Unit Five. Evolution of Animal Life


20. History of the Vertebrates


20.8. Mammals Adapt to Colder Times


Characteristics of Mammals

Most large land-dwelling vertebrates are mammals. The mammals (class Mammalia) that first evolved about 220 million years ago side by side with the dinosaurs would look strange to you, not at all like modern-day lions and tigers and bears.




They share three key characteristics with mammals living today:

1. Mammary glands. Female mammals have mammary glands, which produce milk to nurse the newborns. Even baby whales are nursed by their mother’s milk. Milk is a very-high-calorie food (human milk has 750 kcal per liter), important because of the high energy needs of a rapidly growing newborn mammal.

2. Hair. Among living vertebrates, only mammals have hair (even whales and dolphins have a few sensitive bristles on their snout). A hair is a filament composed of dead cells filled with the protein keratin. The primary function of hair is insulation. The insulation provided by fur may have ensured the survival of mammals when the dinosaurs perished.

3. Middle ear. All mammals have three middle-ear bones, which evolved from bones in the reptile jaw. These bones play a key role in hearing by amplifying vibrations created by sound waves that beat upon the eardrum.


History of the Mammals

We have learned a lot about the evolutionary history of mammals from their fossils. The first mammals arose from therapsids, pictured above in figure 20.22, in the late Triassic about 220 million years ago, just as the first dinosaurs evolved from thecodont archosaurs. Tiny, shrewlike creatures that ate insects, most mammals were only a minor element in a land that quickly came to be dominated by dinosaurs. Fossils reveal that these early mammals had large eye sockets, evidence that they may have been active at night.




Figure 20.22. A therapsid.

This small, weasel-like therapsid may have had fur like its descendants, the mammals.


For 155 million years, all the time the dinosaurs flourished, mammals were a minor group. At the end of the Cretaceous period, 65 million years ago, when dinosaurs and many other land and marine animals became extinct, mammals rapidly diversified. Mammals reached their maximum diversity during the Tertiary period, about 15 million years ago.

Today, 4,500 species of mammals occupy all the large-body niches that dinosaurs once claimed. Present-day mammals range in size from 1.5-gram shrews to 100-ton whales. Almost half of all mammals are rodents—mice and their relatives. Almost one-quarter of all mammals are bats! Mammals have even invaded the seas, as plesiosaur and ichthyosaur reptiles did so successfully millions of years earlier—79 species of whales and dolphins live in today’s oceans. The major orders of mammals are described in table 20.6.





Other Characteristics of Modern Mammals

Endothermy Mammals are endothermic, a crucial adaptation that allows them to be active at any time of the day or night and to colonize severe environments, from deserts to ice fields. Many characteristics, such as hair to provide insulation, play important roles in making endothermy possible. Also, the more efficient blood circulation provided by a four- chambered heart and the more efficient breathing provided by the diaphragm (a special sheet of muscles below the rib cage that aids breathing) make possible the higher metabolic rate upon which endothermy depends.

Teeth. Reptiles have homodont dentition: All of an individual’s teeth are the same. However, mammals have heterodont dentition, with different types of teeth that are highly specialized to match particular eating habits. It is usually possible to determine a mammal’s diet simply by examining its teeth. A dog’s long canine teeth are well suited for biting and holding prey, its molar teeth are sharp for ripping off chunks of flesh. In contrast, canine teeth are absent in horses; instead the horse clips off mouthfuls of plants with its flat, chisel-like incisors. Its molars are covered with ridges to effectively grind and break up tough plant tissues. Rodents, such as a squirrel, are gnawers and have long incisors for breaking open nuts and seeds. These incisors are ever-growing—that is, the ends may become sharp and wear down, but new incisor growth maintains the length.

Placenta. In most mammal species, females carry their young in the uterus during development, nourishing them by a placenta, and give birth to live young. The placenta is a specialized organ within the womb of the mother that brings the bloodstream of the fetus into close contact with the bloodstream of the mother. The placenta evolved from membranes present in the amniotic egg. Figure 20.23 shows a drawing of a fetus within the uterus. The placenta is to the right side, attached to the umbilical cord. Food, water, and oxygen can pass across from mother to child, and wastes can pass over to the mother’s blood and be carried away.



Figure 20.23. The placenta.

The placenta evolved from membranes in the amniotic egg. The umbilical cord evolved from the allantois. The chorion forms most of the placenta itself. The placenta serves as the provisional lungs, intestine, and kidneys of the embryo, without ever mixing maternal and fetal blood.


Hooves and Horns. Keratin, the protein of hair, is also the structural building material in claws, fingernails, and hooves. Hooves are specialized keratin pads on the toes of horses, cows, sheep, antelopes, and other running mammals. The pads are hard and horny, protecting the toe and cushioning it from impact.

The horns of cattle, sheep, and antelopes are composed of a core of bone surrounded by a sheath of compacted keratin. The bony core is attached to the skull, and the horn is not shed. Deer antlers are made not of keratin but of bone. Male deer grow and shed a set of antlers each year.


Today's Mammals

Monotremes: Egg-Laying Mammals. The duck-billed platypus and two species of echidna, or spiny anteater (figure 20.24a), are the only living monotremes. The monotremes have many reptilian features, including laying shelled eggs, but they also have both of the defining mammalian features: hair and functioning mammary glands. Females lack well-developed nipples, so the newly hatched babies cannot suckle. Instead, the milk oozes onto the mother’s fur, and the babies lap it off with their tongues. The platypus, found only in Australia, lives much of its life in the water and is a good swimmer. It uses its bill much as a duck does, rooting in the mud for worms and other small animals.

