The Aquatic Vertebrates

Transcription

1 The Aquatic Vertebrates Say the word "animal" and most people will think first of a vertebrate. While some invertebrates (especially insects and parasites) have great impact on humans, almost all the animals that humans use (as food or coworkers or pets) are vertebrates. The Chordate Features Despite the special status of vertebrates, remember to relate them to the rest of the animal kingdom. Remember that vertebrates are chordates, and that all chordates are eucoelomate deuterostomes with five common features that are present at least in the embryonic stage: The Vertebrate Features In addition to these features, vertebrates: 1. a notochord 2. a dorsal hollow nerve cord 3. a pharynx with gill slits 4. a postanal tail, and 5. an endostyle, which became the thyroid gland. 1. are highly cephalized, with a brain and a concentration of sense organs at the head end; 2. are segmented; 3. have a backbone (organized around and replacing the notochord) and a skull; 4. have four limbs (either fins or legs); 5. have some other less obvious features, like a closed circulatory system using hemoglobin to carry oxygen, either gills or lungs, and compact kidneys to purify their blood. There are 50,000 species of living vertebrates, recently reorganized into nine classes: Myxini Petromyzontida Chondrichthyes Actinopterygii Actinistia Dipnoi Amphibia Reptilia Mammalia hagfishes (jawless) lampreys (jawless) cartilaginous fish (sharks, etc.). ray-finned fishes (most bony fish) lobe-finned fishes (coelacanths) lungfishes frogs, toads, newts and salamanders snakes, lizards, turtles, crocodiles, and birds mammals As we'll see, some experts no longer consider the Myxini to be vertebrates. So that this lesson will not be overwhelming, we will only do the first seven classes (the "aquatic vertebrates") in this module. Also, we will postpone a discussion of evolutionary relationships until the end of the module. General Vertebrate Physiology Before turning to the various groups, a few general descriptive comments are useful. Air is a far more oxygen-rich medium than water is. Air contains about 270 mg/l of oxygen, and under the best conditions water only contains about 10 mg/l. In addition, oxygen-depleted waters are common in nature. Also, water is 1000 times denser and harder to move than air. Therefore, active aquatic organisms need efficient gas exchange structures.

2 Fish, larval amphibians, and some adult amphibians have highly efficient gills. These active animals require more oxygen than sessile animals do to fuel muscle action and to dispose of carbon dioxide and other byproducts of metabolic activities. Most amphibians and some fish have simple lungs for exploiting much more oxygen-rich air. In addition, all vertebrates have an efficient closed circulatory system. The fishes and the amphibians, like the lancelets, do not reproduce asexually by budding. There are species of fish and amphibians, however, in which the females can reproduce parthenogenetically (from unfertilized eggs). Most fishes and amphibians rely on external fertilization. There are a several variants of dioecious sexual reproduction. Most fish and amphibians are oviparous, laying eggs that hatch outside the female's body. Some sharks and amphibians are ovoviviparous. The oviduct produces eggs, but then these hatch inside the uterus, and "live" young are passed out the cloaca. Finally, a few sharks are authentically viviparous (just like mammals)--the fertilized egg attaches to the uterine wall with a placenta and is nourished by the mother's body. Diversity The Myxini and Petromyzontida. Hagfish and lampreys (respectively) are the only vertebrates without jaws, but they are not closely related. As a matter of fact, while lampreys are definitely vertebrates, hagfish have no sign of a vertebral column, and have only a notochord. Some experts say they should not be considered vertebrates, but molecular data argues that they might have had vertebrae and then lost them. There are 75 or so species of jawless fish (or agnathan). Lampreys are mostly predators and parasites of living fish. They attach to the side of a fish, rasp a hole in the flesh, and suck out blood and other bodily fluids. However, some lampreys have a far less violent lifestyle--their lancelet-like larvae filter-feed until adulthood, and then the adult reproduces and dies without feeding. Some hagfishes feed on sick or dead fish while others eat marine invertebrates. Hagfishes will attack fish trapped in nets and are often pulled in by fishermen with the rest of the catch. The hagfish then secrete large quantities of obnoxious slime, giving rise to its other name--"slimy hag." The lampreys have naked, eel-like bodies, cartilaginous skeletons, no paired fins, and vertebrae consisting of tiny bits of cartilage. Hagfish are so flexible that they can literally tie themselves into knots. Lampreys are found in freshwater as well as marine environments while hagfish are strictly marine. One of the earliest vertebrates was a jawless fish called an ostracoderm. However, it was unlike the modern jawless fish because it was heavily armored and fed by stirring up bottom sediment and filtering the particles through its gills. We will talk about ostracoderms in the "Evolution" section at the end of the module. The Chondrichthyes. The 850 species of cartilaginous fishes include the sharks, rays, skates, sawfishes, and ratfishes. All members of this class have skeletons of cartilage (NOT bone) and five to seven gill slits. Most sharks must swim forward constantly to ventilate their gills, but some others (and all rays) can pump water over their gills even if they are lying on the bottom. One feature possessed by all the shark-like fishes and a few other fishes is the spiral valve. This structure is a fold of tissue that greatly increases the surface area in the hindgut, allowing for increased efficiency in digestion. Most of the members of this class are highly specialized predators with streamlined bodies and fins that enhance stability. However, because the fins can't move very much, the cartilaginous fish are not very maneuverable. Their cartilage skeleton is heavier than bone and not as strong, but has the advantage that it is somewhat flexible. The cartilaginous fish also have no swim bladder, and will sink to the bottom if they stop swimming because they are denser than water. The oil stored in a shark's liver decreases the body's density. You probably remember the large, oily liver of the shark you dissected; a shark's liver may account for 30% of its body's weight.

