Overview of Vertebrate Evolution (p. 434)
20.1
20.2
20.3
The Paleozoic Era (p. 434; Figs. 20.1-20.4)
A. Geologic time is subdivided into large eras that are further subdivided into periods.
B. Periods are subdivided into even smaller divisions of time called epochs that are, in turn,
divided into ages.
C. Virtually all of the major animal groups originated in the sea at the beginning of the Paleozoic
era, during or soon after the Cambrian period (545 to 490 M.Y.A.).
D. However, many of the animal phyla that originated during the Cambrian period have no
surviving relatives.
E. The first vertebrates appeared about 500 million years ago; these were jawless fishes.
F. Invasion of the Land
1. Plants and fungi began to colonize land about 500 million years ago, and were followed
by the arthropods around 410 million years ago.
2. Third came the vertebrates during the Carboniferous period (360-280 M.Y.A.); the
amphibians were the first vertebrates to live on land, and reptiles appeared about 300
million years ago.
G. Mass Extinctions
1. Five mass extinctions have occurred during the history of life on earth.
2. The third and most drastic one occurred at the end of the Permian period, when an
estimated 96% of all marine species went extinct.
3. Mass extinctions left open many opportunities for the evolution of surviving organisms.
4. Little is known about the causes of the major extinctions; some scientists believe that the
Permian mass extinction was caused by an accumulation of carbon dioxide in the oceans
as a result of volcanism and the formation of the large “super-continent” Pangaea.
The Mesozoic Era (p. 436; Figs. 20.5-20.8)
A. The Mesozoic era (248 to 65 M.Y.A.) is divided into three periods: the Triassic, the Jurassic,
and the Cretaceous.
B. During the Jurassic, the super-continent of Pangaea began to break up, sea levels were rising,
and the world’s climate became warmer and wetter.
C. The Mesozoic era was a time of intensive evolution of terrestrial plants and animals.
D. The reptiles continued their success, and vertebrates came to be the dominate animals on the
earth; the flowering plants evolved in the early Cretaceous.
E. Reptiles diversified into many forms; from reptile ancestors, dinosaurs, mammals, and birds
evolved.
F. Dinosaurs and mammals evolved about 200 to 220 million years ago, but dinosaurs were
dominant, both in number and size.
G. Throughout the Mesozoic era, speciation continued, and the number of species present on
earth today is at an all-time high.
H. What Happened to the Dinosaurs?
1. At the end of the Mesozoic era, the dinosaurs disappear all at once, and were eventually
replaced by the mammals.
2. What caused the rapid extinction of dinosaurs 65 million years ago is not known.
3. Physicist Luis Alvarez hypothesized that a large meteorite hit the earth and sent up a
cloud of dust, severely limiting photosynthesis and killing off the dinosaurs.
4. During the past decade, a large crater was found off the coast of Yucatán.
5. When scientists radio-dated the impact date, it was found to coincide with the
disappearance of the dinosaurs.
The Cenozoic Era (p. 439; Table 20.1)
A. The early Cenozoic era was a time when the climate was warm and humid.
B. Then the world's climate went through a period of drying and cooling off, with the polar ice
caps and the major glaciers in place about 13 million years ago.
C. Forests receded rapidly during this time, and deserts formed, separating the vast equatorial
forests.
D. The development of such sharp differences in habitat led to rapid speciation.
E. Many large mammals evolved during the Cenozoic, including mammoths and saber-toothed
tigers.
The Parade of Vertebrates (p. 440)
20.4
Fishes Dominate the Sea (p. 440; Figs. 20.9–20.14; Table 20.2)
A. Over half of all vertebrates are fishes, the most diverse and successful of the vertebrate
groups.
B. Fish provided the ancestral base for invasion of land by amphibians.
C. Characteristics of Fishes
1. Fish use gills to extract dissolved oxygen from the water; oxygen gas diffuses from the
water into their blood.
2. All fishes have an internal skeleton with a vertebral column and protective cranium
(skull) made of bone or cartilage.
3. Fish have single-loop blood circulation.
4. Fish have nutritional deficiencies and are unable to synthesize certain amino acids; an
inability transferred to their descendants.
D. The First Fishes
1. The first vertebrates, the jawless fishes known as ostracoderms, evolved in the ocean 500
million years ago.
2. They were highly successful bottom-feeders, thriving for 100 million years until
predatory fishes evolved.
3. One group of jawless fishes, the agnathans, survive today as hagfish and parasitic
lampreys.
E. The Evolution of Jaws
1. Jawed fishes evolved around 410 million years ago.
2. Early jawed fishes had both jaws and paired fins, adaptations that allowed them to be
predators.
3. Spiny fishes were predators with small plates of bone on their scales; placoderms were
huge with massive heads and heavy, bony plates.
4. Early sharks and bony fishes evolved, and shared the seas with spiny fishes and
placoderms for over 150 million years.
5. Eventually, spiny fishes and placoderms went extinct, and the sharks and bony fishes
have dominated the seas for the past 250 million years.
