Comparing Chordates

1. Comparing Chordates • A red-billed oxpecker perches on an impala • These chordates have a mutually beneficial relationship • Oxpeckers pick ticks and other external parasites off the impala, obtaining food while ridding their host of parasites

2. Comparing Chordates

3. Chordate Evolution • Ever since the first chordates appeared more than 500 million years ago, they have been evolving • During this continual process, chordates developed an incredible variety of adaptations • Some of these traits—scales or hair, for example—are relatively simple • Others—such as a four-chambered heart or an amniotic egg—are far more complex • All these adaptations were tested and shaped by natural selection

4. Chordate Origins • Much of what scientists know about the origins of chordates comes from studying the embryos of living organisms • Such studies suggest that the most ancient chordates were closely related to echinoderms • Do scientists know what these early chordates looked like? • Surprisingly, the answer is yes

5. Chordate Origins • The variety of fossilized organisms preserved in the rich Cambrian deposits of Canada's Burgess Shale includes a peculiar organism called Pikaia, shown in the figure at right • When Pikaia was first discovered, it was thought to be a worm • On closer inspection, scientists determined that Pikaia had a notochord—a flexible, supporting structure that is found only in chordates • Pikaia also had paired serial muscles that were arranged in a manner similar to those of today's nonvertebrate chordates, such as lancelets • On the basis of fossil evidence, scientists now classify Pikaia as an early chordate

6. Chordate Origins • To better understand the early evolution of chordates, biologists study a nonvertebrate chordate that is alive today—the tunicate • The tadpolelike larvae of tunicates are the simplest living animals to havea notochord, a dorsal hollow nerve cord, a tail that extends posterior to the anus, and pharyngeal pouches—key features common to all chordates • Today, biologists are studying the genes that control the development of these features

7. Pikaia • This is a reconstruction of Pikaia, a soft-bodied animal that lived during the Cambrian Period • Which features did Pikaia have that were characteristic of chordates?

8. Pikaia

9. The Chordate Family Tree • The chordate family tree has its roots in ancestors that vertebrates share with tunicates and lancelets • The cladogram in the figure at right shows chordate phylogeny—how the different groups of living chordates are related to one another and to their invertebrate ancestors • It also shows the evolution of distinctly vertebrate features, such as jaws and limbs • Notice that the fishes—from hagfishes to lungfishes—include six different groups with long and separate evolutionary histories • On the other hand, modern amphibians, reptiles, birds, and mammals share much more recent common ancestors • Where do extinct groups, such as dinosaurs, fit into the chordate phylum? • The answer may be found in the fossil record

10. Chordate Cladogram • The phylum Chordata includes both vertebrates and nonvertebrate chordates • All of these subphyla share a common invertebrate ancestor • This cladogram shows the phylogenetic relationship of modern chordate groups to that common ancestor • The different colored lines represent the traditional groupings of these animals, as listed in the key • The red circles indicate some of the important chordate adaptations • Such adaptations are the results of evolutionary processes, including natural selection

11. Chordate Cladogram

12. Evolutionary Trends in Vertebrates • The hard body structures of many vertebrates have left behind an excellent fossil record • As a result, scientists know a great deal about vertebrates' evolutionary history • In addition, scientists infer evolutionary trends by studying the characteristics of chordates living today

13. Adaptive Radiations  • The number of species within each chordate group has changed over geologic time • Look at the figure Chordate Cladogram again • The red circles in that figure represent the origin of certain adaptive features • For example, one notable event in chordate evolution was the development of jaws • Another event was the development of paired appendages, including pectoral and pelvic fin or limb girdles • Paired appendages allowed chordates, such as the salamander in the figure at right, to move more efficiently • Over the course of evolution, the appearance of new adaptations—such as jaws and paired appendages—has launched adaptive radiations in chordate groups • An adaptive radiation is the rapid diversification of species as they adapt to new conditions

14. Chordate Movement • Amphibians were the first chordates to have four limbs • Limbs allowed animals like this modern tiger salamander to crawl on land • A rapid increase in the number and diversity of land vertebrates followed the evolution of four limbs

15. Chordate Movement

16. Convergent Evolution  • Adaptive radiations sometimes produce species that are similar in appearance and behavior, even though they are not closely related • This trend is called convergent evolution • Convergent evolution occurred many times during chordate evolution when unrelated species encountered similar ecological conditions and evolved similar adaptations • For example, convergent evolution has produced flying vertebrates as different as birds and bats

17. Chordate Diversity • Living chordates are extremely diverse, as shown in the figure • Yet, the species of chordates that are alive today are a small fraction of the total number of chordate species that have existed over time • Today, vertebrates make up about 96 percent of all living chordate species and account for more than 50,000 species throughout the world • The six living groups of chordates are the: • Nonvertebrate chordates • Fishes • Amphibians • Reptiles • Birds • Mammals • Of these, the largest group by far is the fishes

18. Chordate Diversity • This pie chart shows the diversity of chorates • The area of each slice represents the relative number of living species in each group of chordates • The inner circle shows the six major chordate groups and gives the percentage of species contained in each • The outer circle breaks down each major group and shows the number of known species • Of the total number of fish species, what percentage is represented by the ray-finned fishes?