3 You may remember from lab that shark skin is covered with teethlike placoid scales. The shark's teeth are also scalelike, and grow in rows. When one tooth is lost, another replaces it. Most sharks are predators and have an acute sensory system. A hunting shark relies mainly on its sense of smell. The blacktip reef shark can detect a part of fish meat in 10 billion parts of seawater. As the shark gets closer than about 15 m, it may also rely on vibrations picked up in the water by the lateral line system, and on low-frequency sounds detected by its inner ear. Shark vision is good, but is mostly used only at very short ranges, perhaps 3 m or less. At very close range, the shark uses organs called the ampullae of Lorenzini to detect electrical weak fields. The instinct to attack an electrical field is so strong that sometimes sharks will attack metal objects (even boats and submarine cables) because corroding metal produces an electrical field. After it reaches its prey, a shark attacks ferociously. Humans bite down with a jaw pressure of about 150 pounds per square inch; a blacktip reef shark attacks its prey with 44,000 pounds per square inch. After locking its jaws, it shakes its whole body to rip out a huge chuck of flesh with its serrated teeth. Don't think that all cartilaginous fish are fearsome carnivores, however. Of 350 species of sharks, only 30 species are known to have attacked humans. Many sharks are less than 15 cm long, and others are sluggish bottom dwellers. The whale shark is the largest living fish: almost fifty feet in length with a weight 13 tons, and it filters plankton from the sea. Also, rays are quiet bottom dwellers that feed mostly on mollusks. They may have venomous spines in their tails, but these are strictly defensive weapons. One final word on sharks: each year, many more people kill sharks than sharks kill people. Sharks are not evil killing machines. Like other large predators (lions, bears and even alligators), they have a vital place in nature, but probably cannot survive in the long term unless we go out of our way to protect them. The Actinopterygii. Almost all familiar bony fish belong to the Actinoptyerygii. The 49,000 species of "ray-finned" bony fish are the most numerous and diverse of all living vertebrates. Bony fish range in size from the "stout infantfish" whose adult size is only 0.3 inches long, to the ocean sunfish, that can be 13 feet long and weigh over 3,000 pounds. The bony skeleton of ray-finned fish is light and strong, and their jointed fins provide superb maneuverability. Their skin is covered with bony dermal scales, and their gills are covered by a bony plate called the operculum, and can be ventilated even if the fish is at rest. They can also precisely regulate their buoyancy with their swim bladder. The fanlike fins are supported by flexible elements called fin rays, and sometimes these are modified into venomous spines for defense. Many ray-finned fish have well developed eyes and a keen sense of smell. Also, like sharks, they have a row of sensitive pits along their sides called the lateral line system. This sensory system, closely associated with the ear, can detect the position and motions of nearby prey and other fishes as well as the relative motions of their own bodies in the water. The Actinistia and Dipnoi. A relatively few bony fish have fleshy fins rather than fins with fin rays. These are the lobe-finned fishes (the Actinistia) and the lungfishes (the Dipnoi). The lungfish are restricted to a few species that live in temporary ponds and streams in the southern hemisphere. As their home waters contract, they come to the surface to gulp air. Just before the water disappears, they burrow into the mud to await its return. The coelacanth is the only species of lobefin left alive. It has no lungs (although its ancestors did), and is a sluggish, deep-sea fish. It is uncertain whether the lobefins or the lungfishes were the first to set foot on land, but mitochondrial DNA evidence seems to favor the lungfishes for this important role. Amphibian legs probably came from lungfish fins.

