The first mammals evolved on Earth during the early Jurassic period approximately 200 to 175 million years ago. These early mammals evolved from a common ancestor they shared with reptiles (Fig. 5.22A). Early mammals, like their reptile ancestors, were tetrapods and amniotes (Fig. 6.4). Tetrapods are vertebrate animals with four limbs as well as their evolutionary descendants. Snakes and whales lack four limbs but are still considered tetrapods because they evolved from animals with four limbs. Amniotes are tetrapods with that produce an amnion or watertight membrane surrounding the embryo developing inside an egg. Mammals and reptiles (including birds) are the only two extant groups of amniotes.
The term tetrapods is introduced in Amphibians; the term amniotes is introduced in Reptiles.
Most reptiles give birth to their offspring by laying eggs externally. Early mammals likely also laid eggs. One branch of basal mammals that survives to this day is the monotremes, a group of mammals that lay eggs. The entire group consists of the platypus and four species of echidnas. Figure 6.4.1 shows an example of echidnas (Fig. 6.4.1 A) and a platypus (Fig. 6.4.1 B). Other branches of early mammals evolved and diversified, but most went extinct. The only mammal branches to survive to this day are the basal monotremes, marsupials, and placentals. Placentals are mammals with a placenta (see Structure and Function for a more detailed discussion about placenta), an organ that connects the developing embryo directly to the mother. Examples of placental mammals include rodents, bats, primates (including humans), dogs, horses, rabbits, and whales. Approximately 94 percent of all extant mammal species are placentals (Fig. 6.5). Marsupial mammals are characterized by their front pouches used to protect their newborn offspring. Examples of marsupial mammals include kangaroos, wallabies, wombats, the koala, and the Tasmanian devil. These three extant mammal groups—monotremes, marsupials, and placentals—are monophyletic, meaning the members of each group descend from one common evolutionary ancestor.
Mammals are a diverse group of vertebrate animals that includes between 5,000 and 5,500 species. These mammal species vary greatly in terms of their sizes and shapes. The two largest groups of mammals are the rodents and bats (Fig. 6.5). Approximately 42 percent of all extant mammal species are rodents, animals like mice, rats, squirrels, porcupines, and beavers. The vast majority of mammal species are terrestrial, meaning they live exclusively on land. There are three major groups of marine mammals, or mammals that live primarily in ocean environments: cetaceans, sirenians, and pinnipeds.
Cetaceans are the group of aquatic mammals we know more commonly as whales. Cetaceans are readily identified by their smooth streamline shape and lack of hind limbs. They remotely resemble large fish, although whales and fish are not closely related. Cetaceans fall into two groups: the “toothed whales” or odontocetes (Fig. 6.6 A), and the “baleen whales” or mysticetes (Fig. 6.6 B).
Odontocetes are distinguished from other whales by their hard sharp teeth (Figs. 6.7 B, 6.7 C, and 6.7 D). Examples of odontocetes include the bottlenose dolphin (Fig. 6.6 C), sperm whale (Fig. 6.3 B), and killer whale (Fig. 6.6 A). Mysticetes are whales readily identified by the baleen in their mouths in place of hard teeth. Baleen is the system of bristly comb-like structures in the mouths of mysticete whales (Fig. 6.7 A). They are used to filter particles of food—such as small fish or tiny crustaceans—as they swallow mouthfuls of seawater. Baleen can grow to over four meters long in some mysticete whale species, most notably the bowhead whale (Fig. 6.6 D).
Odontocete whales tend to be smaller in size than mysticete whales. All dolphins and porpoises are considered to be odontocetes or toothed whales. Most cetacean species are marine, although some dolphin species can be found in large freshwater rivers.
Sirenians are the group of marine mammals that include the manatees (Fig. 6.8 A) and the dugong (Fig. 6.8 B). Also known as sea cows, sirenians have elongated tapering bodies with short fore limb flippers and a single wide flat tail fin. Like cetaceans, they do not have external hind limbs and live exclusively in aquatic habitats. Unlike all other groups of marine mammals, sirenians are primarily herbivores, eating seagrasses and algae. Only four species of sirenians are extant today. One large sirenian species was the Steller’s sea cow, which grew to nine meters long (Fig. 6.8 D). This species went extinct in the late 1700s due to overhunting for food and skins, which were used in shipbuilding.
