Transitions Of Reptiles To Mammals Essays -- essays research papers

Transitions of Reptiles to Mammals A long long time ago, in a galaxy not too far away, was a little blue planet called Earth, and on this world not a single mammal lived. However a lot of time has past since then and we now have lots of furry creatures that are collectively called mammals. How did they get their? Where did they come from? These are the kinds of questions that led me to my subject of choice. I will endeavor to provide examples, using specific transitional fossils, to show that mammals have evolved from a group of reptiles and were simply not placed here by unknown forces. Before I begin, I would like to define some terms so that nobody gets left in the dust. The term transitional fossil can be used in conjunction with the term general lineage, 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 younger group. Each step in the sequence consists of some fossils that represent certain genus or family, and the whole sequence often covers a span of tens of millions of years. A lineage like this shows obvious intermediates for every major structural change, and the fossils occur roughly (but often not exactly) in the expected order. However, usually there are still gaps between each of the groups. Sometimes the individual specimens are not thought to 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 the actual ancestor, since the have similar intermediate characteristics. Where Does It All Begin ? Mammals were derived during the Triassic Period ((from 245 to 208 million years ago) It began with relatively warm and wet conditions, but as it progressed conditions became increasingly hot and dry.) from members of the reptilian order Therapsida. The therapsids, members of the subclass Synapsida (sometimes called the mammal-like reptiles),generally were unimpressive in relation to other reptiles of their time. Synapsids were present in the Carboniferous Period (about 280 to 345 million years ago) and are one of the earliest known reptilian groups. Although therapsids were primarily p... ...ng the origin of major new adaptive types. To simplify definitions and to allow the strict delimitation of the Mammalia, some authors have suggested basing the boundary on a single character, the articulation of the jaw between the dentary and squamosal bones and the attendent movement of accessory jaw bones to the middle ear as auditory ossicles. The use of a single character allows the placement in a logical classification of numerous fossil species, other mammalian. characters of which, such as the degree of endothermy and nursing of young and the condition of the internal organs, probably never will be evaluated. It must be recognized, however, that if the advanced therapsids were alive today, taxonomists would be hard-put to decide which to place in the Reptilia and which in the Mammalia. References Carroll, R. 1988. Vertebrate Paleontology and Evolution. W.H. Freeman and Co., New York Gingerich, P.D. 1977. Patterns of Evolution in the Mammalian Fossil Record. Elsevier Scientific Pub. Co. Gingerich, P.D. 1985. Species in the Fossil Record: Concepts, Trends, and Transitions. Paleobiology. Rowe, T. 1988. Definition, Diagnosis, and Origin of Mammalia. J. Vert. Paleontology.