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Old Sunday, April 19, 2009
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Default Mammals


There are approximately 4,260 different mammalian species that have been discovered to date, although this figure varies because not all scientists agree that certain organisms are a distinct species.
In addition, new species are always being discovered, therefore this figure of how many different mammals exist is always changing.
Mammals are all warm-blooded, and all mammals are vertebrates (meaning they have vertebrae, forming a spine), but there are also other animals, like birds, that have these characteristics, so there are additional traits that set mammals apart.

Characteristics of Mammals

Mammals have six key characteristics that can be seen in each and every mammal, and it’s these traits that set mammals apart from other types of creatures:
1. Mammals produce milk to feed their young. Female mammals possess a modified sweat gland – a mammary gland – that is activated by hormonal changes that occur with pregnancy. In fact, this trait is what inspired the term “mammal,” a derivation of “mammary.”
2. Mammals all have one single bone comprising their lower jaw. In all other animals, more than one bone comprises the jaw.
3. All mammals have three tiny bones in the middle portion of the ear.
4. All mammals have a diaphragm. The mammal's diaphragm is a thin muscular wall that separates the upper and lower portions of the torso.
5. All mammals have fur or hair. Hair or fur is a characteristic that's only seen in mammals. All mammals develop fur or hair at some point during their development, though not all keep their fur or hair throughout their lifespan.
6. Mammals have a unique heart. The heart of a mammal is unique in that it has one primary artery leaving the heart bending to the left, whereas other animals either have multiple arteries in the heart or the heart's main artery bends in a different direction.

Categories of Mammals

Within the class of animals considered mammals, there are three categories: eutheria, metatheria and prototheria.
The three categories of mammals can be described as follows:
1. Eutheria - Eutheria are mammals possessing a placenta, like a human or dog.
2. Metatheria - Metatheria are also known as marsupials or pouch-bearing mammals like the kangaroo.
3. Prototheria - Prototheria are also known as monotremes or egg-laying mammals like the duckbill platypus.

Exclusive Traits of Mammals

In addition, there are a few characteristics that are exclusive to mammals, meaning only animals have these traits. But, in each case, there are some mammals that don't have these traits, which is why they're different from the characteristics of mammals (the mammal characteristics are seen in each and every mammal).
· The vast majority of female mammals have a placenta, used to protect and nourish the offspring prior to birth. Marsupials and monotremes do not have a placenta.
· In their lifetime, a mammal will not have more than two sets of teeth. Typically, mammals grow one set of teeth as juveniles, and then a new permanent set grows in as they near adulthood.
· A mammal is warm blooded, meaning it has the ability to generate its own body heat and maintain a steady body temperature, despite ambient temperature changes.
· Mammals also have a separation between their mouth and nasal cavity. Other animals, like reptiles do not have an upper palate; this allows the nasal cavity to remain open regardless of whether there is something inside the mouth.

Multituberculates - An Extinct Category of Mammals

In addition to the three categories of mammals — eutheria, metatheria and prototheria — there was once a fourth mammal category that is now completely extinct.
Multituberculates are a category of mammal that arose during the late Jurassic period 160 million years ago and they survived up until about 35 million years ago.
Multituberculates have no living descendants today, but fossil records indicate that they were similar to modern rodents.
Multituberculates were named for their teeth. These mammals had one pair of incisors on the lower jaw and their molars had numerous cusps forming numerous rows of teeth. These mammals also lacked canine teeth on the upper jaw, like many rodents of today.

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