Carnivorans are group of mammals largely distinguished by their dense fur, sharp claws, and canine teeth. Examples of carnivorans (order Carnivora) include dogs, cats, bears, seals, and skunks. Note that term carnivoran is used to describe a biological classification: members of the order Carnivora. This term is distinct from the functional term carnivore, which describes a meat-eating animal. The giant panda is an example of an herbivorous carnivoran. Most carnivoran species are terrestrial. However, one particular carnivoran group is exclusively marine: the pinnipeds.
Pinnipeds include seals, sea lions (Figs. 6.9 A, 6.9 B, 6.9 C, and 6.9 F), and the walrus (Fig. 6.3 C). Although pinnipeds are very well adapted to living in the ocean, they must return to land to find a mate and give birth to their young. There are 33 extant species of pinnipeds. They are distributed throughout the world, although most species are found in colder climates.
Three other species within order Carnivora are considered to be marine mammals: the polar bear (Fig. 6.9 D), the sea otter (Fig. 6.3 A) and the marine otter (Fig. 6.9 E). Polar bears are considered marine mammals because they spend a majority of their time swimming in the ocean or hunting seals on sea ice. The polar bear’s species name, Ursus maritimus, means “ocean bear” in Latin. Polar bears are found exclusively in the Arctic. Two otter species—belonging to the weasel family of mammals—are also considered marine in nature. The sea otter (Enhydra lutris) lives in coastal kelp forests in the northern Pacific ocean basin from Russia to southern California. The marine otter (Lontra felina) belongs to the same genus as freshwater river otters. This species occurs predominately along the southeastern Pacific coasts of Peru and Chile. In contrast to other marine carnivorans, the marine otter spends much of its time out of the water foraging for food along rocky coastlines (Fig. 6.9 E).
Both fossil and molecular evidence indicate that marine mammals did not evolve or descend from one single ancestral group. Although cetaceans, pinnipeds, sirenians, otters, and polar bears are all mammals, they evolved from separate branches of the mammal line (Fig. 6.4). All of these separate branches evolved from terrestrial mammal groups. Cetaceans evolved from a group of mammals that include hoofed or “even-toed” ungulates such as cattle, deer, and sheep. The hippopotamus is the closest living ancestor to the cetacean group. Sirenians share a common evolutionary ancestor with modern elephants. Pinnipeds share common ancestry with other carnivorans such as dogs and cats, but are most closely related to the weasels, otters, and skunks. Although many of the adaptations for ocean survival seen in these marine mammals appear similar in form and function, each of these marine mammal groups evolved adaptations for life in an ocean environment independently.
Convergent Evolution
Evolutionary biologists are interested in the physical form of structures on an organism. They are also interested in how these structures function or work to improve the survival and reproduction of the organism. Form and function are so closely tied together that, sometimes, unrelated organisms develop similar structures. We call this phenomenon convergence or convergent evolution. You can see convergence in very different animals that live in similar habitats or have similar lifestyles.
A bat is a mammal, and yet it appears to have flying structures in common with many birds and insects. All of these flyers have wings. If we take a closer look at these structures, though, we will see that they are not as similar as they might appear. The wings of bats are supported by bones that are basically elongated finger bones, whereas the wings of birds are covered with feathers, a structure lacking in bats. Insects have no bony supports within their wings, and the membranous structures are actually part of the exoskeleton, composed of chitin and proteins. Even though all of these animals use their wings to fly, when we look closely at them, we see that the wings themselves are very different and, taken along with other characteristics of the organisms (birds and insects do not have hair, nor do they feed their young with milk), do not indicate a recent common ancestor.
Whales are aquatic mammals that, like fish, propel themselves through the water with fins. Even though whales have fins, they are not fish. If we look at the fins of whales and the fins of fish we will see some substantial differences. The fins of whales are supported by the same kinds of bones that support the wings of bats. In fact, if we look at these structures closely, they provide evidence for a closer relationship between bats and whales than between bats and birds or between whales and fish. Fish fins are very different than whale fins. Fish fins are supported by bones, but the origin and structure of the bones supporting the fins are very different than those found in whales.
Convergent evolution can make it challenging to determine the evolutionary relationships between animal groups. Even though organisms may change a great deal over time, clues to their ancestry remain in structures that are homologous, that is, that have the same origin. As previously mentioned, bat wings and whale fins appear to be very different, but if you examine the underlying bone structure, you will see that they all come from the same bones that are also found in chimpanzee hands or horse hooves.