Crystal Eyes

In 1972 Kenneth M. Towe, of the Smithsonian Institution in Washington, D.C., demonstrated the efficiency of the Phacops kind of trilobite eyes--by taking photographs through them. The fat, biconvex lenses of the phacopid eye were designed to bring bright beams to a focus. If you hold a clear glass marble up to the light and peer through it, you can get some idea of the process: you will see an upside-down world, all bent and distorted. But in Towe's photos, the trilobite images seem to be much clearer than that. How could this be? The problem with light traveling through a convex lens to a focus is that different rays travel different distances through the lens according to their trajectory. This means that the rays are bent to different degrees. The result is a fuzzy focus. Euan Clarkson and University of Chicago physicist Riccardo Levi-Setti discovered that something strange had happened to the calcite in the lower part of each Phacops lens: magnesium atoms were present in just the right quantity to correct the spherical aberration. For every bend to the left, there was a compensating bend to the right. This corrective layer made a bowl within the lens; the trilobite had thus manufactured what modern opticians term a doublet. The animals with these eyes may have seen more complete images of an object than their hexagonal-lensed fellows. All this 400 million years ago.

The trilobite whose lenses I started counting had the hexagonal lens design and was a particularly goggle-eyed species, with peepers puffed up like little bladders. The eyes bulged out on either side of the head in the manner of those slightly grotesque ornamental goldfish that have such a thyroidal look. I named this shrimp-sized animal Opipeuter, having recruited the help of a classicist friend to find out the Greek for "one who gazes." The lenses of Opipeuter's eyes were tiny, but unlike those in the crescent-shaped eyes of most trilobites that lived on the seafloor, Opipeuter's lenses faced in all directions, even downward. This trilobite must have been a free swimmer rather than a bottom dweller. Ancient oceans could have swarmed with trilobites, just as krill throng in modern-day seas. These elongated trilobites were the remote ecological equivalents of shrimp, moving through the water column or even on the surface.

A number of different trilobites proved to have this free-swimming design. Fossils of one, the Cyclops-eyed Pricyclopyge, are common in dark Ordovician mudstones 400 million years old. These sediments were originally deposited in relatively deep water, to which these swimmers were apparently confined. Could one somehow test the difference in life habits between shallow- and deep-dwelling trilobites by examining their eyes? Tim McCormick, now at the University of Glasgow, and I, following techniques used to determine how light intensity influences the eyes of living arthropods, were able to insinuate ourselves into the daily lives of our fossil trilobites by making a series of careful measurements on eye construction. We were able to show that Pricyclopyge had eyes constructed in the same fashion as crustaceans that still live in the deeper part of the water column in oceans today. So it seems that trilobites were indeed able to swim at various depths.