The concept of flight is generally considered synonymous with birds, so much so that most people would be surprised to learn that there are currently about 11 known families of extant birds who do not have the ability to fly (McCall). Among these 11 families, approximately 98 species are still alive or have disappeared in recent decades (McCall). Another surprising fact about flightless birds is that flightlessness as a trait did not appear suddenly in flying vertebrates; they all took millions of years to fly (Paul). All flightless birds belong to the class Aves, which itself belongs to the phylum Chordata and the kingdom Animalia (flightless birds). Within the class Aves, there is a superorder known as Palaeognathae, which includes most flightless birds (University of California Museum of Paleontology). Examples of well-known Paleognath birds include ostriches and kiwis (University of California Museum of Paleontology). This group is divided into two groups, tinamiforms and ratites (University of California Museum of Paleontology). Flightless birds not found in the superorder Palaeognathae are classified as Neognathae, another superorder that includes penguins, puffins, and rails (University of California Museum of Paleontology). In the literature, all these birds are often referred to as terrestrial or aquatic flightless species. It is generally accepted that although these bird species no longer fly today, they all come from an ancestor that flew at some point (Los Angeles County Museum of Natural History Foundation). It should also be noted that flightlessness can be observed on a short time scale, as a rapid evolution rather than one taking thousands...... middle of paper ......27. Web.Lloyd, Robin. Slaughtered flightless bird theory. September 8, 2008. the web. March 1, 2014. McCall, Robert A., Sean Nee, and Paul H. Harvey. “The role of wing length in evolution.” Evolutionary Ecology (1998): 569-580. Web.McNab, Brian K. “Energy conservation and the evolution of flightlessness in birds.” The American Naturalist (1994): 628-642. Web.Paul, Gregory S. Dinosaurs of the air: the evolution and loss of flight in dinosaurs and birds. Baltimore: The John Hopkins University Press, 2002. Print. Roff, Derek A. "The Evolution of Flightlessness: Does History Matter?" Evolutionary Ecology (1994): 693-657. Web. Natural History Museum of Los Angeles County founded. Birds must fly...or do they? June 1, 2008. the web. February 27, 2014. University of California Museum of Paleontology. Introduction to the Paleognaths. August 20, 1995. The web. March 1 2014.