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called a wishbone. Many of these dinosaurs look just like a bigger, slightly more lizard inspired archaeopteryx.

So Archaeopteryx had wings, but how well did they work? To our best guesses, it flew better than a domestic chicken but worse than a pigeon. This then leaves us to speculate the purpose of first developing the ability to become airborne. Scientists mainly follow two hypotheses on this point, being that early birds were arboreal or cursorial. The cursorial hypothesis states that early birds ran across the ground and used the first of their flight ability to jump, evading obstacles in their path, or obtaining a burst of speed to capture a meal or avoid being one. This hypothesis is backed up by their leg proportions, and also our knowledge of birds' running velociraptor cousins. The arboreal hypothesis is that birds took to the trees and used their wings to aid far jumps from tree to tree. It is supported by the perching ability of the feet of early bird specimens discovered, and also by the discovery of another early bird, Microraptor Gui, a Chinese specimen that had fully developed flight feathers on both its arms and legs, which may have used these double wings to glide like a flying squirrel.

With archaeopteryx, we have the basics of a bird, and can watch as, having achieved flight most successful adaptations were geared towards improving it. The first change to have we see is in a Cretaceous bird called Rahonavis, which had true pneumatic bones, meaning they were not only hollow, but filled with air sacs to help lighted the bird, also implying a more familiar avian metabolism. After that, with the group Pygostylia early birds lost their lizard tails, it being replaced by the short pygostyle all birds have today. Confuciusornis was an early example of this change, but it still had clawed wings and a sternum poorly designed for flight. Those changes came next, with the cretaceous group Ornithothoraces, "bird chested", which saw the thorax change and become keeled, and the fingers of the clawed wings began to fuse into a structure called the carpometacarpus, which could be held more rigid for flight. By this time, Ornithothoraces was enough like the modern bird that if you saw one today you wouldn't look twice, provided you didn't notice the teeth.

Here the family tree split. Two groups Enantiornithes and Ornithuromorpha both spread over the globe. Enantiornithes was characterized by a shoulder joint opposite that we see in modern birds, and unfused

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