Chordates Including the Vertebrates

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Finally, we come to the phylum that includes ourselves. The chordates incorporated evolutionary innovations that made it possible for them to grow to great size, forming the largest animals on land (dinosaurs and elephants) and in the water (whales). The four unique characteristics of the phylum Chordata are:

1. They have the presence of a notocord at some point in their development. The notocord is a flexible skeletal rod that runs the length of the organism. It remains in adult lampreys and hagfish, but in other chordates it is mostly replaced by vertebrae.

2. They have a single, hollow tubular nerve cord, located dorsal to the notocord, usually enlarged at the anterior end to form a brain.

3. They have paired openings in the anterior digestive tract called gill slits at some point in their development. In humans these are present in the embryonic stage.

One pair remains after this stage and forms the Eustachian tubes, which connect the throat to the middle ear. It is the Eustachian tube that allows you to equalize the pressure in your ear by swallowing.

4. The notocord, nerve cord, and segmented muscles continue into a tail that extends beyond the anus. In humans the tail is present only briefly in the embryo.

Other important, although less unique characteristics are: They have segmented muscles in an otherwise unsegmented body. In humans these muscles lie along the spinal column. Chordates have a closed blood system with a ventrally located heart, and they have a complete digestive system.

Not all chordates are easily recognizable as being closer relatives to us than, say, echinoderms. Two subphyla within the Chordates are marine invertebrates (Table 8.3). The subphylum Urochordata constitutes the tunicates. These include about 2000 species of filter feeders. Many, called sea squirts, are sessile. One group, called the salps, form meters-long colonies of transparent lemon-shaped bodies. Salps are brightly bioluminescent. The phylum Cephalochordata includes the 25 or 30 species of lancelets. Also called amphioxus, they resemble a small, translucent fish without fins or mouth or eyes to mark the head. They are common in sandy coastal waters around the world. Four species are found off the coast of North America.

If you ask someone to name any animal that comes to mind, they will probably think of a member of the third chordate subphylum: the vertebrates. The unique characteristic of the subphylum Vertebrata (also called Craniata) is that they possess a cartilaginous or boney endoskeleton with a cranium, or brain case. Bone is a living tissue, not just calcareous deposits. As the name implies, almost all vertebrates have a spine made of a series of bones called vertebrae. The segmented muscles attach to the vertebrae. The only exception is the cartilaginous fishes (such as sharks). The subphylum Vertebrata includes eight classes that are placed in five groups, each of which contributes new adaptations:

1. The fishes comprise four classes with about 30,000 species. They are the oldest vertebrates and represent the development of the skull, bones, jaws, fins, and vertebrae.

2. The class of amphibians has about 3000 species. They innovated with legs, fully functioning lungs, a chambered heart, and separate circulation systems for the lungs and the rest of the body.

3. The class of reptiles has 6500 species. Their innovations include dry, scaly skin, an expandable rib cage, and the transfer of legs to beneath the body.

4. The birds (class Aves) include 9600 species. They represent the development of warm-bloodedness, although many now believe this first occurred among the dinosaurs. They also uniquely have feathers, air sacs, and hard-shelled eggs.

5. The class of mammals has about 5000 members. They independently developed warm blood, plus their unique characteristics of body hair, a placenta for nurturing embryos (in most orders), and mammary glands with milk production for nurturing juveniles.

Hagfish and lampreys are in two classes that together make a superclass called the jawless fish. Hagfish are blind scavengers that find prey by smell and attach themselves by the mouth. They are the only vertebrates that keep their osmotic equilibrium with the

TABLE 8.3 Classification of Living Members of Phylum Chordates

Subphylum Class Order Family Genus Species

Urochordata (tunicates; sea squirts and salps)

Cephalochordata (lancelets)

Vertebrata

Myxini (hagfish (43 spp.)

Cephalaspidomorphi (jawless fish, e.g., lamprey) Chondrichthyes (cartilagenous fish: sharks and rays, 850 spp.) Bony fish (45 living orders, about 30,000 spp.) Amphibia (3900 spp.)

Caudata (salamanders, 360 spp.) Anura (frogs and toads, 21 families, 3450 spp.) Reptilia (13 orders, including the extinct dinosaurs) Testudines (turtles) Squamata (lizards and snakes) Crocodilia (crocodiles and alligators) Aves (birds) (27 living orders) Mammalia

Nonplacental mammals:

Monotremes (only 3 spp., including duck-billed platypus); Marsupials (kangaroos, opossums, koalas, 260 spp.) Placental mammals:

Insectivora (shrews, hedgehogs, moles, 390 spp.)

Macroscelidea (elephant shrews, 15 spp.)

Dermoptera (flying lemurs, 2 spp.)

Chiroptera (bats, 986 spp.)

Scandentia (tree shrews, 16 spp.)

Xenarthra (anteaters, armadillos, sloths, 30 spp.)

Pholodota (pangolins, 7 spp.)

Lagomorpha (rabbits, hares, pikas, 69 spp.)

Rodentia (gnawing mammals: squirrels, rats, 1814 spp.)

Cetacea (whales, dolphins, porpoises, 79 spp.)

Carnivora (dogs, wolves, cats, bears, weasels, 240 spp.)

Pinnipedia (sea lions, seals, walruses, 34 spp.)

Tubulidentata (aardvark, 1 spp.)

Proboscidea (elephants, 2 spp.)

Hyracoidea (coneys, 7 spp.)

Sirenia (sea cows and manatees, 4 spp.)

Perissodactyla (odd-toed hoofed mammals:

horses, asses, zebras, tapirs, rhinoceroses, 17 spp.) Artiodactyla (even-toed hoofed mammals: swine, camels, deer, hippopotamuses, antelopes, cattle, sheep, goats, 211 spp.) (Perissodactyla and Artiodactyla are called ungulates) Primates (233 spp.):

prosimians (lemurs, tarsiers), monkeys, apes, humans Great Apes

Homo

Homo sapiens Homo habilis (extinct) Pan (chimpanzees) Gorilla

Lesser Apes surrounding seawater instead of controlling their internal condition. The lamprey is a destructive eel-like parasite. It attaches itself to live fish by a suckerlike mouth and uses sharp teeth to rasp at the flesh of the victim. Accidental introduction of the sea lamprey Petromyzon marinus to the Great Lakes in the nineteenth century led to devastation of one commercial species of fish after another. Larvicides are applied to streams where they breed. This, combined with fish stocking programs, are producing a recovery of the fisheries.

The cartilaginous fish, which include sharks and rays, surprisingly evolved from boney ancestors. They track prey by smell and use an interesting lateral line system of sensors along the side that detect vibrations and bioelectrical fields produced by all animals. The electric ray can produce a low-voltage but high-current field from large organs alongside its head. The power output can be several kilowatts and can stun prey or repel predators.

The boney fish include most of the familiar fresh- and saltwater fish. They have developed ray fins and a swim bladder, a gas-filled chamber below the spine that the fish uses to control buoyancy. The importance of this can be seen by considering the sharks, which lack a swim bladder and therefore must swim continuously to keep from sinking to the bottom of the sea.

Among amphibians, the life cycle of frogs comes close to proving a famous biological principle proposed in the nineteenth century which states that ontogeny (the development of an individual from embryo to adult) recapitulates (repeats) phylogeny (the evolutionary development of a species). This principle, termed recapitulation, is invoked to explain embryonic features that disappear in the adult, such as gill slits in human embryos. As new features evolve, they are added to the embryonic development. But the principle turns out to have too many exceptions to be generally valid. Evolution sometimes deletes characteristics, as well as adding them, so the ontogeny is not an accurate record of the organism'sevolution. What we observe in frogs is the development of an aquatic species into a terrestrial species. For example, all frogs hatch from the egg resembling fish, having gills, a tail, and no limbs. This early stage is called the tadpole. Gradually, the tadpoles sprout limbs, grow lungs and eyelids, and the tail shortens, until the adult frog is finished.

