Eight Different Classes of Vertebrates Help (page 3)

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Updated on Aug 29, 2011

Class Mammalia: Backboned Animals With “breasts”

True to their name, mammals are vertebrates that have mammary ( MAM -ah-ree) glands within their “breasts” ( mamm ). We, as human beings, of course, share such mammalian (mah- MAY -lee-un) characteristics as the nursing of the young with milk secreted by the mammary glands of the mother’s breasts. Mammals also have bodies that are covered with hair, rather than with feathers (although hair, like feathers, is rich in the waterproof keratin proteins).

Mammals are like birds in that they are endothermic (as was noted back in Chapter 3). This means that their body “heat” ( therm ) is regulated from “within” ( endo -). (Thus, endothermic is a close synonym or relative of the word homeothermic.)

Another similarity between birds and mammals is their ancestry. It is quite likely that both classes of “higher” vertebrates evolved from the same “lower” class – the reptiles. While Archaeopteryx and other early birds flew over the heads of the dinosaurs, the first mammals were probably small and rodent-like. They probably hid in the forest and ate insects, or scurried to hide in holes below the huge, lumbering feet of the dinosaurs!

The Three Major Groups Of Mammals

After an ancient comet or asteroid struck the Earth and created a massive cloud of dust and debris, the lack of sunlight and progressive cooling and drying of the climate probably contributed to the extinction of the dinosaurs. With these “Terrible Lizards” or Kings of the Reptiles gone from the scene, the small, rodent-like mammals had a giant niche (nich) or empty “nest” of environmental roles and functions ready to be filled.

During the Cenozoic Era or “Age of Mammals” (Chapter 3), stretching from about 65 million years ago up to the present, three major groups of mammals have extensively evolved. These are called the monotremes ( MAHN -uh-treems), marsupials (mar- SOO -pee-als), and the placentals (plah- SEN -tals) (see Figure 12.8).

The Chordata: Animals with a “Chord” in Their “Back” Wide Diversity in Backboned Creatures: Eight Different Classes of Vertebrates The Three Major Groups Of Mammals

Fig. 12.8 The great story of the mammals.


The monotremes are literally mammals with a “single” (mono-) “hole” (trem)! This odd name reflects the fact that monotremes have just a single, common opening for both their urinary and reproductive tracts. So, besides excreting urine, the monotremes also lay their eggs through this single outlet. As the only group of egg-layers, the monotremes are considered the most primitive type of mammals (closest to the reptiles). Today, they are mainly represented by the platypus ( PLAT -uh-pus) and the spiny anteaters. The platypus is named for its “flatfootedness,” but an even more obvious characteristic is its prominent duckbill. The duckbilled platypus lives in Australia and New Guinea, mainly feeding upon insects.


Marsupials are mammals with “little pouches” (marsupi). In the wilds of North America, the most familiar marsupial is probably the opossum. But as captives in zoos, we usually associate pouches with the kangaroo. The koala bear of Australia is another famous marsupial. Rather than hatching from eggs (like the monotremes), the young marsupials are viviparous (vy- VIP -urus) – “born” ( par ) “alive” ( vivi ). But, since they are born almost naked and at a very early stage, the young marsupials must finish their development within their mother’s pouch.


Finally, most modern mammals are placentals. In everyday English, this exactly means that their young are nourished by a “flat cake” ( placent ), actually an internal organ called the placenta (plah- SEN -tuh). During development, the youngster in its embryo and later fetal ( FEE -tal) stage is attached to the placenta organ by an umbilical (um- BILL -uh-kul) or “pertaining to the navel” cord . The fetal blood circulates through the umbilical cord and into the placenta, where it releases waste products and picks up glucose, oxygen, and other nutrients from the mother’s bloodstream. And instead of developing within a pouch, the fetus develops within the mothers’ uterus ( YEW -ter-us); that is, her “womb” ( uter ).

Placental Mammals Orders of Classifications

Order Insectivora and Chiroptera

This very large group of modern placental mammals is usually subdivided into a number of highly distinctive Orders. The core insectivores (in- SEK -tuh-vors) or Order Insectivora ( in -sek- TIH -vor-ah), for instance, are the basic group of “insect devourers” ( vores ). The core insectivores include the moles and shrews. Also eating insects are the members of Class Edentata ( ee -den- TAY -tah), or mammals “without” ( e -) “teeth” ( dent ). The toothless (or nearly toothless) Edentata involve such long-tongued mammals as the anteaters and armadilloes , and such “slow”-moving plant-eaters as the sloths ( SLAWTHS ). Members of the Order Chiroptera (keye- RAHP -ter-ah) have their “hands” ( chir ) or forelimbs modified to form “wings” ( pter ). The night-flying, insect-eating bats obviously belong to this Order.

Order Rodentia and Lagomorpha

Order Rodentia (row- DEN -shah), quite differently, embraces the “gnawers” with big, chisel-like teeth. Rats, mice, hamsters, beavers, squirrels, and porcupines are all rodents . The Order Lagomorpha ( lag -uh- MOR -fuh) encompasses the huge group of long-eared, chisel-teethed, “hare” ( lago ) “shaped” ( morph ) mammals, such as the modern rabbits and hares.

Order Artiodactyla and Perissodactyla

When looking at toes rather than teeth or ears, we consider the Order Artiodactyla ( ar -tee-oh- DAK -tuh-luh) and the Order Perissodactyla (puh- ris -uh- DAK -tuh-luh). The Artiodactyla are four-footed, hoofed mammals with an “even-number” ( artio ) of “toes” ( dactyl ), usually either two or four per foot. This even-numbered toe group claims the camels, deer, pigs, cattle, and sheep. The Perissodactyla, in marked contrast, are those hoofed mammals having an “uneven” ( perisso ) number of “toes” ( dactyl ) on each foot. Horses and rhinoceroses are familiar members of this uneven-toed Order.

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