Mammals are mammals that are warm-blooded and aquatic in nature, with both ears on one side of the body and a tail in the form of an extended muscular limb used in swimming. Most mammals are placid, although there are a few prominent exceptions such as the elk, badger, bear, pouched deer, squirrel and porcupine. Most are small in size, weighing no more than about two-hundred grams, although some increase to three hundred and five hundred grams in weight. They have hair in the form of hair tufts or bristle, with the exception of the well known and popular ladybird.
Classification of mammals
The classification of mammals also includes four subclasses, the order Rodentia having twelve subclasses, the order Eutheria having twelve, the family Allcidae having three, the clade Cervicalis having one, and the family Cynuridae having three. Within these orders and suborders, there are further classifications such as the clade Metatheria having three types of clades, and the clade Lari has only one. There are also two subclasses within the order Rodentia, the order Eutheria having two different clades and the order Perissus having the oldest known type of mammal.
Two broad classifications
There are many classifications of mammals, but the most commonly used one is the classification of mammals into two broad classifications: those that are placental and those that are marsupials. Marsupials are those species that retain their original anatomy after they have evolved in the sea or freshwater. The placental classification of mammals refers to those species that have met all their needs through sexual reproduction without any need for additional equipment like the placenta. The marsupials include all placental species that are the descendants of a single species that lived on the land until they later became aquatic.
The second classification of mammals is that of the terrestrial vertebrates. This classification includes all vertebrates that breathing through the skins pores and have evolved in the earth. It also includes amphibians, which include all fishes, reptiles, and non-amphibital birds and mammals. The earthworms and their relatives are an important part of this classification. This is also the system that includes all mammals with a tail.
Classification of mammals into two major classifications is based on the extent of their distribution and their classification into separate sections is based on fossil evidence. This system has many advantages over other classification systems, including those based solely on fossil evidence. The fossils represent the only examples of actual survival and evolution of these species. They also provide very precise information about the characteristics of these species, their appearance, behavior, age and population at different times.
Multituberculate
The first major classification is based on fossil evidence and includes five subclasses, including the Multituberculate (which includes four subclasses and one arch). The other five subclasses are broadly classified into two categories, those with a placenta and those without a placenta. These include the Prototheria, Eutheria, Neornithes, metathesis and the Anura. The classification of mammals based on fossil evidence provides an excellent basis for classifying and diagnosing any disease or physical anomaly in a creature. The molecular studies of living creatures on the other hand are used for investigating the relationships among species.
The third classification of mammals is based on molecular studies and classifies twenty varieties of mammal that are closely related to humans and other primates. These include the Old Primate Family and the Old Mouse Family. The classification of mammals into chondritons, monotremes and synopsis is based on fossil evidence and is also proposed by molecular studies. The classification of mammals into placental and postnatal is also proposed by molecular studies.
Terrestrial and aquatic-water species
The classification of mammals into terrestrial and aquatic-water species is based on fossil evidence and is widely accepted. The fossils consist of teeth, jaws, snouts, horns, scales, tail and jaws of dinosaurs and pre-historic creatures. The classification of mammals into Laurasian, Permian and Paleocene is based on fossils. The classification of mammals into Cenozoic, Devonian and Jurassic is based on fossils from these periods.