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.
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 predators by nature, some adaptations included a herbivorous species as well, they were generally small active carnivores. Primitive therapsids are present as fossils in certain Middle Permian deposits; later forms are known from every continent except Australia but are most common in the Late Permian and Early Triassic of South Africa.
The several features that separate modern reptiles from modern mammals doubtlessly evolved at different rates. Many attributes of mammals are correlated with their highly active lifestyle; for example, efficient double circulation of blood with a completely four-chambered heart, anucleate and biconcave erythrocytes (blood cells), the diaphragm, and the secondary palate (which separates passages of food and air and allows breathing during mastication (chewing) or suckling). Hair for insulation correlates with endothermy (being warm-blooded), the physiological maintenance of individual temperature independent of the environmental temperature, and endothermy allows high levels of sustained activity. the unique characteristics of mammals thus would seem to have evolved as a complex interrelated system.
Transitions to New Higher Taxa
Transitions often result in a new "higher taxon" (a new genus, family, order, etc.) from a species belonging to different, older taxon. There is nothing magical about this. The first members of the new group are not bizzare, they are simply a new, slightly different species, barely different from the parent species. Eventually they give rise to a more different species, which in turn gives rise to a still more different species, and so on, until the descendents are radically different from the original parent. For...
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
Rowe, T. 1988. Definition, Diagnosis, and Origin of Mammalia. J. Vert.
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