INTRODUCTION: WHY COLOR?

As we split open a slab of shale, especially if it is of Cambrian age, trilobites often appear in brilliant shades of ochre, against a gray-greenish background, the color of the matrix encasing them. The color of the trilobite shell, or carapace, has little to do with the original color of the living trilobite. It is the result of chemical processes, such as mineralization, which took place after the burial and decay of the dead animal or its molting exuviae. These processes are called diagenesis and are part of a broader set of burial events that depend on the environment and fall under the comprehensive name of taphonomy. Taphonomy involves the nature of the minerals making up the shale and of those dissolved in the surrounding water or infiltrated after burial and fossilization. And yet, the color that our vision perceives has an emotional impact that makes our "treasure hunt" for trilobites both attractive and addictive.

Unfortunately, the rigor of the scientific paleontological literature has predominantly denied such emotional responses by erasing any trace of color with whitening coatings, which reduce the graphical reproduction of the subject matter to black-and-white photography. However, there is a reason for the whitening process: the aerosols used for the whitening carry an electrical charge that makes the coating stick preferentially to the tips of protruding details, thus enhancing somewhat the resulting informative three-dimensional contrast. Such whitening has been the fate of many images presented in my earlier trilobite books.

The rise of digital photography and the fact that color printing is now more affordable make it possible to exploit to its fullest the splendid complexity of human vision, which, with its trichromatic sensitivity, unique among primates, connects directly to the emotional response that color images may elicit as well as to artistic expression. Is this of added value over black-and-white images when looking at trilobites? It is my contention that, indeed, color images of trilobites convey more information about the fine structure of the object, such as that enhanced by aerosol whitening, than is usually appreciated. Under illumination of a color reproduction similar to that used in taking the picture (e.g., daylight or halogen lighting,), the response of the eye retina contains a subtle contribution from a phenomenon called chromostereopsis. The latter is an optical phenomenon whereby different colors may elicit different depth perceptions. Much has been discussed in the literature in regard to this phenomenon and how the brain processes the signals from the retina in regard to color processing hand in hand with the perception of form and boundaries. The net result is that color images seem to stand out with some illusion of relief. (An excellent and updated summary of this visual phenomenon can be found on the web under "Chromostereopsis" in Wikipedia, The Free Encyclopedia.)

My input on this issue is a comparison I made between black-and-white images of trilobites whitened with white aerosols of magnesium oxide (the old trick to enhance 3-D contrast mentioned above) and color images of the same prior to whitening (an example of this comparison is shown in plate 1). This comparison showed clearly that the structural information delivered by both images is essentially identical, thus confirming that the color image does provide the relief evidence sought by paleontologists for publication. On this ground, to publish my trilobite pictures in color has more to offer than just the entertainment of browsing through a coffee-table book.

Now I must turn to the choices I made about which trilobite images to include and which of their geographic origins to highlight. On both grounds, my choices were greatly biased. As to the choice of depositional location, this was dictated by preconceived notions of accessibility, renown, and sometimes also by exotic appeal. A certain predilection for the most ancient geological settings, the Lower and Middle Cambrian, will transpire here—a predilection perhaps fostered by my encounters with my mentor, the late Franco Rasetti, who regarded Cambrian trilobites as the most rewarding of his attention. However, the principal bias results from my choice of illustrating complete, fully articulated trilobite specimens. Complete specimens are a rare occurrence in the fossil record of trilobites, which restricts greatly the taphonomy of their burial, which, in turn, is responsible for their ultimate appearance, inclusive of their color. Complete outstretched trilobites, with all sutures intact, indicate sudden burial of living individuals in a tranquil environment, at some depth in the sea, remote from the disturbance of wave motion. Such episodes of sudden burial are known to occur as a result of gentle but massive deposition of silt in suspension in so-called turbidity waves, the result of surface storms and sometimes volcanic ash fallout. The sediments often reveal a cyclic or rhythmic alternating repetition of mineral content, as documented, for example, in the layers of the Manuels River Formation of Conception Bay, Newfoundland—which I studied long ago with my colleague Jan Bergström of Sweden. After the death of a trilobite, several pathways lead to fossilization of the remains, which depend critically on the chemistry of the local environment. With few by now famous exceptions, like the Burgess Shale of British Columbia and the Chengjiang fauna of China, where soft tissues are beautifully preserved, the decay of the biopolymers making up the soft tissues, generally fostered by bacterial action even in anaerobic marine sediments, leaves no trace of internal, nonmineralized structures. Only the trilobite exoskeleton, the carapace, partially mineralized by calcite, often escapes complete dissolution, and this may survive unaltered over geological eras. However, over time, mineralogical alterations, diagenesis, still conspire to modify the chemical nature of the fossilized carapace, leading to a multitude of final appearances, and in particular colors, of the fossil trilobite.

Thus, the brilliant ochre colors frequently encountered in the carapace of trilobites preserved in clay-mineral shales are the result of a three-step taphonomic-di-agenetic process, favored by a marine environment rich in dissolved iron hydroxide and oxygen starved. In the first step, residual organic matter is digested by sulfate-reducing anaerobic bacteria yielding hydrogen sulfide (HS-). In the second step, HS- combines with iron ions (Fe2+) to yield iron sulfide FeS2 (the mineral pyrite, fool's gold). These two steps may occur within a few weeks after burial. The third step, which may occur over millions of years, involves the oxidation of the pyrite to the minerals limonite (FeO2, yellow ochre) and hematite (FeO3, red ochre). Sometimes, the pyrite replacement survives over eons, and we find the trilobites amidst a multitude of pyrite nodules, in particular in carbon-rich black...