4 The Amphibia. We are now at a crossroads of development the invasion of land by the aquatic vertebrates. A terrestrial life imposed numerous changes on the circulatory, respiratory, and skeletal systems. Amphibians have nostrils for breathing with the mouth closed, and most of them have lungs even though 70% of gas exchange occurs through the moist skin. Gills are present, but only in the larval stages in most cases. The two-chambered heart of fish gave way to the three-chambered amphibian heart, and the beginning of separate pulmonary and systemic circuits for the blood. Finally, the flimsy skeleton of fish was replaced with a more substantial one because water could not longer be depended upon to bear most of the body's weight. Although amphibians have become at least semi-terrestrial creatures, one vast habitat remains uncolonized: there are no marine amphibians. All amphibians are carnivorous in the adult stage. In the larval stage, most are herbivorous. This change in food sources may limit competition between adults and larvae, just as a similar change in food between adults and larvae did in the insects. Although some frogs, salamanders and caecilians are exclusively terrestrial, reproduction ties all amphibians to fresh water, or at least moist conditions like decaying leaf litter, because the amphibian egg cannot withstand drying. Some amphibians get around this limitation in remarkable ways, even to the extent of incubating eggs in their mouths or stomachs or keeping them in the oviducts until after hatching. Because amphibians are soft-bodied, often slow, and have no physical defenses like stingers or powerful jaws, some rely on deposition of poison in their skins for protection from predators. The "poison arrow frog" of South America is a well-known example. Columbian Indians can poison the tips of their blowgun darts by merely rubbing them across the frog's back. Many poisonous amphibians are brightly colored to warn away predators; this is called aposematic coloration. After all, the frog would "rather" keep predators away than kill them and die in the process. There are three living orders in the class Amphibia: Caudata Anura Gymnophiona salamanders frogs and toads caecilians The salamanders are probably closest to the early amphibians. We are very close to their world center of diversity; Tennessee has more species than Europe and Asia combined. The frogs, with their strong rear legs, have departed from this early model, and are much more successful (3,500 species as opposed to only 350 for the salamanders). Frogs and toads are found on every continent except Antarctica, and inhabit not only land and fresh water, but also trees. Numerous adaptations to their leaping/hopping form of locomotion can be seen in their skeletons; for example, their lower vertebrae have been fused to create a more rigid pelvic girdle. One of their most successful adaptations is ability to capture insects by flicking out a sticky tongue. Frogs and toads are also known for their complex social structures, maintained by vocalizations to attract the opposite sex and warn away rivals. Ten or twelve species may be calling at once in a pond, but each species' sensory system picks up mainly the frequencies and tones used by its own species. The purpose of the call, the sex, and the size of the caller can all be determined from the call. Caecilians are highly modified and specialized tropical amphibians. They have lost their legs and are either aquatic or burrowers in moist soil. Evolution As the "Invertebrate Chordate" module said, the earliest, tunicate-like chordates were filterfeeding in Cambrian seas 590 million years ago. Eventually, a tunicate larva became sexually mature and permanently mobile. By a series of unknown steps, by 500 million years ago this evolved into an

5 armored, jawless fish called an ostracoderm. Most ostracoderms lived in fresh water. They stirred up bottom sediments and filtered the particles through their gills. While you will often hear lampreys called "primitive," they lack armor and have a feeding strategy (attacking bony fish) very unlike that of their ostracoderm ancestors. Incidentally, although the lamprey has a cartilaginous skeleton, this is not a primitive condition, since the ostracoderms had bony skeletons. The cartilaginous skeletons of both agnathans and sharks originated later in evolution. By about 400 million years ago, the ostracoderms had given rise to armored, jawed fish called acanthodians and placoderms and also to the jawed ancestors of the cartilaginous and bony fish. Jaws evolved from the ostracoderm gill arches. They allowed a much more flexible lifestyle because a jawed fish could eat anything it could bite off, not just particles that would filter through its gills. By 425 to 450 million years ago, the cartilaginous and bony fish had diverged from each other. It is thought that both the cartilaginous and bony fish arose in fresh water at about the same time in the Devonian period, but the cartilaginous fish returned to the sea early, and the bony fish radiated into the ocean only later in their evolution. You may remember the lecture discussion of how the marine bony fish are less concentrated than their environment and must drink seawater and excrete salts to maintain their ionic content. Also, contrary to what you might think, the earliest bony fish had both lungs and gills, so lungs were not a later innovation. In most bony fish, the lung later became the swim bladder, used for regulating buoyancy. We believe that the amphibians arose from an air-gulping lungfish in the Devonian period, million years ago. Perhaps recurrent droughts selected for fish that could crawl from one drying pool to another on their sturdy, fleshy fins. In 2006, a transitional animal named Tiktaalik was discovered in northern Canada. This was a fish from 375 million years ago, but had strong fins, gills and nostrils for breathing air, and a distinct neck. The first true amphibians were large, salamander-like animals called labyrinthodonts. Some labyrinthodonts were as big as crocodiles. The amphibians had reached essentially their present form by about 260 million years ago. In addition, the labyrinthodonts gave rise to the cotylosaurs, the first reptiles.

