The phenomenon known as character displacement develops when the ranges of 2 or more closely related species overlap geographically. Differences between the species are accentuated in the zone of sympatry, but remain less pronounced in the parts of their ranges outside that zone. Characters displaced may be morphologic, ecologic, behavioral, or physiologic. Biologists have shown that size is one of the most common morphologic characters displaced. A moderate difference in size--on the order of 1.3--appears to be sufficient to cause obligated feeding on different kinds of food, and thus permits sympatric species to avoid competitive elimination.

Although character displacement in modern faunas has been well documented, to my knowledge no examples have been described from the fossil record. Analysis of several extensive collections of Cambrian fossils suggests that size displacement was common among agnostid trilobites. Where 2 or more agnostid trilobites are found together, the interspecific ratio of maximum size usually is close to 1.3. Also, maximum size generally is more uniform where given species are separated than where they are found together. The similarity of these patterns to those displayed by certain modern faunas indicates that sympatric agnostids were structurally specialized to feed on resources of different sizes. This conclusion helps to explain how superficially similar species of Cambrian agnostids may have coexisted without competitive elimination. The examples further indicate that character displacement was operating early in the history of metazoans.

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