The Evolution Of Trilobites Essay, Research Paper

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 ( 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 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 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.

The trilobites played an important role in the marine community and were quite successful during the Ordovician era. . They were prevalent in both pelagic and benthic areas of the sea in their endeavors to capitalize on foraging locations. They ranged in size from 2-7cm long but have been found to be as big as 20cm. In the Ordovician community, trilobites were less abundant than many of the other marine invertebrates.

So what happened to the trilobites? The fossil records suggest an increase in the diversity of the trilobites as they evolved with their changing environment. With this speciation, one would expect trilobites to be adapted and well protected from extinction. There are many theories about the demise of the trilobites. Many paleontologists believe that the decline in trilobite diversity is due to the rise of predators such of the jawed fish. This seems reasonable due to the abundance of predatory fish and sharks in the Permian period and beyond. Some researchers believe that the way they molted may have been a contributing factor to their extinction as well. Brandt (1997) explains that inconsistent molting patterns have been observed in the fossil records that suggest it may have put them at risk for injury. Modern day crustaceans always molt in the exact same pattern to avoid getting an appendage stuck in the newly shed skin. This pattern observed in the trilobites may provide clues into the lives and deaths of these creatures. The most common and accepted theory involves the mass extinction of the late Permian period. The lowering of sea levels and the change in sea water composition have been proposed as reasonable contributing factors to the largest mass extinction of our time (Freeman et al. 1998). This may have caused ecological and biological stresses on the group. Loss of habitat, due to decreasing water levels, may have resulted in a significant loss in numbers. The abruptness of the mass extinction left very little time for speciation, adaptation or endurance. This may have led to a decrease in the number of trilobites and the increase in predatory fish may have been too much for the group to endure.

Today, modern crustaceans such as the shrimp, crabs and lobster, and other arthropods such as the spider family are the cousins of the late trilobites. The diversity of today s arthropods is a mere fraction of the Ordovician period. This may be partly due to rise of the human species and the loss of much of the habitat required for such diversity. Man s existence has changed the environment and affected many of the inhabitants as well. The endangered species of today are numerous and a real concern to many people. It leaves many of us asking if they have stopped evolving or have humans just made it impossible for many species to adapt?