6 The Terrestrial Vertebrates The reptiles, birds and mammals are the only fully terrestrial chordates. Not only are they well adapted for terrestrial locomotion, feeding, etc., but they can also reproduce on land. Also, because we encounter them more than we encounter fish or amphibians, these are the most familiar chordates. Requirements for Moving to Land The move to land required several changes. First, the amphibian tendency towards a sturdier and land-adapted skeleton was continued. Often this involved moving the legs directly beneath the body (as in a dog) instead of allowing them to sprawl out to the sides (as in a salamander). Second, since a moist skin loses a lot of water, the skin became dry and gas exchange occurred exclusively through lungs. The lungs themselves became much more efficient than the rudimentary amphibian lungs, and reached their peak of efficiency in the birds. Third, the circulatory system evolved from the 3-chambered amphibian heart to the 4- chambered heart of advanced reptiles, birds and mammals. This arrangement allowed separation of pulmonary and systemic blood. Fourth, the kidneys also became adapted to eliminate wastes without excessive loss of water. Fifth, the dry-shelled "amniote" egg allowed reptiles and bird embryos to develop in a bubble of fluid, exchange gases with the air, and deposit wastes inside the egg instead of relying on surrounding water to remove them. The "amniotes" (reptiles, birds, and mammals) are thought to all descend from one amniote ancestor. It is interesting to note that while many amphibians spend most of their lives on land but must return to water to mate, even aquatic reptiles like alligators, sea turtles and marine lizards must lay their eggs on land. Only one small group of sea snakes is viviparous and gives birth in the ocean. General Characteristics of Terrestrial Vertebrates Almost all reptiles are carnivorous, but there are a few herbivores among the lizards and turtles. Birds and mammals have very diverse feeding habits (consider a lion, a plankton-eating whale, a cow, etc.). All terrestrial vertebrates have a well developed esophagus and a stomach that is actively involved in digestion. Some mammals (the ruminants) have very complex and highly specialized stomachs. In terrestrial vertebrates there is a distinct division (the pylorus) between the foregut (mouth to stomach) and the hindgut (intestine to anus.) Reptiles, birds, and mammals all utilize internal fertilization, and have copulatory organs. Sexual reproduction is by far the rule, although a very few lizards reproduce parthenogenetically. No vertebrate reproduces by budding from somatic cells. Finally, secondary sexual characteristics are often present. These characteristics may take the form of body size or proportions, plumage (as in birds), mammary glands (as in mammals), or antlers or horns (as in ruminants). One last general comment relates to type of metabolism. Mammals and birds are "warmblooded" endotherms. Most fishes, amphibians, and modern reptiles are ectothermic, meaning that they have a low metabolic rate and cannot maintain a body temperature warmer than their environment with body heat alone. Aside from the fact that many reptiles and amphibians do maintain a fairly constant body temperature by basking in the sun and hiding in the shade, the ectothermic way of life has advantages and disadvantages. We are familiar with the disadvantages. Reptiles seem to be inactive most of the time. They cannot remain active in cold weather, and they are comically sluggish in cool temperatures. Although they are capable of sudden bursts of speed (financed by anaerobic respiration), their endurance is poor and they are very slow to recover from exhaustion.

7 On the other hand, ectothermy has some advantages. Ectotherms have about 10% of the food requirements of endotherms of the same size, convert food to biomass many times more efficiently, and can tolerate extended anaerobic conditions (like under the ice of a pond) or extended periods without food (like a desert dry season). Finally, because surface/ volume ratio increases as a body gets smaller, endotherms cannot be smaller than a certain size (about 3 g for the smallest shrews and hummingbirds) because at smaller sizes, body heat would be lost too fast to be replaced. On the other hand, loss of body heat is no problem for ectotherms, and many of them are very small. This gives them access to literally a whole world of prey too small to be worthwhile for endotherms. Therefore, ectothermy is not an inferior way of life, but merely a different one. Since we are including only terrestrial vertebrates in this lesson, the groups break down into two classes: 1. Reptilia - turtles species lizards species snakes species crocodilians - 22 living species birds species 2. Mammalia - mammals species The birds used to be in Class Aves, separate from the reptiles. However, cladistic analysis now convinces us that the birds should be moved into the class Reptilia because they diverged from the other reptiles so recently. In fact, the birds are the closest relatives to the extinct dinosaurs. The Reptilia Almost all reptiles are carnivorous, but there are a few herbivores among the lizards and turtles. Reptile lungs are fairly simple, with more internal surface area than amphibians, but less than mammals. Reptiles generally inhale by expanding their ribs and sucking in air (negative pressure breathing). This system contrasts with amphibians and lungfish, which swallow air for inhalation (positive pressure breathing). Turtles cannot move their ribs and rely instead on the internal movement of bones to increase and decrease the volume of air in their body cavities. Snakes and some lizards with long slender bodies have only one developed lung. We will cover three orders of modern reptiles: Testudines Crocodilia Squamata turtles alligators, crocodiles, and birds lizards and snakes The Testudines. The turtles are an extremely ancient group. Their hard shell is their trademark and certainly one of the secrets of their success. Turtles are also long-lived animals, with large sea turtles and tortoises living about as long as humans. One of the most interesting aspects of turtle biology is how sea turtles find their way on migrations. Green turtles migrate 2200 km their feeding grounds on the coast of Brazil to tiny nesting beaches on Ascension Island in the South Atlantic. Perhaps they navigate using the same cues as birds: the magnetic field of the earth, polarized light, and the sun and stars. The Non-Bird Crocodilia. Crocodilians are large reptiles (a type of Madagascar crocodile can reach a length of 9 m!). They are remarkably inactive most of the time, but are capable of impressive bursts of speed (even on land) when they are after a meal. They swim with sideways sweeps of their broad tail and mainly eat fish. Their hearts have four distinct chambers, like those of birds and mammals. There are 21 species of crocodilians alive today, including alligators, crocodiles, caimans and a narrowsnouted Asian form called the gavial. Unfortunately, hunters after their hides have exterminated them

