GEOCLASSICS - Trilobite Index

Trilobites, meaning 3 lobed, were the first multiple celled animals with a hardened exoskeleton to exist on earth! They dominated the seas 570 million years ago and steadily declined in numbers until they became extinct 350 million years later.

Whatever their size, all trilobite fossils have a similar body plan, being made up of three main body parts: a cephalon (head), a segmented thorax, and a pygidium (tail piece). Trilobites were among the first of the arthropods, a phylum of hard-shelled creatures with multiple body segments and jointed legs (although the legs, antennae and other finer structures of trilobites only rarely are preserved). They constitute an extinct class of arthropods, the Trilobita, made up of nine orders, over 150 families, about 5000 genera, and over 15,000 described species. New species of trilobites are unearthed and described every year, making trilobites the single most diverse group of extinct organisms.

The smallest known trilobite species is just under a millimeter long, while the largest include species from 30 to 70 cm in length (roughly a foot to two feet long!). Most trilobites are about an inch long, and part of their appeal is that you can hold and examine an entire fossil animal and turn it about in your hand. Collectors are especially interested in their eyes: explained further below.

Trilobite Eyes
Trilobites developed one of the first advanced visual systems in the animal kingdom. The majority of trilobites had a pair of compound eyes, made up of many lensed units. At least one suborder of trilobites, the Agnostina, are thought to be primarily eyeless. None have ever been found with eyes.

The advantage of good eye design
Compound eyes in living arthropods such as insects are very sensitive to motion, and it is likely that they were similarly important in predator detection in trilobites. It has also been suggested that stereoscopic vision was provided by closely spaced, but separate eyes. Trilobite eyes had rigid, crystalline lenses and an internal doublet structure (two lens layers of different refractive indices acting in combination) for focusing problems that result from rigid lenses.

Three types of trilobite eyes
There are three recognized kinds of trilobite eyes: holochroal, schizochroal, and abathochroal. The first two are the major types, with the great majority of trilobites bearing holochroal eyes, and the distinctive schizochroal eye a recognized innovation of the Phacopida. Holochroal eyes are characterized by close packing of biconvex lenses beneath a single corneal layer that covers all of the lenses. These lenses are generally hexagonal in outline and range in number from one to more than 15,000 per eye. Schizochroal eyes on the other hand are made up of a few to more than 700 relatively large, thick lenses, each covered by a separate cornea. The abathochroal eye is seen in only a few Cambrian trilobites and is somehat similar to the schizochroal eye, but differs in some important respects: the sclera is not thick, and the corneal membrane does not extend downward, but ends at the edge of the lens.

USA

MOROCCO

RUSSIA

TRILOBITES