Mammals

Reptiles arose from amphibians, and birds arose from reptiles. However, mammals, such as the Armadillo shown above in my backyard, did not arise from birds. Mammals, like birds, arose from early reptiles, though from a different reptilian stock than the birds. Here are the main "new inventions" nature came up with that made us mammals more complex beings than amphibians, reptiles and birds:

Mammals, like birds, but unlike their reptilian ancestors, are warm-blooded -- their bodies stay warm, even when surrounded by cold air.

Mammal bodies are covered with hair, which helps the body stay warm; even the Armadillo above has hairs.

Mammal teeth come in a variety of shapes, known as incisors, canines, premolars and/or molars, with each tooth type specialized for stabbing, cutting, tearing, or grinding of food. Reptilian teeth aren't nearly as diverse.

Mammal ears are relatively complex, the middle ear containing three bones, as opposed to only one bone in the middle ear of reptiles and birds.

Mammal brains are much larger than those of reptiles, enabling greater intelligence and consequent flexibility of behavior.

Female mammals possess mammary glands -- called teats in many species and breasts among humans -- which secrete nourishing milk for the newborn.

Modern mammals don't lay eggs, as reptiles and birds do. This enables mammal females to remain active as the embryo undergoes its early development.

Archaeopteryx, the earliest-known fossilized bird, was a curious mixture with both reptilian and bird features. The same sort of "missing link" exists for the mammals, except that this link is still alive. There's a very primitive group of mammals found in Australia and New Zealand known as monotremes which do lay eggs, just like their reptilian ancestors. Only two monotreme species survive: the Duck-billed Platypus , and the Spiny Anteater. Though these creatures lay eggs, their females also produce milk. The Duck- billed Platypus also has a very unmammal-like duck-bill. Some scientists have seriously suggested that monotremes, instead of being reptile-like mammals, are mammal-like reptiles!

To give you a feeling for the diversity of the non-egg-laying subclass of mammals, here's a list of the best-known orders:

INSECTIVORA: moles and shrews
CHIROPTERA: bats
PRIMATES: monkeys, apes, humans
EDENTATA: armadillos, anteaters
LAGOMORPHA: rabbits, hares
RODENTIA: mice, rats, squirrels, porcupines
CETACEA: whales, dolphins, porpoises
CARNIVORA: dogs, cats, raccoons, skunks, seals
PROBOSCIDEA: elephants
PERISSODACTYLA: horses, zebras, rhinoceroses
ARTIODACTYLA: pigs, deer, cattle, hippopotamuses

Other entire orders of mammal exist, which you've probably never heard of. For instance, there's an order for the flying lemur, one for the pangolin, another for the aardvark, and several others -- some 17 orders in all.

anatomy of a chipmunk's lower jaw After you've spent some time tramping in the fields and woods, you are surprised by how often you find a mammal's skeletal skull or lower jaws, with no hint of the rest of the animal. For example, the other day in the mulch below my azaleas I found the lower jaw at the right. It was only about 1Ľ-inch long (33 mm) and at first I thought it was a rat's jaw bone. However, using the Key-Guide to Mammal Skulls and Lower Jaws, I "keyed it out" to belonging to a chipmunk. While using the key I had to learn a few interesting terms, such as condyle, coronoid process and ramus, and notice how they related to one another, but I enjoyed that!

Clearly, once we learn "mammal basics" by becoming familiar with our backyard mammals, we have a whole world of interesting critters out there to meet!

You may want to review some mammal-oriented books available at Amazon.com by clicking here.