8 from some areas, and few today live long enough to reach their maximum size. To supply hides, alligators are being raised on aquaculture farms in the southern US. The Squamata. Lizards, with 3300 species, are almost as diverse as mammals. Most lizards are small (the average is about 5 cm long), but the largest, the Komodo monitor lizard, is 3 m long and may weigh over 200 pounds. Many of the smaller lizards are arboreal. Most lizards are tropical or subtropical, but some live in the temperate zone and above the timberline in mountains; the marine iguana even dives up to 10 m deep in the ocean to eat seaweed. Lizards are tend to be small and rather defenseless, and predator defense is an important problem for them. For most lizards, camouflage and darting quickness suffice, but some use tail autotomy. When threatened by a predator, some lizards can constrict their tail muscles and pinch their tail off. Then the detached tail writhes and occupies the predator while the lizard makes its escape. This strategy has been called "your tail or your life." A human with no arms or legs would be helpless and pitiable, but limbless snakes are silent, effective predators. Limblessness in snakes probably arose when some lizards adopted a burrowing existence, but now most snakes forage on the surface. Very much like sharks, snakes track their prey through odors, vibrations in the ground, and (for warm-blooded prey) emanations of heat. Smaller snakes like the garter snake eat earthworms and grubs that are swallowed whole, but the prey of larger snakes must be subdued, either by constriction or by venom. Constrictors are specialized by having powerful muscles and very short vertebrae (allowing tight coils). Some boas have 300 vertebrae. Venom probably evolved because it's safe for the snake--prey is struck, allowed to escape and die not far away, and the snake tracks it down and ingests it. One of the remarkable aspects of snakes is their ability to swallow prey much bigger in diameter than they are. Their skulls are very loosely constructed, and the connection of the lower jaw to the skull and of the two lower jaw bones to each other are particularly elastic. How large a piece of raw meat could you swallow without choking? Could you swallow a one pound steak in one piece? Would you attempt to swallow a whole chicken? Then you'll probably be impressed to learn that a large python can swallow an animal weighing up to 150 pounds! The Birds Birds are a type of crocodilian, but are so different from the other crocodilians that they warrant a paragraph or two of their own. Birds are beautiful, diverse (with almost twice as many species as mammals), and they fly. Flight opens up many possibilities, since a flying animal can pursue flying prey like insects, can easily escape earthbound predators, and can migrate long distances over both land and water (golden plovers make a yearly migration of 25,000 miles!). Flight is a very demanding activity, however. To fly, birds had to have a much lighter skeleton than a reptile, bones strengthened by fusion, powerful flight (breast) muscles, feathers to catch the air and provide lift, sharp eyes, and a brain providing excellent and fast muscular coordination (imagine the disasters awaiting a klutzy bird as it flies between the branches of a tree). What is not so obvious is that a flying bird needs a better power plant than a reptile. Birds are endotherms. Rather than avoiding exertion and relying on anaerobic respiration to supply peak needs, birds maintain a high metabolic rate with aerobic respiration. The fastest bird, the Asian spine-tailed swift, flies 100 mph in level flight, with a blood temperature of 112 F! Birds can do this because of their circulatory systems and their gas exchange systems. The circulatory system is much like a mammal's, with a four-chambered heart that beats fairly fast because of the small size of most birds (an exercising chickadee's heart beats 1000 times per minute!).

9 On the other hand, bird lungs are much more efficient than mammalian lungs. As mammals, we have alveolar lungs. We breathe into and out of a series of blind sacs (the alveoli), and some stale air always remains in the alveoli. Birds use a system of air sacs as well lungs. The air sacs inflate and deflate as the bird breathes, and the system maintains a one-way flow of air through the lungs during both inspiration and expiration. Air flows not into alveoli, but rather through a series of tiny tubes called parabronchi (giving the name parabronchial lungs). Parabronchial lungs are continuously flushed with fresh air, just as there is a continuous flow of fresh water over the gills of a fish. Perhaps this is why some remarkable geese can fly over the Himalayas, at an altitude that would leave even a superbly conditioned human mountain climber gasping and dying. Air sacs provide other services to birds. Birds have high metabolic rates, and cannot sweat. The movement of air into and out of the sacs cools the body. In addition, the air sacs extend even into the bones, making them lighter. Some contend that the birds are actually living dinosaurs. Birds still retain several features reminiscent of reptiles - scales on their legs, some internal structures, and their habit of laying eggs. The feature that sets birds apart from all other organisms, however, is the presence of feathers. Feathers provide an aerodynamic surface, insulation and (in many species) sexual displays. The Mammalia Of course, we reserve our greatest interest for our own class, the Mammalia. Mammals range in size from the pigmy shrew (which weighs as much as a penny) to the 150 ton blue whale. They can be found in every habitat from the arctic to the tropics, from the mountains to the oceans, and from the swamps to the deserts. In addition to four-chambered hearts and endothermy, all mammals have these five characteristics: mammary glands in the female for milk production, hair, specialized teeth with efficient jaws, a muscular diaphragm, and a well-developed cerebral cortex. Mammals have excellent sensory systems including vision, hearing, smell, and touch and well-developed brains for responding to these sensations. Mammalian lungs are much more finely and evenly divided than the lungs of reptiles. Mammals inhale by a highly efficient suction mechanism. The mammalian coelom is separated by the muscular diaphragm into a thoracic cavity and an abdominal cavity. Contraction of the diaphragm and expansion of the rib cage increases the size of the thoracic cavity thus sucking air into the lungs. The three subclasses of mammals are based on reproduction: the egg-laying monotremes like the platypus and echidna, the pouched marsupials like the kangaroo, and the placentals like the rabbit. The Monotremes. The monotremes (or subclass Protheria) consist of only a few species that are clearly mammals, but lay reptilian-style eggs. However, after hatching, the young are suckled. The monotremes also maintain lower body temperatures than most mammals. The monotremes probably branched from the rest of the mammals very early in mammalian evolution. Monotremes are confined to Australia and New Guinea. The Marsupials. The marsupials (or subclass Metatheria) give birth to live young, but the young must crawl to a pouch to feed on milk and complete their development. We usually associate marsupials (especially kangaroos) with Australia, but we believe that marsupials originated in North America, and remained diverse in Australia due to their isolation from competition with placental mammals. The only marsupials in North America and South America are opossums. We might regard marsupials as primitive, but their reproduction has certain advantages. Once a placental mammal becomes pregnant, she is saddled with an enormous energetic investment that cannot be ended if conditions turn bad. But a marsupial only makes a small investment by the birth of the young, and can remove them from her pouch if the environment deteriorates. In some cases, a

