Transitions Of Reptiles To Mammals Essays -- essays research papers

Transitions of Reptiles to Mammals     A long long time ago, in a galaxy non too far away, was a little blueplanet cal direct Earth, and on this world not a single mammal lived. unless a lotof time has past since then and we now select lots of furry creatures that arcollectively 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 willendeavor to provide examples, using specific transitional fossils, to show thatmammals have evolved from a group of reptiles and were simply not fit(p) here byunknown forces.     Before I begin, I would like to define some circumstances so that nobody getsleft in the dust. The term transitional fossil can be used in conjunction withthe term general railway line, together they help explain the how one species becameanother."General lineage"This is a sequence of similar genera or families, linking an older to a verydi fferent younger group. Each step in the sequence consists of some fossils that follow certain genus or family, and the whole sequence often covers a span oftens of millions of years. A lineage like this shows obvious intermediates forevery major structural change, and the fossils chance roughly (but often notexactly) in the expected order. However, usually there are still gaps betweeneach of the groups. Sometimes the individual specimens are not thought to bedirectly ancestral to the next-youngest fossils (e.g. they may be "cousins"" or"uncles" rather than "parents"). However they are assumed to be closely relatedto the material ancestor, since the have similar intermediate characteristics.Where Does It All Begin ?     Mammals were derived during the Triassic Period ((from 245 to 208million years ago) It began with relatively warm and wet conditions, but as itprogressed conditions became more and more hot and dry.) from members of t hereptilian order Therapsida. The therapsids, members of the subclass Synapsida(sometimes called the mammal-like reptiles),generally were unimpressive inrelation to other reptiles of their time. Synapsids were present in theCarboniferous Period (about 280 to 345 million years ago) and are one of theearliest known reptilian groups. Although therapsids were primarily p... ...ng the origin of major new adaptive types. To simplify definitions and toallow the strict delimitation of the Mammalia, some authors have suggestedbasing the boundary on a single character, the articulation of the jaw betweenthe dentary and squamosal bones and the attendent movement of accessory jawbones to the middle ear as auditory ossicles. The use of a single characterallows the placement in a logical classification of numerous fossil species,other mammalian. characters of which, such as the degree of endothermy andnursing of young and the condition of the internal organs, probably never willbe evaluated. It must be recognized, however, that if the advanced therapsidswere alive today, taxonomists would be hard-put to reconcile which to place in theReptilia and which in the Mammalia.ReferencesCarroll, R. 1988. Vertebrate Paleontology and Evolution. W.H. Freeman andCo., New YorkGingerich, P.D. 1977. Patterns of Evolution in the Mammalian fogey Record.Elsevier Scientific Pub. Co.Gingerich, P.D. 1985. Species in the Fossil Record Concepts, Trends, andTransitions. Paleobiology.Rowe, T. 1988. Definition, Diagnosis, and Origin of Mammalia. J. Vert.Paleontology.