| Summary: | With the rapid evolution of reptilian forms during Triassic and Jurassic times, many reptilo-avian forms may have evolved. Two examples of such evolution have been preserved in the Jurassic sediments of Germany (Archaeopteryx and Archaeornis). An anatomical feature, characteristic of birds, and not reported for either of these forms, is a well-developed sternum (Wetmore, 1951). Since this is an important avian characteristic, neither of these bird-like animals should be con-sidered to be related directly to the ancestors of modern birds. Therefore, other yet undiscovered forms may be regarded as more properly comprising the Archaeornithes, and these ancestral forms may be found to have lived on the Antarctic continent at a time when that land supported a flourishing tropical vegetation and highly varied vertebrate fauna. To explain the uniform presence of a very well-developed sternum in birds would be difficult if this structure were not already present in some ancestral reptile or amphibo-reptile. While a sternum is found in amphibians, there is
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