Jim Baker, age 13, of Sahuarita, Arizona, for his question:
What are trilobites?
The last of the trilobites departed some 200 million years ago. But there was a time when they climbed to the very top of the tree of life. At the peak of their long dynasty, the various species of trilobite accounted for some 60 per cent of all creatures living on the earth. Perhaps they departed without regret, for they handed on life to a variety of quite different creatures that still populate the world.
The word trilobite refers to a body with three lobes, or sections. The trilobite who thrived so long ago had head, thorax and tail sections, somewhat like a modern insect. His soft body wore a protective shell, rather like that of a shrimp. These resemblances are not surprising, for the trilobite was the remote ancestor of our modern shrimps, insects and all other members of the vast Phylum Arthropoda. He and his kinfolk were ancient arthropods that lived in the sea when its waters were less salty than they are today.
The Cambrian Period of around 500 million years ago is called the Age of Trilobites because at that time these arthropods were the most advanced creatures on earth. The earliest ones were smallish, perhaps no bigger than beetles. Each had lots of short, thin legs, arranged in a row along each side. He scuttled along on the silty seabed, grubbing for food. The shell on his back protected him from the few creatures likely to attack him. But at first it was stiff and unbendable.
Through millions of generations, the descendants of the original trilobites branched out in different forms. Life in the mild seas was easy and, as more worms and lesser creatures developed, there was more food. Many trilobites grew bigger and some were giants, 27 inches long. And through the ages, many trilobites modified their stiff shells. The grooves became pliable and the crusty creatures were able to coil around to hide their softer undersides. This was a successful safety measure, for other creatures were growing larger and hungrier.
Trilobites of one sort or another survived through the entire Paleozoic Era that lasted some 330 million years. Their long and successful dynasty declined and finally became extinct during the Permian Period, about 200 million years ago. The fossils of some 2,000 species have been indentif ied and this is but a small percentage of the total.
The original trilobites departed, but some of their species changed so much that they became different species. They were the remote ancestors of our shrimps and lobsters, our crusty crabs and teeming insects. The ancient story of the trilobites was traceable because so many of their durable shells became fossilized. As trilobites grew, they shed their tight shells for bigger ones and countless millions of these discarded shells were preserved in limestone and other rocks.
As a rule, extinction is a sad story of final farewell. But we cannot grieve too much for the ancient trilobites, for their descendants still survive though greatly changed. Some 350 million years ago, one branch of the clan became scorpions. These bold fellows were the first animals to leave the seas for life on the dry land, and so, the trilobites participated in the greatest advance of life on earth.