THE Origin of Mammals

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Surprisingly,such varied mammals as mice,elephants,whales and men,have evolved from the same type of animal,small and shrew-like,that live almost 200 million years ago.These animals (called Morganucodom by some zoologists and Erythrotherium by others) had some characteristics that are found in reptiles and others that are typically mammalian.We know from studying the fossil remains of mammals that the Mammalia evolved from early reptiles and,although these two groups are distinct today,at some period during the history of the vertebrates,intermediate groups must have lived.Morganucodon is one of these intermediate from.

The mammals living today are classified into three groups or orders;the Monotremata or monotremes;the Marsupialia or marsupials;and the Placentalia or placentals.The Montremata is contained in the subclass Theria.To understand why this is so,we must trace the evolutionary history of the mammals.

The dominant land vertebrates some 250 million years ago were the reptiles.Of the tremendous diversity of from then alive,only the crocodiles,turtles,tortoises,snakes and lizards remain today.These animals,like their ancestors,are limited in three important ways;they reproduce by laying eggs,they have little control over their body temperature,and they have small brains and are therefore incapable of very complex behaviour patterns or of adapting their behaviour to changed circumstances.

It is very likely one of the reptilian features of Morganucodom was that it laid eggs.When the first monotremes evolved from Morganucodom,they too laid eggs.We know this because the last surviving monotremes,the duckbill or platypus and several species of Spiny anteater,lay eggs even today.The monotremes exhibit many other reptilian features,as in bones of the shoulder girdle and the shape of the limb bones,the cartilaginous cup in the eye,found in reptiles but not in mammals,and the shape of the cochlear bone of the inner ear.Another reptilian feature is the possession of a cloaca through which the genital,urinary and faecal products pass.

Within a hundred million years of their origins,a new method of reproduction evolved in mammalian stack.Instead of growing inside eggs,the young were born at an early stage of development and thereafter were carried attached to the maternal teats,usually within a pouch on the mother’s abdomen.This early birth was necessary because there was either no placenta or no well-developed placenta through which the embryo could be nourished from the mother’s body.

This method of reproduction was a distinct advance over the monotreme method,for it afforded more protection fro the yuong at the most crucial early days of their lives,and it is still employed by members of the second order of mammals,the mammals,the Marsupialia.Like the monotremes,the marsupials also have a few reptilian featured,but the fact that they are warm-blooded and have a covering of hair,emphasises that they can be no doubt that they are mammals.

The third mammalian order,the Placentalia,contains the vast majority of mammals living today.In all,the young are retained inside the mother for a long period prior to birth,during which the developing embryo derives all its nourishment from the mother.It was the evolution of this extremely successful method of reproduction that enable the placentals to colonize a great variety of habitats:bats in the air;whales and dolphins in t he sea;otters and beavers in rivers;and mice and men on land.

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