Trilobites are the ubiquitous fossil critter of the Paleozoic. They were amongst the first animals with eyes, and the only ones who ever evolved lenses using crystals of the mineral calcite. Like their distant descendants the insects, they had compound eyes of many lenses, each like a pixel on a screen. Each dot in the eye of the trilobite in the photo is one sub-eye, containing a fossilised crystal. There are some disadvantages to this kind of eye. While our lenses are made of a soft substance, and have tiny muscles to adjust the shape for focussing at different distances, theirs were rigid and impossible to focus. Using minerals as lenses also creates a variety of optical problems, which trilobites solved in incredible ways.
Calcite is the main constituent of chalk, limestone and most biomineralisation (such as mollusc shells), and our earlier post on Viking spar touched on some of its optical properties. Its most famous characteristic is called double refraction, which you can experience by putting a calcite rhombus onto a page and viewing the doubled text through the crystal. This happens because it splits the light into 2 rays that travel through at different speeds and angles, producing the double image. In all doubly-refractive minerals there is also one direction in which the light is not split, usually that looking down the long axis. Trilobites solved their double vision problem by aligning all the crystals in their compound eyes in that one precise direction, making them behave optically like glass instead of calcite.
This is just the simplest trick in the trilobite armoury. They needed to see nearby food items while remaining watchful for distant predators. To accomplish this they selectively doped different areas of their calcite crystal lenses with impurities as they biomineralised them into being, creating what opticians call a doublet structure. This makes the single crystal become two lenses by changing its refractive index (RI) in different parts of the lens. All crystals bend light rays as they pass through and RI is a measure of how much each mineral does this.
With two different refractions in your lens you can bend the rays in 2 ways, one for close up and another for far away. This chemical structure gave them the ability to focus at many distances, called depth of field, and also compensates for a type of distortion called spherical aberration (seen when an object bulges at the edge of a magnifying glass). Early man-made lenses such as those by Hooke and Huygens in early microscopes use the same trick, as does the modern geologist's hand lens.
There are two main kinds of trilobite eye, holochroal ones have many close packed hexagonal lenses (1-15000), while schizochroal eyes had fewer (1-700) larger thick lenses. The greater the density of lenses, the more sensitive to movement they are, and the keenest eyes undoubtedly belonged to predators. The sheer variety of shapes and sizes discovered reflects the wide diversity of ecological niches these beasties filled. Some were so big they dominate the head, and are curved to provide a wide (up to 360 degrees) field of view, while others belonging to bottom mud dwellers were raised on stalks, to check for approaching predators.
Ain't the highways and byways of evolution wonderful places, filled with awe inspiring truths?
Image credit for Hollardops mesocristata, from the early Devonian Tazoulat Formation in north Africa: Houston Museum of Natural Science; Daderot
Rotating image: http://www.nhm.ac.uk/
For those with paywall access:http://www.sciencemag.org/
and for those who read paper, this is invaluable: Richard Fortey's 'Trilobite: eyewitness to evolution', by the retired expert on these beasties from the London Natural History Museum.
Our post on Mineral evolution:
and Viking spar discussing calcite's use in navigation and double refraction: