Transitions Of Reptiles To Mammals Essay

2603 words - 10 pages

a long boring read, lots of techno words needs work on footnotes and referencesTransitions of Reptiles to MammalsA long long time ago, in a galaxy not too far away, was a little blue planet calledEarth, and on this world not a single mammal lived. However a lot of time has past sincethen and we now have lots of furry creatures that are collectively called mammals. Howdid they get their? Where did they come from? These are the kinds of questions that ledme to my subject of choice. I will endeavor to provide examples, using specifictransitional fossils, to show that mammals have evolved from a group of reptiles and weresimply not placed here by unknown forces.Before I begin, I would like to define some terms so that nobody gets left in thedust. The term transitional fossil can be used in conjunction with the term generallineage, together they help explain the how one species became another.'General lineage':This is a sequence of similar genera or families, linking an older to a very different youngergroup. Each step in the sequence consists of some fossils that represent certain genus orfamily, and the whole sequence often covers a span of tens of millions of years. A lineagelike this shows obvious intermediates for every major structural change, and the fossilsoccur roughly (but often not exactly) in the expected order. However, usually there arestill gaps between each of the groups. Sometimes the individual specimens are not thoughtto be directly ancestral to the next-youngest fossils (e.g. they may be 'cousins'' or'uncles' rather than 'parents'). However they are assumed to be closely related to theactual ancestor, since the have similar intermediate characteristics.-1-Where Does It All Begin ?Mammals were derived during the Triassic Period ((from 245 to 208 million yearsago) It began with relatively warm and wet conditions, but as it progressed conditionsbecame increasingly hot and dry.) from members of the reptilian order Therapsida. Thetherapsids, members of the subclass Synapsida (sometimes called the mammal-likereptiles),generally were unimpressive in relation to other reptiles of their time. Synapsidswere present in the Carboniferous Period (about 280 to 345 million years ago) and are oneof the earliest known reptilian groups. Although therapsids were primarily predators bynature, some adaptations included a herbivorous species as well, they were generally smallactive carnivores. Primitive therapsids are present as fossils in certain Middle Permiandeposits; later forms are known from every continent except Australia but are mostcommon in the Late Permian and Early Triassic of South Africa.The several features that separate modern reptiles from modern mammalsdoubtlessly evolved at different rates. Many attributes of mammals are correlated withtheir highly active lifestyle; for example, efficient double circulation of blood with acompletely four-chambered heart, anucleate and biconcave erythrocytes (blood cells), thediaphragm, and the...

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