In the subdivision of the animal kingdom into major groups or phyla, the trilobites are included in the phylum Arthropoda. The history of these highly developed invertebrates begins abruptly with the beginning of the Cambrian period and extends to modern times. No animals with skeletons or shells are found among the well-preserved traces of Precambrian life forms, and since the hard parts of arthropods (and many other phyla) are an integral part of their organization, we must infer that they were not in existence as yet.
Trilobites are amongst the oldest fossil arthropods. Familiar arthropods are the insects, scorpions, spiders, centipedes and millipedes, shrimps, lobsters, crabs, and others with jointed legs. Although they first inhabited the seas, the arthropods have adapted themselves most efficiently to every life habitat.
The position of the Arthropoda, and of the Trilobita in particular, in the animal kingdom is illustrated in figure 1. Here the principal groupings into classes and subclasses are traced through, by vertical lines, the Phanerozoic time scale (the ensemble of geologic eras with a fossil record), from their first appearance to present times. Boxes indicate the higher ranking of classes, often related, into phyla. This representation is very schematic and does not show the variations in the number of species encompassed by each phylum as a function of time. In the histogram at the base of figure 1, however, we can appreciate a cumulative comparison of the relative abundance of known extinct and living species. The Arthropoda actually represent a much more prolific group of animals than is shown in such a histogram. In fact, the one million or more species of insects, representing seventy percent of all species of extinct and living animals, had to be removed from the plot so that the remnant fractions could become visible. This may come as a surprise to many, but from the point of view of a statistician of living species, we live today in the age of insects! It would be misleading to
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