F. Sharks
1. Sharks evolved into fast swimmers because their bony skeleton was replaced with a
lighter one made up of cartilage.
2. Sharks and their close relatives, the rays, are members of the class Chondrichthyes, with
750 species found worldwide.
3. Sharks are well-adapted to their predatory life due to sophisticated sensory adaptations.
4. Reproduction among the Chondrichthyes is the most advanced among fishes.
G. Bony Fishes
1. Bony fishes evolved a swim bladder to allow buoyancy.
2. Bony fishes, members of the class Osteichthyes, are the most numerous of any vertebrate
group, with at least 30,000 different species.
3. They have also developed a specialized lateral line system to help them detect changes in
the environment.
20.5
20.6
20.7
20.8
Amphibians Invade the Land (p. 444; Figs. 26.15, 20.16, 20.17; Table 20.3)
A. Amphibians likely evolved from a group of bony fishes called the lobe-finned fishes.
B. Characteristics of Amphibians
1. Characteristics of amphibians that allowed them to successfully invade the land include
legs, lungs, cutaneous respiration, pulmonary veins, and a partially divided heart.
C. History of Amphibians
1. For the first 100 million years of their evolution, amphibians were the dominant
vertebrates on land.
2. By the mid-Permian period, fully terrestrial groups of amphibians had evolved, some
very large and with extensive body armor.
3. The amphibians of today must reproduce in water and are themselves not completely
terrestrial.
Reptiles Conquer the Land (p. 446; Figs. 20.18, 20.19; Table 20.4)
A. Characteristics of Reptiles
1. Reptiles evolved from amphibians, and were more fully terrestrial.
2. Most reptiles lay a water-tight amniotic egg that contains a food source and several
protective membranes.
3. Retaining water is possible through the evolution of the reptilian dry skin that is covered
with tough, water-tight scales.
4. Thoracic breathing allows greater breathing capacity in reptiles.
5. The lungs, heart, and leg arrangement are also more efficient in reptiles
B. Around 7,000 species of reptiles exist today, including turtles, crocodiles, snakes, lizards, and
tuataras.
C. History of Reptiles
1. Reptiles evolved around 300 million years ago; three lineages formed.
2. In one lineage, predatory reptiles called the pelycosaurs evolved, with their long, sharp
teeth and sails to help control temperature; therapsids replaced the pelycosaurs as the
dominant land vertebrate, and one group eventually gave rise to the mammals.
3. A second lineage gave rise to the ancestors of turtles.
4. A third lineage of reptiles gave rise around 230 million years ago to the ancestors of
snakes, lizards, and tuataras, to marine reptiles (ichthyosaurs and plesiosaurs), and to the
archosaurs.
5. Early archosaurs resembled crocodiles, but later forms called thecodonts were the first
reptiles to be bipedal.
6. Early archosaurs gave rise to four groups: dinosaurs, many of which grew to immense
sizes; crocodiles, which are virtually the same now as they were then; pterosaurs, the
flying reptiles, and birds.
7. Dinosaurs were the most successful of all land vertebrates but became extinct around 65
million years ago, along with the marine reptiles and pterosaurs.
Birds Master the Air (p. 450; Figs. 20.20, 20.21; Table 20.5)
A. Birds evolved from dinosaurs 150 million years ago, but did not become numerous until the
pterosaurs went extinct.
B. Characteristics of Birds
1. Birds have feathers, hollow bones, and a well-developed collarbone for flying.
2. Birds are endothermic, able to maintain their own body temperature.
C. History of Birds
1. The oldest bird of which there is a clear fossil is Archaeopteryx, a reptilian creature with
feathered wings and tail and a wishbone, which dinosaurs lack.
2. Diverse birds were present by the early Cretaceous.
3. About 8,600 species of birds are in existence today.
Mammals Adapt to Colder Times (p. 453; Figs. 20.22-20.24 Table 20.6)
A. Characteristics of Mammals
1. Mammals are so named because they possess mammary glands that produce milk to
nourish their young; they also are the only vertebrates with hair.
B. History of the Mammals
1. This group evolved 220 million years ago, with the earliest mammals likely shrewlike,
nocturnal, insectivores.
2. Mammals were a minor group during the time of the dinosaurs, but rapidly diversified
when dinosaurs and many other land and marine animals became extinct 65 million years
ago.
3. Over 4,500 species of mammals exist now, half of them rodents, and one-quarter of them
bats.
C. Other Characteristics of Modern Mammals
1. Mammals, like birds, are endothermic.
2. Mammals have heterodont dentition, unlike reptiles, which have homodont dentition.
3. In most mammals, females carry their young internally, nourished by the placenta.
4. Keratin is the structural protein in hair, claws, fingernails, and hooves; horns are
composed of a bony core surrounded by a sheath of keratin, while antlers are made of
bone.
D. Today’s Mammals
1. The egg-laying mammals or monotremes have many reptilian features (such as laying
shelled eggs) as well as the defining mammalian features (hair and mammary glands).
2. The pouched mammals or marsupials differ from other mammals by their pattern of
embryonic development.
3. Most species of mammals today are placental mammals.