TRILOBITES

Trilobites lived in the ancient seas for 340 million years, first appearing during what is known as the Cambrian Explosion, when oxygen released by plants made development of sophisticated animals possible. These widespread marine animals were outnumbered by many other soft-bodied species, but their hard shells, or carapaces, made good fossils. These facts, and the variety of forms they evolved over time, make them excellent index fossils to the drift of continental masses.

Trilobites belong to Arthropodia, the phylum of creatures with jointed legs such as spiders, insects, shrimp, and their relative, the modern horseshoe crab, which trilobites resemble. Their name, three lobed, refers to the plainly visible division of their bodies into the center axis -- called an axial lobe -- and the pleural lobes to either side. Most of them were not large, though some reached 70 centimeters (a little over two feet) in length.

Physically, trilobites are notable for two innovations. The first one is their carapace, which incorporated the mineral calcium carbonate to give it strength. This resulted in the growing trilobite having to molt, or shed its carapace, many times during its life. The resulting cast-off shells are part of the fossil record along with the remains of the creatures themselves. The second -- and possibly the most crucial from a human perspective -- innovation is that they are the first life form to develop eyes. While these were compound eyes, very similar in form to the ones insects have today, ultimately they led to the binocular vision which is our primary way of perceiving the world.

Trilobites reached the apex of their development by the end of the Cambrian Period, about 500 million years ago, but they were abundant through the end of the Permian Period (230 million years ago), when about 90 to 95% of marine species became extinct.


Information for this article came, in part, from Trilobites, by Riccardo Levi-Setti.

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