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Desert Diary
Biology/Amniotes and Anamniotes


There is a single scheme used by all biologists to classify organisms. Some of you will recognize the sequence from bigger to smaller groups: kingdom, phylum, class, order, family, genus, and species, and remember that each rank has subdivisions, such as suborder and subfamily.

However, there also are a large number of other classifications used informally for communication among biologists. You may be familiar, for example, with the term "tetrapod", which separates all the four-legged vertebrates and their descendants from the others. Thus tetrapods include all the amphibians, reptiles, birds, and mammals, but not the various fish groups.

A less familiar classification divides vertebrates into two groups: the anamniotes and the amniotes. This separates the vertebrates into the various fishes and amphibians on the one hand from the reptiles, birds, and mammals on the other. The amniotes have a special membrane, the amnion, enclosing the embryo—as one biologist put it, giving each embryo its own private swimming pool. The amnion freed us from the necessity of returning to water to reproduce, and allowed the amniotes to conquer the land.
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Contributor: Arthur H. Harris, Centennial Museum, University of Texas at El Paso.

Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.