Bilaterally symmetrical animals
CLASS: Mammals
Features of true mammals include advancement in brain development, skin hair and sweat glands. Mammals also developed mammary gland, but this is only present in the female.
Mammals also have bones within each of their ears. These bones are thought to have evolved from unneeded jaw bones, and are an integral part of mammals hearing.
Mammalia appeared on Earth around 220 million years ago, somewhat of a delay since the last significant split in the relationship between land vertebrates. This delay is most probably attributable to the Great Dying, which caused a mass extinction of Earth life around 250 million years ago.
Subclasses of the mammalia are allotheria, prototheria and theria.
Alloteria were a form of mammal distinguishable by having molar teeth with two cusps at the front and back of each tooth. These mammals have become extict.
Prototherians are often considered to be ancestral to the therians in that they have retained the primitive braincase formation and the linear cusp alignment on their teeth.
The therians give birth to live young, as opposed to eggs. Their ears are external, and their ankles have adapted to allow for versatile land movement.
Humans belong to the theria subclass.
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ALLOTHERIA Extinct mammals with uniquely cusped molar teeth. |
PROTOTHERIA Mammals with more primitive braincase and teeth structure. |
Mammals that give birth to live young. |
