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ORDOVICIAN PERIOD

488-444 million years ago

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THE DIGESTIVE SYSTEM
The early gnathostomes modified their digestive system to accomodate more diverse diets allowed by their jaws. These changes included a modification of a portion of the intestine to form a stomach and the evolution of teeth.

Early gnathostomes (after the placoderms) evolved dermal denticles which are homologous to teeth (Romer, p. 333). Prior to the evolution of teeth, SPARCL1 gave rise to amelogenin, enamelin, and ameloblastin early in the history of gnathostomes (Kawasaki, 2004). Gills were no longer involved in feeding (as in lower chordates, lamprey larvae, and fossil ostracoderms) (Romer, p. 357). Gnathostomes evolved lips (Torrey, 1979, p. 317) and the hypophyseal pouch of the stomodeum was incorporated into the mouth (Kardong, p. 490). The muscularis layer of the gastrointestinal tract included inner circular and outer longitudinal muscle layers (Stevens, p. 20). The gnathostome pancreas resulted from a fusion of exocrine and endocrine regions which were previously separate (Kardong, p. 519). The liver increased in size and became divided into lobes (Weichert, 1970, p. 194). The ancestors of gnathostomes evolved a stomach which subsequently developed a pyloris and a fundus region (Romer, p. 378). Digestive enzymes such as pepsin and elastase were utilized in digestion (Stevens, p. 171). The rectum is also defined in gnathostomes.

THE RESPIRATORY SYSTEM
In respiration, the medulla is responsible for rhythmic breathing and adaptations to hypoxia include an increase in hematocrit (Hoar, 1970)