Types of Animals on Earth Essay

1102 Words5 Pages
Earth is the only known planet up to date which sustains life. A few millions years ago, Earth was home to the one of the most vicious and devastating creatures well-known to mankind ̶ dinosaur. Wrapped in a cocoon of evolution, Earth, along with most living beings, had endured metamorphosis, transforming the world to what it is now. Gigantic deadly dinosaurs went extinct, whilst only a minority that survived mutated into smaller-sized animals. Animals that roam the Earth are classified into three major categories, namely mammals, reptiles and amphibians, where each category is further divided into their respective orders (refer to Figure 1 in Appendix 1). The first major type of animal is mammal. There are more than 4,000 different…show more content…
Mammals are sub-divided into two distinct groups: Prototheria and Theria (Eisenberg & Redford, 1999). Members of the subclass Prototheria lay eggs like most non-mammalian vertebrates. However, they feed their newborn with mammary gland secretions like all other mammals. They lack nipples, but the skin over their mammary glands exude milk for their babies. The Prototheria are also referred to as monotremes, which literally means that they have one opening for excretion and reproduction. This is similar to birds and reptiles. On the other hand, all other living mammalian species, including humans, are in the subclass Theria. They have in common the fact that they give birth to live young. Therian mammals apparently did not evolve from the Prototheria. The relatively primitive prototherian reproductive system evidently evolved after their evolutionary line separated from the other early mammals. The next type of animals to be discussed is reptile. The modern reptiles representing only a small part of those present in ancient times. Back to the history there are 16 orders of reptiles but only four have living representatives which include three of the six described subclass (Royce E. Ballinger & John D.Lynch, 1983). Unlike the mammal which
Open Document