19. Chordate Diversity

20. Controlling Body Temperature • On a spring morning, after a cold night, a tortoise lies on a rock basking in the sun • Nearby, a snake slides out of its burrow beneath a rotting stump • In a tree overhead, a young robin puffs up its downy feathers • As you walk out of the water after an early swim, your skin gets goose bumps and you shiver • All these activities are examples of the different ways that vertebrates control their body temperature

21. Body Temperature and Homeostasis • Recall from Chapter 2 that many of the chemical reactions that are important in metabolism are influenced by temperature • For this reason, essential life functions can be carried out most efficiently when an animal's internal body temperature is within a particular “operating range” • For muscles to operate quickly and efficiently, for example, their temperature can neither be too low nor too high • If muscles are too cold, they may contract slowly, making it difficult for the animal to respond quickly to events around it • If an animal gets too hot, on the other hand, its muscles may tire easily and other body systems may not function properly

22. Body Temperature and Homeostasis • Because most chordates are vertebrates, and mechanisms for controlling body temperature are well developed among vertebrates, this section will focus exclusively on that group • The control of body temperature is important for maintaining homeostasis in vertebrates, particularly in habitats where temperature varies widely with time of day and with season • Vertebrates have a variety of ways to control their body temperature • All of these ways incorporate three important features: • Source of heat for the body • Way to conserve that heat • Method of eliminating excess heat when necessary • In terms of how they generate and control their body heat, vertebrates can be classified into two basic groups: • Ectotherms • Endotherms

23. Ectothermy • On cool, sunny mornings, lizards often bask in the sun • This doesn't mean that they are lazy! • A lizard is an ectotherm, which means that its body temperature is mainly determined by the temperature of its environment • Most reptiles, fishes, and amphibians are ectotherms—animals whose body temperatures are controlled primarily by picking up heat from, or losing heat to, their environment • Ectotherms often warm up by basking in the sun, and may cool down by seeking shelter in underground burrows

24. Ectothermy • Ectotherms have relatively low rates of metabolism when they are resting • Thus, their bodies do not generate much heat • When active, an ectotherm's muscles generate heat, just as your muscles do • However, because its body lacks effective insulation, the heat is lost to the environment fairly easily

25. Endothermy  • An endotherm is an animal whose body temperature is controlled from within • Birds and mammals are endotherms, which means they can generate and retain heat inside their bodies • Endotherms have relatively high metabolic rates that generate a significant amount of heat, even when they are resting • Birds conserve body heat primarily through insulating feathers, such as down • Mammals have body fat and hair for insulation • Mammals can get rid of excess heat by panting, as dogs do, or by sweating, as humans do

26. Comparing Ectotherms and Endotherms • In an absolute sense, neither endothermy nor ectothermy is superior • Each strategy has advantages and disadvantages in different environments • For example, endotherms move around easily during cool nights or in cold weather because they generate and conserve their own body heat • That's how musk ox live in the tundra and killer whales swim through polar seas • But the high metabolic rate that generates that heat requires a lot of fuel • The amount of food needed to keep a single cow alive would be enough to feed ten cow-sized lizards!

27. Comparing Ectotherms and Endotherms • Ectothermic animals, such as the gila monster, need much less food than similarly sized endotherms • In environments where temperatures stay warm and fairly constant most of the time, ectothermy is a more energy-efficient strategy • But large ectotherms run into trouble in habitats where temperatures get cold at night or stay cold for long periods, such as boreal forest biomes • It takes a long time for a large animal to warm up in the sun after a cold night • Most large lizards and amphibians live in warm areas such as tropical rain forest biomes

28. Evolution of Temperature Control • There is little doubt that the first land vertebrates were ectotherms • But there is some doubt as to when endothermy evolved • Although modern reptiles are ectotherms, some biologists hypothesize that at least some of the dinosaurs were endotherms • Others hypothesize that endothermy evolved a long time after the appearance of the dinosaurs, so that all the dinosaurs were ectotherms • Evidence suggests that endothermy has evolved more than one time • It developed once along the evolutionary line of reptiles that led to birds and once along the evolutionary line of reptiles that led to mammals

29. Form and Function in Chordates • The nonvertebrate chordates that are alive today represent a simple and ancient stage in the development of chordate body systems • However, the fact that the organ systems are simple does not mean they are inferior • After all, lancelets and tunicates have survived to the present day, so their body systems are well equipped to perform the essential functions of life

30. Form and Function in Chordates • Among vertebrates, organ systems exhibit a wider range of complexity than those of nonvertebrate chordates • Many adaptive radiations of vertebrates have produced a variety of specialized organ systems that perform essential functions and maintain homeostasis • The complexity of vertebrate organ systems can be seen in the different ways that vertebrates feed, breathe, respond, move, and reproduce

31. Feeding • Feeding and digestion help maintain homeostasis by providing the body with a continuing supply of needed nutrients • Most tunicates, and all lancelets, are filter feeders • These chordates remove small organisms called plankton from the water that passes through their pharynx • A few adult tunicates feed on deposited material from the surface of the sediments on which they dwell

