Science at a Distance


	Phylum: Arthropoda Class: Trilobitae
Trilobites were probably the first animals to see our planet. A little over 590 million years ago a segmented annelid worm completed its evolution through pro-arthropod forms and into a shape and structure that would successfully dominate animal life for the next 200 million years. One of the features that made this "jointed feet" creature so numerous and diverse was its ability to see. Although life had been around for close to three billion years before trilobites, none of the previous life forms had ever seen the rocks, plants, mud, or oceans in which they lived. Trilobites were the first.
	Arthropods ("jointed feet") are a very old phylum whose members share a common body plan. In a typical arthropod, a series of segments, each with two jointed "legs", are connected together in a repetitive manner to create a centipede-like organism. In primitive arthropods most of the segments were identical, but as their evolution progressed, some segments fused together, some became smaller, and others began to specialize. The result was a more complex organism with a wide variety of abilities and futures.
Every arthropod is protected and supported by an exoskeleton. Often compared to a suit of armor, an exoskeleton is a hardened crust secreted by the skin. In modern arthropods the surface is waxy and waterproof. Beneath the wax is a stiffening layer that gives form and strength to the creature. Usually this is made of modified chitin, but in crabs it can also contain calcium. Next to the skin is a softer, flexible layer of chitin which extends through the joints between the segments and between the parts of the legs, allowing the creature to move. Trilobites show all these features. Segmented and with an exoskeleton, a trilobite had a body consisting of a dorsal (top) shield which was strengthened and calcified and a ventral (bottom) shield which was not. It is the top of the trilobite that usually fossilized. This dorsal shield is divided into three parts, a headpiece or cephalon, a series of segments in the middle, the thorax, and a tailpiece or pygidium. It is tempting to think that the word 'trilobite', meaning 'three lobes', refers to these three regions, but it does not. Trilobites also have three lobes running lengthwise along their bodies, and this is the feature after which they are named. Trilobite fossils would have been known by Mendel and Darwin. Fossilized dorsal shields were first found in 1698, and traces of the ventral shield showing the appendages were discovered in 1825. These fossils revealed a pair of jointed legs (coxae) on each segment. Each 'leg' was divided into a lower part which some trilobites used for walking, and an upper part with feather-like gills used for breathing, and in some cases for swimming. Fossilized tracks of walking trilobites have been found and show an unspecialized, but very successful way of getting around. All trilobites lived in the sea. Small ones dug in and buried themselves into the mud, others crawled along the ocean floor, and yet others swam efficiently. They ate everything from detritus, to the dead bodies of other creatures, and some of them were efficient predators. They probably ate one another. Large spines are common on many species and were probably there for protection. Swimmers had large eyes, some with 360 degree vision. They could see where they had been as efficiently as where they were going. Despite a long an successful reign in the oceans of the world, extinction finally caught up with the trilobites at the end of the Devonian period, about 350 million years ago. A few species lingered longer, but the golden age was over. Cephalopods, predatory invertebrates such as the Nautilus, were becoming more and more numerous and enjoyed a good meal of plankton eating trilobites. Sharks, efficient carnivores, were rapidly dominating the Devonian seas, and bony fish with crushing jaws and strong teeth were feeding on slow moving trilobites of all kinds with increasing frequency. A body plan which has served them well for a very long time was finally superseded by others and the trilobites slipped gracefully into the fossil record. Anyone who collects fossils today has at least one trilobite in their collection.

RETURN

Phylum: Arthropoda Class: Trilobitae

Phylum: Arthropoda
Class: Trilobitae