The distributions of trilobite species were controlled by a combination of habitat preferences and paleogeographic constraints, which tend to limit their extent. The Lower Ordovician trilobite Carolinites genacinaca Ross, 1951, however, had a remarkable range unequaled among polymerid trilobites; it has been recognized on all Ordovician paleocontinents. Its distribution has been explained by an epipelagic mode of life, based on evidence from functional morphology, analogy with modern pelagic crustaceans, and geological occurrence. In such a case, morphological identity throughout its range might be anticipated, if all occurrences can be postulated to be members of a single pandemic population. Rotational superimposition has been used to compare variation within samples drawn from Alberta, Spitsbergen, and Australia with a benchmark population from the western United States. All are morphometrically similar. By any criterion, specimens identical to the benchmark population are found within the Alberta, Spitsbergen and Australia samples, which represent the extremes of the species' geographic range. A lone cranidium from France, previously referred to Carolinites vizcainoi, may be a juvenile of C. genacinaca or C. tasmaniensis; its differences are consistent with ontogeny. A small number of specimens from Siberia and central China show differences in cranidial proportions from the Utah specimens that may be the result of preservational factors and/or photographic technique, or may represent genuine morphological disparity; this could be clarified if more specimens were to become available. This study suggests that C. genacinaca was ubiquitous in the epipelagic environment in a belt that encircled the planet between paleolatitudes of approximately 30 degrees N and 30 degrees S.

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