10 kangaroo pursued by dingoes (wild dogs) will take the smallest young from her pouch and fling it over her shoulder, allowing the mother and remaining young to escape. It seems heartless to sacrifice the smallest, but that is the offspring that has consumed the least investment. The Placental Mammals. Placental mammals (subclass Eutheria) dominate the mammalian world. When we think of placental mammals we probably think of ourselves or dogs or horses, but the most successful mammalian order is the rodents. You will probably be surprised to learn that it is thought that our order, the primates, is very closely related to the rodents. Another diverse group (1/3 of all eutherians) are the small insectivores (such as shrews) and bats. Two other great evolutionary lines of placental mammals are the herbivores (including rabbits, horses, cattle, deer, elephants and even whales) and carnivores (probably the smartest mammals aside from the primates). Some unusual mammals show remarkable convergent evolution with other taxa. For example, porpoises and whales resemble sharks and bats resemble birds because either fast swimming or flight imposes much the same solutions on any animal attempting them. Convergent evolution between unrelated mammals filling the same ecological niches on different continents is less spectacular but just as interesting. While North American was home to placental wolves, ocelots, flying squirrels and ground hogs, Australia evolved Tasmanian wolves, native cats, flying phalangers and wombats, all marsupials. In some cases, these mammal pairs even look alike. Evolution was able to work an adaptable mammalian body plan into similar, well-adapted end results, even from vastly different starting points. Evolution of the Terrestrial Vertebrates All terrestrial vertebrates evolved from reptiles, which were the dominant land animals for 200 million years. However, the surviving reptiles are like a family that was once rich and powerful, but now is living in a tiny apartment crowded with pictures of old mansions and illustrious ancestors. About 300 million years ago, a lizardlike group called the cotylosaurs ("stem reptiles") had diverged from the amphibians. The cotylosaurs gave rise to six lineages, three of which have persisted: anapsids diapsids synapsids The anapsids gave rise to turtles. The diapsid reptiles radiated spectacularly into the dinosaurs, pterosaurs (flying reptiles), crocodilians and birds. Birds apparently developed from small, bipedal dinosaurs relatively recently, some 160 million years ago. This is why taxonomists now maintain that there cannot be a Class Reptilia that excludes the birds. The oldest known bird-like creature-- Archaeopteryx--resembled those dinosaurs, but had feathers. Archaeopteryx was about the size of a small pigeon, and probably couldn't fly, so maybe it used its "wings" for gliding from trees or as nets for catching insects. Some biologists think that feathers first evolved in small dinosaurs for thermal insulation or mating displays, not flight. The last group of ancestral reptiles, the synapsids, radiated into several extinct groups, but also into a group of dog-sized reptiles called therapsids by about 200 million years ago. By the mid-permian period, the therapsids were big, dominant predators, but then they declined and all but some small therapsids died out as the dinosaurs ruled the earth. These small therapsids (predators, insectivores and herbivores) became the first rodent-sized mammals, probably in the early Jurassic period. By the end of the Age of Dinosaurs, three of the four original orders of mammals had gone extinct. The only mammals left were very small, insectivorous and nocturnal. Nocturnal habits selected for endothermy and acute senses, but it seemed as if the mammals were on their way out. However, the mammals got a big break--the dinosaurs died out, and the humble mammals radiated tremendously. This radiation was aided after about 40 million years ago, when the climate became cooler and drier, selecting for the spread of grasslands and the advent of giant mammalian grazers. Since "the Age of Mammals," the diversity of mammals (especially large ones) has actually declined, due in no small part to man's hunting and destruction of habitat.

