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 lead Earth, and on this world not a case-by-case mammal lived. However a lotof time has past since then and we now have practically of furry creatures that arecollectively 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 limited transitional fossils, to show thatmammals have evolved from a group of reptiles and were simply not postal serviced here byunknown forces.     Before I begin, I would wish to define some terms so that nobody gets left-hand(a) in the dust. The term transitional fossil can be employ in conjunction withthe term general lineage, together they help explicate the how one species becameanother."General lineage"This is a sequence of similar genera or families, linking an older to a verydifferent younger group. Each tonus in the sequence consists of some fossils thatrepresent certain genus or family, and the hale sequence often covers a span oftens of trillions of years. A lineage like this shows obvious intermediates forevery major structural change, and the fossils occur roughly (but often notexactly) in the expected order. However, usually there are whitewash gaps betweeneach of the groups. Sometimes the individual specimens are not panorama to bedirectly ancestral to the next-youngest fossils (e.g. they may be "cousins"" or"uncles" sooner than "parents"). However they are assumed to be closely relatedto the true ancestor, since the have similar intermediate characteristics.Where Does It All Begin ?     Mammals were derived during the Triassic occlusive ((from 245 to 208million years ago) It began with relatively warm and wet conditions, but as itprogressed conditions became increasingly h ot and dry.) from members of thereptilian 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 in the first place p... ...ng the origin of major new adaptive types. To simplify definitions and toallow the stiff delimitation of the Mammalia, some authors have suggestedbasing the boundary on a single character, the articulation of the shell 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 salmagundi of numerous fossil species,other mammalian. characters of which, such as the degree of endothermy and treat of young and the condition of the internal organ s, probably never willbe evaluated. It must be recognized, however, that if the advanced therapsidswere alive today, taxonomists would be hard-put to decide which to place in theReptilia and which in the Mammalia.ReferencesCarroll, R. 1988. Vertebrate Paleontology and Evolution. W.H. Freeman andCo., sensitive 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.