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A model of Archaeopteryx lithographica on display at the Oxford University Museum of Natural History. It is surmised that this creature lived 155 to 150 million years ago (during the Jurassic Period) and was the size of today's eagles.
Birds share hundreds of unique skeletal features with dinosaurs. A study published in a July 2005 issue of the journal Nature suggests that dinosaurs had a bigger, more complex system of air sacs similar to that in today's birds.
A contributor to Wikipedia writes: "Research since the 1970s indicates that theropod dinosaurs are most likely the ancestors of birds; in fact, most paleontologists regard birds as the only surviving dinosaurs and some believe dinosaurs and birds should be put together under one biological class."
On the survival of birds and not dinosaurs, creatures smaller than dinosaurs had a greater ability to survive than did dinosaurs. Among these smaller creatures were birds, which could flee from predators by taking flight. According to evolutionary biologists, described in Wikipedia:
Scientists in general accept that an asteroid, 4 to kilometers in diameter, that crashed into the Gulf of Mexico around 65 million years ago created a chain of events that led to the extinction of dinosaurs. This may have included a diminution of plants. One of the advantages of being smaller was the greater availability of food: smaller creatures did not need to consume so large of a volume of food to survive. Continuing with speculation, maybe the large plant eating creatures died out because with an environmental catastrophe they lost their food supply, and when they died so too did those who ate them: the carnivores. Larger creatures were also at a disadvantage in finding quick shelter.
URL: http://www.fsmitha.com/time-birds.html