The Archaeopterygidae is a group including only the first known bird, Archaeopteryx, from the Late Jurassic of Germany. Archaeopteryx is represented by several excellent, two-dimensional

(flattened) fossil skeletons, many of which include skulls and/or feather impressions. Archaeopteryx has been called the most valuable of all fossils because of its importance in understanding the evolution of species. At the time of its discovery in the early 1860s, Charles Darwin had only recently proposed his theory of evolution by natural selection. Darwin himself relied primarily on observations of living species to explain his case for evolution. Evolution is a process that typically requires millions of years, however, and his opponents argued, even then, that anatomical transitions from one species to another were difficult to ascertain from the observation of living species alone. Archaeopteryx provided another kind of evidence entirely, blending the features of reptiles with those of birds from the very annals of time, demonstrating in a single fossil taxon that one species could likely evolve into another and that the process of avian evolution had been taking place for tens of millions of years.

Diagnostic traits that were found in these first recognized birds included small, sharp teeth in upper and lower jaws, five fused sacral vertebrae, and lack of a bony breastbone that were specific to Archaeopteryx. These same creatures had other obviously dinosaurian features such as a long, bony tail and sharply clawed fingers and toes, but those traits were less unique to the Archaeopterygidae than the specific nature of their teeth, jaws, and limb girdles.

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