The most abundant frog genus is probably Rana. All frogs, and most adult amphibians, are carnivores. They feed on anything that moves and that is small enough to swallow whole. Some amphibians, such as some salamanders, remain aquatic; others do not have an aquatic stage. However, all have thin water-permeable skins that require moist conditions. This may make them among the terrestrial animals that are most vulnerable to environmental problems. There is considerable anecdotal evidence today that frog populations in many areas are in serious decline. The cause is a mystery, but some of the factors that have been blamed include habitat destruction, acid rain, pesticides, and damage from increased ultraviolet radiation due to destruction of stratospheric ozone.

Reptiles show numerous evolutionary developments. Many of these developments freed reptiles from dependence on moist conditions. Their dry skin limits evaporative loss. Internal fertilization and shelled, waterproof eggs eliminates the need for water for reproduction. In turtles and crocodilians the temperature of incubation of the eggs determines the sex ratio. More efficient respiration allows greater size, up to the 115-kg Komodo dragon. Reptile jaws are capable of applying crushing force. They have higher blood pressure and more efficient circulation. Their lungs are more efficient. Whereas amphibians force air into their lungs with mouth muscles, reptiles developed the ability to suck air in by expanding the rib cage. They excrete nitrogen as insoluble uric acid, further conserving water. The nervous system of reptiles is much more complex than that of amphibians. Crocodilians developed a true cerebral cortex. Placement of the legs below the body instead of at the sides allows better support and motion.

Birds (class Aves) have many distinguishing characteristics other than the obvious ones of having feathers and forelimbs developed into wings. Their necks are disproportionately long. Their skeleton contains air cavities, making them strong, yet light. Their beaks lack teeth. They have a four-chambered heart and are warm-blooded. They excrete nitrogenous wastes as uric acid. Fertilization is internal and they produce eggs with a large amount of yolk. Some birds feed on insects; other invertebrates, such as worms, mollusks, and crustaceans; and vertebrates. About one-fifth feed on nectar. Many eat seeds. The beak is specialized for the type of feeding behavior. Bird behavior is highly developed, including complex activities such as nest building and social activity. The farther from the equator one gets, the higher the percentage of birds that migrate for the winter. The Arctic tern breeds above the Arctic Circle and then migrates to the Antarctic region! Day length seems to be the factor that stimulates migration. Birds can navigate using the sun and stars as cues. Amazingly, it has been proven that they can also detect Earth's magnetic field.

Birds are very sensitive to the use of pesticides in the environment. Some pesticides tend to biomagnify, increasing in concentration as they are passed up the food chain. This has had the greatest impact on birds of prey, such as eagles and osprey. Other contaminants that affect birds include lead shot discharged by hunters in wetlands. Waterfowl ingest them, resulting in lead poisoning. The loss of wintering lands in tropical regions may be a cause of the reduced songbird populations that have occurred in the last 40 years.

The classification of birds and reptiles may soon change. This is because the common ancestor of all reptiles is also the ancestor of the birds. Thus, they should be classified together, or perhaps distinguished as feathered or featherless reptiles. Birds developed from certain dinosaurs. Their closest relatives today are the crocodilians. For the present, however, the traditional classification is retained.

The main distinguishing characteristic of mammals is the presence of hair or fur and the production of milk. Hair or fur provides insulation, giving greater advantage to the warm-blooded metabolism. It has other functions as well, such as camouflage, waterproofing, and sensing (as by whiskers). Hair is produced from dead cells that leave a fibrous protein called keratin, the same substance that forms nails, claws, hooves, and feathers. Mammalian skin contains a variety of glands, including sweat, scent, and mammary glands. The latter produce the milk to nurture the young. Herbivorous animals harbor anaerobic bacteria and protozoans in a stomach chamber to ferment cellulose into sugars, fatty acids, and starches. No vertebrates are able to use cellulose on their own. Carnivores feed mostly on herbivores. The need for hunting behavior has selected for intelligence in these animals. The herbivores, on the other hand, have developed keen senses as protection against predators.

