, the earliest fishes, The first chordates evolved about 550 million years ago. • • called agnathans, had neither jaws nor paired fins. • At that time, many different groups of organisms appeared in the shallow Agnathans did have seas that covered a large portion of Earth s continents. • ’ a backbone, which • According to the fossil record, the first vertebrates were fishes. provided a central axis for muscle attachment. • As their muscles pulled against the backbone, the agnathans propelled themselves along the ocean bottom. The First Vertebrates Evolution of Fishes • Within another 50 million years, jawless fishes had diversified into a great • About 430 million years variety of species. ago, the acanthodians (spiny fishes) appeared. • The major group was the ostracoderms which had • Acanthodians had strong jaws with jagged, bony edges that served as teeth, primitive fins and massive enabling them to hold onto prey. plates of bony tissue on their body. • The development of jaws in fishes was a key evolutionary innovation. • Jawless fishes dominated the oceans for about 100 million years, until they • Jaws are thought to have evolved from gill arch supports made of cartilage were replaced by new kinds of fishes that were hunters. (a lightweight, strong, flexible tissue). Evolution of Fishes Evolution of Fishes About 20 million years after the acanthodians appeared, the placoderms The spiny fishes had internal skeletons of cartilage, although some fossils • • evolved. indicate that their skeletons also contained some bone. • Placoderms were jawed fishes with massive heads armored with bony plates. • Their scales also contained small plates of bone. • By the end of the Devonian period, almost all of the early fishes, including the placoderms, had disappeared. • After dominating the seas for almost 50 million years, they were replaced by swifter swimmers, the sharks and bony fishes. Sharks and Bony Fishes Sharks and Bony Fishes •By the end of the Devonian period, almost all of the • Sharks have a skeleton composed of cartilage. early fishes had disappeared. • Calcium carbonate is deposited in the outer layer of the cartilage. •At about the same time • A thin layer of bone covers this reinforced cartilage. (400 million years ago), • The result is a very light but strong skeleton. sharks and bony fishes appeared. •Sharks and bony fishes, which are superior swimmers, thrived in the Devonian seas. •Most have streamlined bodies that are well adapted for rapid movement through the water. Sharks and Bony Fishes Modern Fishes • Bony fishes have a skeleton made of bone, which is heavier and less flexible • Today there are more fishes than any other group of vertebrates. than cartilage. • Today’s fishes belong to one of three major groups: the agnathans, the • Bony fishes have a swim bladder, which compensates for this increased cartilaginous fishes and the bony fishes. weight. • The agnathans • This gas-filled sac (hagfishes and buoys them in the the lampreys) water, just as an resemble the air-filled balloon early jawless fishes. buoys a swimmer. Modern Fishes Evolution of Amphibians • The first cartilaginous fishes (sharks) and the bony fishes evolved at about the • The first group of vertebrates to live on land was the same time, 400 million years ago. amphibians, which appeared about 370 million years ago. • These two groups of fishes likely evolved from the same early, jawless fishes that gave rise to the acanthodians and the placoderms. • Amphibians probably shared • The shark’s a common ancestor with relatives, the modern lungfishes and skates and rays other lobe-finned fishes. evolved later. • The pattern of bones in an amphibian’s limbs bears a strong resemblance to that of a lobe-finned fish. • While several species of lobe-finned fishes exist today, those species thought to be ancestral to amphibians are extinct. The ‘Age of Amphibians’ Evolution of Amphibians • Although amphibians first appeared in the Devonian period, they increased greatly in numbers during the Carboniferous period. • During this time (The age of Amphibians) the number of amphibian families increased from 14 to about 34. • By the late Carboniferous period, much of what was to become North America was covered by low-lying tropical swamplands. • Amphibians thrived in this moist environment, sharing it with early reptiles. The ‘Age of Amphibians’ Adaptations for Life on Land • In the Permian period that followed, amphibians reached their greatest • Life on land is quite different from life in the water. Therefore, a number of diversity, increasing to 40 families. major adaptations allowed some species to successfully invade the land. • In the early Permian period, a remarkable change occurred among – Legs. Legs support the body’s weight as well as allow movement from place to place. amphibians, many of them began to leave the marshes for dry uplands. – Lungs. The delicate structure of a fish’s gills depends on water for support. On land, lungs carry out gas exchange. • By the middle Permian, – Heart. Walking on land requires a greater expenditure of energy than swimming. Land 60 percent of all animals tend to have higher metabolic rates than aquatic animals. This requires greater amphibian species amounts of oxygen. The structure of the vertebrate heart allows oxygen to be were living in dry delivered to the body efficiently. environments. Modern Amphibians Modern Amphibians • All of today’s amphibians are descendants of the amphibians that survived into the Jurassic period. • The middle Permian period marked the peak of amphibian success. • They are found in aquatic and moist habitats throughout the temperate and • By the end of this period a tropical regions of the world. new kind of vertebrate, a reptile called a therapsid, • Frogs and toads make up had become common and the largest and most familiar began to replace the amphibians. group of modern amphibians. • By the end of the Triassic period that followed, there were only 15 families • Salamanders and newts are of amphibians. far less numerous. • Caecilians account for less than 1 percent of today’s amphibian species. Evolution of Amphibians Early Reptiles • Fluid loss is a problem for all terrestrial animals that live on land. • The adaptations that permitted amphibians to live on land further developed in reptiles. • Two very important adaptations for terrestrial life evolved in reptiles. • Reptiles were the first animals to have skin and eggs that are both almost watertight. Early Reptiles Evolution of Terrestrial Vertebrates • When reptiles first evolved, about 320 million years ago, Earth was entering a long, dry period. • Early reptiles were better suited to these conditions than amphibians were, and the reptiles quickly diversified. • Within 50 million years, reptiles had replaced amphibians as the dominant terrestrial vertebrates. Evolution of Dinosaurs Evolution of Dinosaurs • One factor that affected dinosaur evolution was the movement of • Beginning about 235 million years ago, dinosaurs dominated life on land for the continents, which altered Earth’s climate. roughly 150 million years. • When the dinosaurs first appeared, all of Earth’s landmasses were • They evolved from the thecodonts, an extinct joined in a single supercontinent called Pangaea. group of crocodile-like reptiles. • There were few mountain ranges over this enormous stretch of • During their long history, dinosaurs changed a great deal because the world land, and the interior was dry. they inhabited changed. • Coastal climates were much the same all over the world, quite • Dinosaurs represent a long parade of change and adaptation. warm, with a dry season followed by a very wet rainy season. • As Pangaea broke apart, the climates of the various landmasses varied. • Some species of dinosaurs could not adapt and became extinct, while new kinds flourished. Breakup of Pangaea Triassic Dinosaurs • The oldest known dinosaur fossils are in rocks from the early Triassic