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ORDOVICIAN PERIOD

488-444 million years ago

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cephalopod chasing fish


THE DIGESTIVE SYSTEM

Although the early vertebrates did modify their digestive systems, they would not be dominant members of marine food chains until the evolution of jaws. They were preyed upon by large invertebrate predators such as cephalopods and sea scorpions.


In the early vertebrates (and perhaps the primitive craniates as well), the coelom formed from mesoderm after it separated from other tissues during development rather than form from segmented pouches from the gut as in echinoderms and lancelets) (Romer, p. 315). Primitive vertebrates evolved oral glands, some of which secreted mucus (Romer, p. 329). Other changes include the reduction in the number of pharyngeal pouches (Weichert, 1970, p.214), the secretion of amylase from the pancreas (Stevens, p.160), and the function of pancreatic lipase as the most important enzyme in the digestion of lipids (Stevens, p.168).