Most mammals have mating seasons, timed to produce young at a favorable season for rearing. Mating is limited by the female, who is receptive to mating only during a brief period in the mating season known as estrus. Old World monkeys and humans have a different cycle, the menstrual cycle. Mammals exhibit three ways of giving birth. Mono-tremes, such as the duck-billed platypus, is a mammal that lays eggs. Animals that lay eggs are called oviparous. The platypus still nurses its young with milk, however. Marsupials are viviparous; that is, they give live birth. However, the newborns are essentially still embryos. For example, the red kangaroo spends about a month in the uterus. When it is born, it climbs by itself through the mother's fur to a pouch, where it attaches itself to a teat to suckle for 235 days until it is capable of living independently.

The third way to give birth is that of the placental mammals, which comprise 94% of all mammals. Gestation in utero is prolonged. The embryo is nourished by a placenta, a membrane structure produced by and surrounding the embryo. The placenta grows thousands of tiny fingerlike projections called villi into the lining of the mother's uterus to absorb nutrients and oxygen from the maternal blood supply without there being an actual exchange of maternal and fetal blood. The fetus is connected to the placenta by the umbilical cord. Once born, the mammal may be more or less dependent on parents for a time.

Humans are members of the primate order. The first primate fossils date back about 40 million years. About 8 million years ago, forests in eastern Africa gave way to savannas, giving an advantage to apes that could stand upright. But it was 4 million years ago that the first fossils that could be called hominids (humanlike) appeared. This species has been named Australopithecus afarensis. One of the best such fossils, discovered in 1974, is of a female that has been named "Lucy." It had a brain the size of a chimpanzee's, about 450 mL. The first species of our genus was Homo habilis, which lived from 2 million years ago until about 1.5 million years ago. This species used stone and bone tools and had a brain size of about 640 mL. This was followed by Homo erectus, which had a brain size about 1000 mL and lived in social groups. Homo erectus disappeared about 300,000 years ago and was replaced by our species, Homo sapiens. About 130,000 years ago a subspecies of Homo sapiens appeared, called the Neanderthals. They had a brain size within the range of modern humans, about 1100 to 1700 mL.

About 30,000 years ago Neanderthal man was replaced by a new subspecies, Homo sapiens sapiens—us! These ancestors of ours were distinct from the Neanderthals in a number of ways. Although Neanderthals made stone blades, those of sapiens were of much higher quality. Sapiens buried their dead with ritual objects, suggesting a belief in an afterlife. Finally, their vocal apparatus and related portions of the brain show increased development. This suggests that early humans may have been capable of language. Language capability confers the ability to do abstract reasoning. Thus, language is the basis of intellect, the one thing that most decisively distinguishes us from the other animals. It is interesting to realize that modern humans coexisted with a distinct subspecies not long before the beginning of recorded history. Many suspect that Neanderthal man was wiped out in conflicts with our ancestors. Others think that the two groups merged by interbreeding.

Some interesting information on our more recent origins has been obtained from studies of mitochondrial DNA. Although we inherit the DNA in our cells' nuclei from both parents, there is also DNA in our mitochondria that we inherit from our mother alone. Mitochondrial DNA can be used to trace genetic heritage because it is not confounded by recombination and the mixing due to sexual reproduction. The diversity in mitochondrial DNA from different ethnic populations has been compared with the divergence that would have occurred due to the rate of mutation from natural effects. From this it has been possible to establish that if the assumptions used in the analysis are reasonable, all humans in the world today are descendents of a single individual that lived in Africa 200,000 years ago.

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