Marsupials: Pouched Mammals. The major difference between marsupials (figure 20.24b) and other mammals is their pattern of embryonic development. In marsupials, a fertilized egg is surrounded by chorionic and amniotic membranes, but no shell forms around the egg as it does in monotremes. The marsupial embryo is nourished by an abundant yolk within the shell-less egg. Shortly before birth, a short-lived placenta forms from the chorion membrane. After the embryo is born, tiny and hairless, it crawls into the marsupial pouch where it latches onto a nipple and continues its development.

Placental Mammals. Mammals that produce a true placenta, which nourishes the embryo throughout its entire development, are called placental mammals (figure 20.24c). Most species of mammals living today, including humans, are in this group. Unlike marsupials, the young undergo a considerable period of development before they are born.



Figure 20.24. Today's mammals.

(a) This echidna, Tachyglossus aculeatus, is a monotreme. (b) Marsupials include kangaroos, like this adult with young in its pouch. (c) This female African lion, Panthera leo (order Carnivora), is a placental mammal.


Key Learning Outcome 20.8. Mammals are endotherms that nurse their young with milk and exhibit a variety of different kinds of teeth. All mammals have at least some hair.


Inquiry & Analysis

Are Extinction Rates Constant?

Since the time of the dinosaurs, the number of living species has risen steadily. Today there are over 700 families of marine animals, containing thousands of described species.

Interspersed, however, have been a number of major setbacks, termed mass extinctions, in which the number of species has greatly decreased. Five major mass extinctions have been identified, the most severe of which occurred at the end of the Permian Period, approximately 225 million years ago, at which time more than half of all families and as many as 96% of all species may have perished.

The most famous and well-studied extinction occurred 65 million years ago at the end of the Cretaceous Period. At that time, dinosaurs and a variety of other organisms went extinct, most likely due to the collision of a large meteor with earth. This mass extinction did have one positive effect, though: With the disappearance of dinosaurs, mammals that previously had been relatively small and inconspicuous, quickly experienced a vast evolutionary radiation, that ultimately produced a wide variety of organisms, including elephants, tigers, whales, and humans. Indeed, a general observation is that biological diversity tends to rebound quickly after mass extinctions, reaching comparable levels of species richness, even if the organisms making up that diversity are not the same.

Today, the number of species in the world is decreasing at an alarming rate due to human activities. We are living during a sixth mass extinction. Some estimate that species are becoming extinct at a rate not seen on earth since the Cretaceous mass extinction.

One thing that the Cretaceous mass extinction and the present-day mass extinction share is that species are becoming extinct for reasons that have nothing to do with what they themselves are like. Is this generally true of extinctions, or are mass extinctions a special case?

Evolutionist Lee Van Valen put forth the hypothesis in 1973 that extinction is indeed usually due to random events unrelated to a species's particular adaptations. If this is in fact the case, then the likelihood that a species will go extinct would be expected to be virtually constant, when viewed over long periods of time.

Van Valen's hypothesis has been tested by evolutionary biologists for a variety of groups. One of the most complete fossil records available for such a test is that of marine echinoids (sea urchins and sand dollars). The fossil echinoid you see in the photo, genus Cidaris, is from the Cretaceous some 75 million years ago. In the graph above you see an examination of the 200 million year fossil record of echinoids. Data are presented as the number of echinoid families that have survived for over a period of 200 million years. The red dashed line shows a theoretical constant extinction rate, as postulated by Van Valen. The blue line is a "best-fit” curve determined by statistical regression analysis.

1. Applying Concepts. What is the dependent variable?

2. Interpreting Data. Which curve best represents the number of echinoid families at the time of the fossil in the photo?

3. Making Inferences. Over the 200-million-year fossil record of echinoids, which of the two lines best represents the data?

4. Drawing Conclusions. Is Van Valen's hypothesis supported by this analysis?



Test Your Understanding

1. Paleontologists divide the earth’s past into the following hierarchical organization, beginning with the largest block of time:

a. age, era, period, epoch.

b. era, period, epoch, age.

c. epoch, age, era, period.

d. period, epoch, age, era.

2. Of the animal phyla, the only two to successfully populate terrestrial habitats in large numbers of species and individuals are the

a. arthropods and the segmented worms.

b. sponges and the chordates.

c. cnidarians and the arthropods.

d. arthropods and the chordates.

3. In the Paleozoic era, the first vertebrate animal group to live successfully on land was the

a. amphibians.

b. reptiles.

c. birds.

d. mammals.

4. Mammals began to diversify, with larger forms evolving

a. after the Cretaceous extinction.

b. during the Triassic.

c. at the same time that large-bodied dinosaurs evolved.

d. All of the above.

5. All fish species, living and extinct, share all of the following characteristics except

a. gills.

b. jaws.

c. internal skeleton with dorsal nerve cord.

d. single-loop circulatory system.

6. Chondrichthyes (sharks) and Osteichthyes (bony fish) have evolved anatomical solutions to increase swimming speed and maneuverability. Which modification is not found in Osteichthyes?

a. a lateral line system

b. buoyancy control through swim bladders

c. an internal skeleton made of cartilage

d. an operculum

7. Adaptations in reptiles do not include

a. an amniotic egg.

b. a layer of scales on the skin.

c. middle ear bones.

d. modifications to the respiratory system.

8. Characteristics that evolved in birds to allow for flight include

a. reptilian-like scales on the legs.

b. a hard-shelled amniotic egg.

c. internal fertilization.

d. thin, hollow bones in the skeleton.

9. Both birds and mammals share the physiological characteristic of endothermy. How do these animals maintain a high body temperature?

a. They live in warm environments.

b. They have high metabolic rates.

c. They fly, which produces heat.

d. They eat a lot.

10. A characteristic unique to most species of mammals and no other vertebrates is

a. endothermy.

b. skin covering for insulation and protection from dehydration.

c. hair.

d. a notochord.