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What Happened to the Trilobites The Ordovician period was a time of great change The Cambrian explosion had resulted in almost every major phylum of organisms evolving in a period of 40 million years Freeman Herron 1998 It was also a time of the great radiation This allowed existing life forms to develop adaptations and variations which led to new species Droser et al 1996 This also allowed a huge diversity of species especially in comparison to today The Ordovician radiation is marked by changes in species genera and families Droser et al 1996 This meant the seas now had many inhabitants with different strategies and niches The shallow seas 30m were quite warm 20C with thick shale and limestone bottoms covered with sediments and dead organisms This allowed many different marine invertebrates to find appropriate habitats Soft-bodied burrowers such as the sea pen Phycodes found the thick sediments ideal for protection from predators and the environment Other inhabitants were more sessile and included the Crinoids stalked sea lilies Bryozoa lacy fans and the Brachiopods bivalves Other members of the community included the Gastropods mollusks and snails corals and Trilobites The waters consisted of few other living organisms Evolution has not yet created land plants jawed fishes or insects Belonging to the order Arthropoda the trilobites consisted of over 1500 genera The trilobites are characterized by their segmented bodies and exoskeleton made of chitin The term Tri-lob-ite meaning 3 lobed things describes their sectioned bodies see Fig1 The cephalon consists of the head region where the sense organs eyes in some species facial suture and glabella were located The compound eye was the first visual system to evolve and consisted of a set of separated optical elements called the ommatidia Oxford Brookes 1999 Two types of eyes were found in some trilobites the holochroal which consisted of closely
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