11 Biology 1110 Vertebrate Worksheet 1. Are all vertebrates chordates? If not, which vertebrates are not chordates? Are all chordates vertebrates? If not, which chordates are not vertebrates? 2. What are some of the features that all vertebrates share? 3. Name the nine classes of vertebrates and give the common names of at least one member of each class. Why do some investigators no longer consider the hagfish a vertebrate? 4. Why do aquatic animals have a harder time getting oxygen than terrestrial animals? 5. How do the aquatic vertebrates carry on gas exchange? 6. Is there any asexual reproduction among the aquatic vertebrates? What are some of the variants of sexual reproduction? 7. Describe the lifestyle of a modern jawless fish. How does this differ from the lifestyle of the ancient jawless fish, the ostracoderms? 8. The layperson makes little distinction between sharks and bony fish, but what are some of the differences between them? Include skin coverings in your answer. 9. Describe how a shark locates its prey.

12 10. Are all cartilaginous fish carnivores? 11. Distinguish between the three classes of bony fish. Which of these types is most diverse and widespread? 12. Your roommate comes home from a fishing trip with a cooler full of fish. He asks you to guess what kinds of fish he caught. Which class of fish would be the least surprising for the average fishing trip? Which class of fish would be the most surprising? Consider the classes Myxini, Petromyzontida, Chondrichthyes, Actinopterygii, Actinistia, and Dipnoi as possibilities. 13. Your roommate doesn t wait for you to guess, and brags he has caught a member of the Dipnoi. Your mouth drops open. If it is true, where must he have gone on this fishing trip? 14. Describe the changes that allowed the amphibians to move (at least partially) to dry land. Why must we stipulate that the move was only partial? 15. Despite your answer above, how can some amphibians (like some toads) live in rather dry environments? 16. Explain aposematic coloration. How does it benefit the animal that practices it? 17. Name the orders of amphibians, and give the common names of some members of each order. 18. Why have the frogs become so successful in comparison to the other amphibians?

13 19. Contrast ostracoderms and placoderms. What were their roles in early vertebrate evolution? 20. Why is it thought that the bony fish moved from the freshwater to the marine environment more recently than the cartilaginous fish did? 21. What group of fish gave rise to the amphibians? When did this happen? What is the role of Tiktaalik in this story? What is the role of the labyrinthodonts? 22. What modifications did the reptiles make to the amphibian skeleton? Skin? Lungs? Circulatory system? Kidney? 23. What is an amniote egg, and why was it an important step in the move to land? 24. Describe the reproduction of the terrestrial vertebrates. 25. Distinguish between endothermy and ectothermy. What are the disadvantages and advantages of ectothermy? 26. Why is it a problem for an endotherm to be very small, while small size is not a problem for an ectotherm? 27. Contrast negative pressure breathing and positive pressure breathing. Which type is used by the reptiles? Birds? Mammals?

14 28. In general, what do reptiles eat? 29. What is tail autotomy why is it an advantage to lizards that use it? 30. What special adaptations do constricting snakes have? Why is venom advantageous to the snakes that use it? 31. How is the bird body adapted to flight? 32. Explain the difference between an alveolar lung and a parabronchial lung. Explain why bird lungs are more efficient than mammalian lungs. 33. In what ways are birds like reptiles? In what ways are they unique? 34. What are the characteristic features of a mammal? 35. Name and describe the three major reproductive groups of mammals. The four major groups of placental mammals. 36. What are the advantages and disadvantages of the marsupial form of reproduction?

15 37. What were the cotylosaurs? What are the three extant (still surviving) lineages they evolved? What living vertebrates came from each of these lineages? 38. Which of the reptilian lineages above gave rise to the most living species? Which one gave rise to the dinosaurs? 39. Why do many biologists say that if there is going to be a single Class Reptilia, it must include the birds? 40. You tell you little brother about the diversity of the vertebrates, and he reacts by begging your parents for a diapsid he can keep as a pet. They agree and give you the assignment of getting the diapsid. You re now in the parking lot of the pet store. Assuming that you don t want to get your little brother a dangerous pet, what will you buy? 41. When did the mammals first appear? From which group of reptiles did they evolve? 42. Why did the module say that at the time of the extinction of the dinosaurs, the mammals appeared to be in decline? Why might it be said that the mammals are in decline again today?

