Abstract

In the Oxfordian-age Corallian beds of southern England, many of the nodosariid species are highly variable in shape. In some instances, the variation of a single species is wide enough to include individuals within the morphological continuum that exhibit the characteristic morphology of two or even three different commonly accepted genera. For example, a population of specimens referable to Lenticulina muensteri (Roemer) from a single rock sample includes individuals showing the characteristics of Lenticulina Lamarck, Marginulinopsis Silvestri, and Planularia Defrance. Studies of populations of Lenticulina tricarinella (Reuss) from the middle and upper Jurassic reveal similar transgression of the variation field of a species across the boundaries of accepted genera. Such anomalies arise from the development of the classification of the Nodosariidae largely on the basis of Cenozoic and Recent material, whereas the family was at its peak of numbers and diversity in the Jurassic and early Cretaceous. Necessary modification of the classification of the family will best be brought about by grouping together species of proven relationship into larger units rather than by defining new genera on the basis of a few key characters.

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