Class Mammalia

click to find the answer to today's question What's special about the 28 teeth of an armadillo?
The Ringtail (Bassariscus astutus) link to an Internet Website is the Arizona State mammal.

Ringtail Class characteristics: link to an Internet Website

The opossum, Didelphis virginiana, link to a local picture is the only marsupial in the northern hemisphere. Marsupials give birth to tiny, embryonic offspring that complete development while attached to a nipple in an external "pouch" on the abdomen of the the female.

Other Arizona mammals are eutherians, placental mammals. The placenta develops from embryonic cells that grow out from tissue surrounding the developing embryo and make contact with the uterine wall. Placental blood vessels absorb nutrients from the mother's body by osmosis, but there is no direct connection between the circulatory system of the embryo and the mother.

The shrew link to a local picture has the distinction of being the only poisonous mammal in North America. While it doesn't have fangs, its saliva has a toxin that paralizes its prey. Shrews are very aggressive and will attack animals several times their size. If shrews were not the smallest mammals in the world, they would be the most feared. Arizona has many species of bats, the only mammals that can fly. link to an Internet Website

For sheer numbers, the Mexican freetail bat, Tadaria mexicana, is the most common. Many of these bats can be found in Arizona. Bats have a weight to surface area ratio that allows them to actually fly. The front legs of bats are modified into wings. Most of the wing surface area comes from a hairless membrane link to a local picture covering the fingers of the hand. The bones of the bat skeleton link to a local picture are not hollow or porous, but they are very small and lightweight.

the flying squirrl The southern flying squirrel, link to an Internet WebsiteGlaucomys volans, is found in southeastern portion of the United States. This animal has a loose fold of skin along each side of the body that expands when the legs are extended. Since they have no wings to produce "lift", they can only glide from branch to branch.

Mammal teeth: link to an Internet Website

The teeth of mammals are highly specialized for their specific types of feeding. Mammals are heterodont, meaning that they have several types of teeth.

Mammal teeth are thecodont, being set in sockets in the jaw bones.

Most mammals are diphyodont, link to an Internet Website having two sets of teeth. They first grow a set of deciduous teeth, which are shed and replaced by permanent teeth as the jaws grow to full size. These "milk teeth" do not include molars. The permanent teeth of many mammals stop growing when they reach full size. These teeth are called rooted teeth. The teeth in some mammals continue to grow throughout life. The incisors of rodents are examples of these rootless teeth.

Mammals can be identified by their dental formula, which is an expression of the number of each type of tooth on one side of the upper jaw over the number of each type of tooth on one side of the lower jaw. For example, a mammal whose dental formula is 3/3, 0/1, 3/3, 2/3 would have a total of 36 teeth. Each side of the upper jaw would have 3 incisors, 0 canines, 3 premolars, and 2 molars. Each side of the lower jaw would have 3 incisors, 1 canine, 3 premolars, and 3 molars.

Dentitions of some common mammals
  • Humans
  • Dogs
  • Cats
  • Raccoons
  • Opossum
  • Skunk
  • Shrews
  • Rabbits
  • Rodents
  • Squirrels
  • Horses
  • Pigs
  • Cows & Sheep
  • Deer
  • Armadillos
2/2, 1/1, 2/2, 3/3
3/3, 1/1, 4/4, 2/3
3/3, 1/1, 3/2, 1/1
3/3, 1/1, 4/4, 2/2
5/4, 1/1, 3/3, 4/4
3/3, 1/1, 3/3, 1/2
3/1, 1/1, 3/1, 3/3
2/1, 0/0, 3/2, 3/3
1/1, 0/0, 3/2, 3/3
1/1, 0/0, 2/1, 3/3
3/3, 0/0, 3/3, 3/3
3/3, 1/1, 4/4, 3/3
0/3, 0/1, 3/3, 3/3
0/3, 0/1, 3/3, 3/3

Day 2

click to find the answer to today's question What two things make the skull of an opossum very easy to recognize?

Mammal classification:

There are less than 950 species of living mammals contained in 16 orders. There are 3 species of monotremes, 57 species of marsupials, and the remainder of the species are eutherians.

The external appearance of a mammal is greatly affected by several environmental factors. Because of this, characteristics of the skull, teeth, and feet are generally used in classification and identification of mammals instead of color, size, etc.

The skulls of mammals are quite distinct and can be used to identify the species of the animal. The drawings below shows the top and bottom of a mammal skull with parts labeled.

  • Details of skull top link to a local picture
  • Details of skull bottom link to a local picture


Mammals usually leave distinctive tracks the can be used to identify the animal and determine a good deal of information about it.






Research Links:

Zoology Class










All the teeth of an armadillo are the same type.










The brain case of the opossum is extremely small compared to the overall size of the skull. The opossum has 50 teeth.