The most widely distributed trilobite species: Ordovician Carolinites genacinaca

The distributions of trilobite species were controlled by a combination of habitat preferences and paleogeographic constraints, which tend to limit th eir extent. The Lower Ordovician trilobite Carolinites genacinaca Ross, 195 1, however, had a remarkable range unequaled among polymerid trilobites; it has been recognized on all Ordovician paleocontinents. Its distribution ha s been explained by an epipelagic mode of life, based on evidence from func tional morphology, analogy with modern pelagic crustaceans, and geological occurrence. In such a case, morphological identity throughout its range mig ht be anticipated, if all occurrences can be postulated to be members of a single pandemic population. Rotational superimposition has been used to com pare variation within samples drawn from Alberta, Spitsbergen, and Australi a with a benchmark population from the western United States. All are morph ometrically 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 cra nidium from France, previously referred to Carolinites vizcainoi, may be a juvenile of C. genacinaca or C, tasmaniensis; its differences are consisten t with ontogeny. A :;mall number of specimens from Siberia and central Chin a show differences in cranidial proportions from the Utah specimens that ma y bl: the result of preservational factors and/or photographic technique, o r may represent genuine morphological disparity; this could be clarified if more specimens were to become available. This study suggests that C. genac inaca was ubiquitous in the epipelagic environment in a belt that encircled the planet between paleolatitudes of approximately 30 degrees N and 30 deg rees S.