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	Fenestrae or skull openings        Fenestrae or skull openings (after Benton 2005).
	Primitive reptiles have long been recognized to have split into two major branches, Sauropsids and Therapsids (the latter now usually called Synapsids). The first branch, the Sauropsids or "lizard faces" (a term coined by Thomas Henry Huxley in 1863) include the forerunners of most reptiles, from dinosaurs to crocodiles and turtles. Huxley distinguished them from Therapsids or "beast faces," including mammals and their extinct relatives. Romer (1956) later subdivided the sauropsids into three basic groups: Anapsids, Diapsids, and Eurapsids, recognizable by the number of openings or fenestrae (Latin for "windows") in the side of the skull, used for jaw muscle attachment (see figure). This anatomical criteria for all reptiles remains in widespread use (Benton 2005).  The Anapsids, with no fenestrae, include many basal reptiles or eureptiles, some lizards, and perhaps also the turtles (molecular studies, however, indicate the placement of turtles within diapsids). The Diapsids, with two fenestrae,  produced the dinosaurs, and more recently, the birds. Huxley (1876) had correctly inferred that birds, who are diapsids, evolved from diapsid lizards including dinosaurs. An offshoot group of the diapsids, called the eurapsids, with two fenestrae fused into one, produced the large marine reptiles called ichthyosaurs, including Pleisiosaurs and Mosasaurs which were common during the Cretaceous period.    The other major branch of early reptiles, the Synapsids, have a single large fenestra on each side of the skull, located behind the eyes (shown at right in figure). The synapsids developed an aorta or arch on the left side only, and strengthened the skull by moving the quadrate bone up and back, eliminating the otic notch. This modification of the jaw (effectively, eliminating the location of the reptilian ear drum, in the otic notch)  proved essential in the later development of more complex anatomy related to mammalian hearing. Synapsid development led to a larger brain, more developed hearing, heterodont teeth, and homeothermy or regulated body temperature.  The group of synapsids known as Therapsids, equivalent to the term "mammal-like reptiles," are considered to be the ancestors of mammals, by way of the Cynodonts ("dog-teeth"), mammal-like synapsids in the Early and Middle Triassic period.    References Benton, M.  2005, pp.104-5 Huxley, T.H.  1863.  The Structure and Classification of the Mammalia. Hunterian Lectures, presented in Medical Times and Gazette. Huxley, T.H.  1876. Lectures on Evolution. New York Tribune. Extra. no 36. In Collected Essays IV: pp 46-138                                     Romer, A.S. 1956, 1966. Vertebrate Paleontology. University of Chicago Press.  : .
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