the evolution of 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 (~20°C), 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 T
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 Fig.1). 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 packed lens units, shaped like a truncated cone. The second eye type was the schizochroal, which was separately incased and maintained in position by the muscular sclera (Oxford Brookes, 1999). The body region (thorax) consisted of many hinged plates, which allowed them increased flexibility. In some species, this made it possible for them to roll into a ball for protection (Fig.2). The third body segment, called the pygidium, consisted of a body plate fused to the abdominal region. This region was the area where spines were situated in some later species. rilobites. The waters consisted of few other living organisms. Evolution has not yet 'created' land plants, jawed fishes or insects. The trilobites playe
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Approximate Pages = 4 (250 words per page double spaced)
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