32. Feeding • The skulls and teeth of vertebrates are adapted for feeding on a much wider assortment of foods, ranging from insects to large mammals, and from leaves to fruits and seeds • Some vertebrates—such as baleen whales, flamingoes, and manta rays—are filter feeders with sievelike mouth structures that enable them to strain small crustaceans and fish from the water • The long bill of the hummingbird and the narrow snout of the honey possum are both adaptations that enable them to feed on nectar • Other vertebrates, such as the crocodile are adapted to eating meat • Many mammals have sharp canine teeth and incisors that they use to tear and slice their food

33. Crocodile Feeding • The blunt, broad jaws and numerous peglike teeth of this crocodile help it catch large prey—such as zebra—even in thick vegetation • How do the mouth structures of a filter-feeding vertebrate differ from those of a carnivore like this reptile?

34. Crocodile Feeding

35. Feeding • The digestive systems of vertebrates have organs that are well adapted for different feeding habits • Carnivores such as sharks typically have short digestive tracts that produce fast-acting, meat-digesting enzymes • Herbivores such as cows, on the other hand, often have long intestines that harbor colonies of bacteria • These bacteria are helpful in digesting the tough cellulose fibers in plant tissues

36. Vertebrate Digestive Systems • The digestive systems of vertebrates are adapted for a variety of feeding modes • As you can see, these systems differ in their degree of complexity

37. Vertebrate Digestive Systems

38. Respiration • Chordates typically have one of two basic structures for respiration, or gas exchange • As a general rule, aquatic chordates—such as tunicates, fishes, and amphibian larvae—use gills for respiration • Land vertebrates, including adult amphibians, reptiles, birds, and mammals, use lungs • However, some animals “break the rules”: • For example, several fishes, such as lungfishes, have both gills and lungs

39. Respiration • Some chordates have respiratory structures in addition to gills and lungs • Many bony fishes, for example, have accessory organs for respiration, such as simple air sacs, that are derived from the gut • All lancelets and some sea snakes respire by the diffusion of oxygen across their body surfaces • Recall that diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration • Many adult amphibians use their moist skins and the linings of their mouths and pharynxes to respire by diffusion

40. Gills • The figure below shows how gills function in chordates • As water passes over the gill filaments, oxygen molecules diffuse into blood in tiny blood vessels called capillaries • At the same time, carbon dioxide diffuses from blood into the water

41. Gill Function  • Fishes and many other aquatic chordates use gills for respiration

42. Gill Function 

43. Lungs • Although the structure of the lungs varies, the basic process of breathing is the same among land vertebrates • Inhaling brings oxygen-rich air from outside the body through the trachea and into the lungs • The oxygen diffuses into the blood inside the lung capillaries • At the same time, carbon dioxide diffuses out of the capillaries into the air within the lungs • Oxygen-poor air is then exhaled

44. Lungs • As you move from amphibians to mammals, the surface area of the lungs increases • Observe this trend in the figure at right • The typical amphibian lung is little more than a sac with ridges • Reptilian lungs are often divided into a series of large and small chambers that increase the surface area available for gas exchange • In mammals, the lungs branch extensively, and their entire volume is filled with thousands of bubblelike structures called alveoli (singular: alveolus) • Alveoli provide an enormous surface area for gas exchange • This lung structure enables mammals to take in the large amounts of oxygen required by their endothermic metabolism • However, because air must move in and out through the same passageways, there is always stale, oxygen-poor air trapped in the lungs of mammals and most other vertebrates

45. Lungs • In contrast, in the lungs of birds, air flows in only one direction • A system of tubes in a bird's lungs, plus air sacs, enables this one-way air flow • Thus, gas exchange surfaces are constantly in contact with fresh air that contains a lot of oxygen • This supply of oxygen enables birds to fly at high altitudes, where there is less oxygen in the atmosphere than at lower altitudes

46. Vertebrate Respiration • Unlike most aquatic chordates, land vertebrates—like salamanders, lizards, birds, and primates—use lungs to breathe • A few aquatic chordates, such as sea turtles and marine mammals, use lungs as well

47. Vertebrate Respiration

48. Circulation • Circulatory systems maintain homeostasis by transporting materials throughout animals' bodies • The first chordates, like tunicates and lancelets of today, probably had simple circulatory systems • Tunicates have short, tubelike hearts with a simple pump but no true chambers • Lancelets have a fairly well-developed circulatory system but no specialized heart

49. Single- and Double-Loop Circulation • As chordates evolved, more complex organ systems and more efficient channels for internal transport developed • The figure at right shows the main transport systems in vertebrates • Those that use gills for respiration have a single-loop circulatory system • In this system, blood travels from the heart to the gills, then to the rest of the body, and back to the heart in one circuit

50. Vertebrate Circulatory Systems • Most vertebrates that use gills for respiration have a single-loop circulatory system that forces blood around the body in one direction • Vertebrates that use lungs have a double-loop system • The hearts of fishes have two chambers • Amphibians and most reptiles have three-chambered hearts • Crocodilians, birds, and mammals have hearts with four separate chambers