16 Biology 1110 Vertebrates 1. At some stage of their development, all vertebrates have 1. a postanal tail. 2. a skull and vertebral column. 3. four limbs. 4. All of these. 2. Which organism is not paired with its proper taxon? 1. rainbow trout--actinopterygii 2. bullfrog--actinistia 3. hagfish--myxini 4. crow--reptilia 3. Some sharks and amphibians seem to give birth to live young rather than laying eggs because they are 1. viviparous, and the eggs hatch inside the uterus. 2. oviparous, and the young are nourished by a placenta. 3. ovoviparous, and the eggs hatch inside the uterus. 4. parthenogenetic, and the young are budded from somatic cells. 4. Aside from the fact that they have no jaws, you could distinguish an agnathan from all other modern fish by the fact that the agnathan 1. lacks a skull. 2. lacks paired fins. 3. reproduces by asexual cysts. 4. is entirely sessile. 5. Some biologists contend that the Myxini should not be considered true vertebrates because they don't have 1. a backbone. 2. a skull. 3. a blastula stage. 4. a true brain. 6. Chondrichthyes and Actinopterygii are different in many ways, but one feature you could not use to tell them apart is 1. their type of fins. 2. the lack of a swim bladder in the Chondrichthyes. 3. their type of skeleton. 4. the presence of a lateral line system in the Actinopterygii. 7. As it closes in on prey, a shark uses its ampullae of Lorenzini, which detect minute 1. electrical fields. 2. temperature variations. 3. concentrations of amino acids. 4. disturbances in the water. 8. The name Actinopterygii refers to a characteristic of the... in a type of 1. brain... cartilaginous fish. 2. fins... bony fish. 3. mouthpart... bony fish. 4. reproductive organs... amphibian. 9. Amphibians had to make numerous changes to make the transition to land; one of these changes was not development of 1. a stronger skeleton. 2. lungs. 3. a three-chambered heart. 4. an egg that could resist drying. 10. Adult amphibians are 1. herbivorous. 2. carnivorous. 3. filter-feeders. 4. deposit feeders. 11. You could tell a member of the Caudata from a member of the Gymnophiona because the Caudata would have 1. legs. 2. a mouth. 3. a tail. 4. poison spines. 12. The reptiles are better adapted to a terrestrial existence than the amphibians in several ways, but one of these changes was not a change in 1. skin. 2. kidneys. 3. diet. 4. lungs.

17 13. The amniote or "land" egg of reptiles allowed 1. deposition of wastes inside the egg. 2. the embryo to be surrounded by fluid. 3. gas exchange with the air. 4. All of these. 14. Ectothermy has the disadvantage that the animal..., but it has the advantage that the animal 1. is immobile in cold temperatures... doesn't need much food. 2. must eat fatty, high-energy foods... can be much more active. 3. cannot conserve water very well... can put up with long periods of fasting. 4. has poor endurance... can be active in a broad range of temperatures. 15. The group of vertebrates with the most species is the 1. lizards. 2. birds. 3. rodents. 16. Which organism is not paired with its proper order? 1. snapping turtle--testudines 2. iguana--squamata 3. alligator--crocodilia 4. rattlensnake--sauria 17. Most non-bird reptiles are..., however a few of the larger lizards and turtles are 1. herbivorous... detritivorous. 2. herbivorous... carnivorous. 3. carnivorous... herbivorous. 4. detritivorous... carnivorous. 18. The great advantage of bird lungs over mammalian lungs is that bird lungs 1. have a one-way flow of air. 2. are much larger in relation to body size. 3. have a blood supply with higher pressure and faster flow rate. 4. have hemoglobin in the cell membranes rather than in red blood cells. 19. The differences between the Protheria, Eutheria and Metatheria relates to their type of 1. skull. 2. reproduction. 3. excretion. 4. brain ventricles. 20. Mammals have several characteristics, but one of them is not 1. teeth specialized for different chewing tasks. 2. a muscular diaphragm. 3. milk production. 4. positive-pressure breathing. 21. The anapsid reptiles gave rise to the..., and the synapsid reptiles gave rise to the 1. birds... mammals. 2. turtles... mammals. 3. mammals... crocodilians. 4. dinosaurs.. birds. 22. Which of the following pairs does not consist of an evolutionary lineage and its founding group? 1. agnathans... ostracoderms 2. amphibians... labyrinthodonts 3. birds... acanthodians 4. mammals... therapsids

18 Biology 1110 Vertebrates Correct Correct. A bullfrog is an amphibian, not a lobe-finned fish No. A viviparous organism (like a mammal) would not lay an egg. 2. No. Oviparous means they lay eggs. 3. Correct. 4. No. Parthenogenetic means reproduction from unfertilized eggs, not budding from somatic cells No, agnathans do have skulls. They're not bony, and they may not have a complete roof over the brain, but they do have skulls. 2. Correct. Agnathans have one fin that circles the rear of the body along the midline. 4. No. Agnathans are mobile, not sessile Correct. They just have a notochord. They do have a skull, a blastula, and a brain, however Correct. Both cartilaginous and ray-finned fish have lateral line systems Correct. 4. No. This sounds like the lateral line system Correct. Actinopterygii are ray-finned fish. 9. 1, 2 and 3. No. They did all this. 4. Correct. They never did develop an egg that resisted drying out. That's why their reproduction is tied to water, or at least wet places Correct. Think of a frog eating insects Correct. The Gymnophiona are the caecilians, and have no legs. The salamanders do have legs Correct. Their diets are the same--mostly carnivorous Correct Correct. 3. No. There's nothing about ectothermy that interferes with water conservation, but ectotherms can tolerate long periods without food Correct. The birds have almost 9000 species. But even they are put in the shade by the bony fish (15,000 species) Correct. Snakes and a lizards are members of the Squamata Correct. Think of insect-eating lizards, mouse-eating snakes, fish-eating alligators. But some lizards and turtles are herbivores Correct. Bird lungs are smaller in relation to body size than mammalian lungs, and the other answers are false Correct. The three groups mentioned are the monotremes, placentals and marsupials Correct. All the terrestrial vertebrates have negative-pressure breathing Correct. The third group, the diapsids, gave us the birds, dinosaurs, flying reptiles and crocodilians Correct. The acanthodians were an early type of jawed fish.