Among the remains of early life on earth, the fossil record we find buried in ancient sedimentary rocks bears evidence of an extraordinary group of marine creature, the trilobites. The position of these invertebrates in the evolution of the animal kingdom is extraordinary because of their early ascent to a high level of functional complexity, described in fascinating detail by their persistent and ubiquitous fossil remains. Trilobites could see their immediate environment with amazingly sophisticated optical devices in the form of large composite eyes, the first use of optics coupled with sensory perception in nature. As a unique feat in the history of life, their eye lenses were shaped to correct for optical aberrations, with design identical to that proposed (quite independently of any knowledge of trilobites) by Descartes and Huygens.
Although we can only hold the petrified remains of this long-extinct form of life, what was preserved of trilobites constitutes a record of immediate and striking impact, a still life we can interpret and recognize. It is this extravagantly rich still life I want to illustrate in this book, preserving as much as possible of the excitement of a voyage back in time to the dawn of life.
Trilobites share with many other invertebrates in the drama of the "Cambrian explosion": the rapid evolution of amazingly sophisticated and diverse life forms at the beginning of the Cambrian period, some 570 million years ago. In a sort of chain reaction triggered by plant life through a large-scale photosynthetic release of oxygen in the atmosphere, the pattern and rate of biological evolution was tremendously accelerated. Energy-releasing, oxygen-burning processes became available to the evolution of animal life. The newly discovered powerhouse fostered experimentation with ever more demanding levels of body activity and multiplication of forms. During a relatively short (geologic) time, perhaps a few million years, different new schemes of bodybuilding emerged that involved the construction of internal and external skeletons or exoskeletons. These could support increasingly complex body structures and functions. The genetic radiation of new life forms found coundess environments devoid of enemies in which to adapt and prosper.
Amidst this revolution, trilobites evolved to appear suddenly in the fossil record, when they had already reached a full degree of development and differentiation. Compound eyes are already present in the earliest trilobites. The preservation of trilobites as fossils in their burial rock is related to the fact that the living animal possessed an already mineralized exoskeleton. Somehow, trilobites learned early in the game to metabolize calcium carbonate in building sturdy shields. This was most likely another consequence of the explosive increase in the level of oxygen in the atmosphere that made carbonate more available.
Indeed, the fact that trilobites heralded the advent of advanced life forms in the fossil record can be regarded as a most peculiar event. Trilobites were not the only inhabitants of the primordial seas. Snapshots of the life forms that coexisted with early trilobites about 540 million years ago in the Middle Cambrian are provided by the soft-bodied fauna unusually preserved in the Burgess Shale of British Columbia. This assemblage reveals an extraordinary diversity of extravagant marine animals that left no record in ordinary sedimentary rocks due to the lack of hard body parts. An even older and similar record, dating at about 570 million years of age, the Lower Cambrian, has been recently discovered in Chengjiang (Yunnan Province), Southern China. Most of these peculiar life forms, even if never seen again, could be recognized as belonging to animal groups or phyla that carried through to recent times. The trilobites appear as a minority of species in the Burgess and Chengjiang faunas. And yet, because of their mineralized exoskeleton, they have left with us the most compelling evidence of their remarkable functional complexity. The high point of the trilobites' differentiation of forms was reached toward the end of the Cambrian period, some 500 million years ago. They became extinct at the end of the Permian period, the borderline between the Paleozoic and Mesozoic eras, around 230 million years ago. Dinosaurs were still largely in the making at that time. Long before the age of fishes, which began about 400 million years ago, the trilobites had already survived evolutionary decimation by about 170 million years.
The span of time which saw the birth, development, and disappearance of trilobites and their age is indeed staggering. However, the fact that their general appearance is not so dissimilar from that of living arthropods—the horseshoe crab, for example—makes trilobites less forbidding than some other forms of extinct life. The widespread abundance of trilobites, particularly during the Cambrian period, is evident from layers of sedimentary rocks which are occasionally coated with trilobite remains. The presence of particular genera in layers of determinate geologic age makes trilobites very important index fossils. The presence of identical forms in rocks of identical age and composition on locations now separated by oceans is telltale evidence of the drift of the continents on the earth's crust.
The fascination of the trilobites' age, their role in early life on our planet, the optimization of their visual organs, the ingenuity of their life adaptation to the environment, and their value as geological markers explain why trilobites are important to both the professional scientist and the amateur fossil collector.
Was this article helpful?