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Studies of the morphology and ultrastructure of Turborotalia pachyderma (Ehrenberg) support the concept that this species arose from T. continuosa (Blow) in the upper part of Neogene Zone 12; both species occur together up to the upper Miocene zones N16 or N17, which is the upper limit of T. continuosa. Early chambers in T. pachyderma have the highly arched, laterally directed aperture of T. continuosa. This is a lineage quite distinct from that leading from T. globorotaloidea (Colom) to the Neogloboquadrina dutertrei (d'Orbigny) group in the upper Miocene (N17-N18). The pachyderma group invariably has a well-developed thickened imperforate lip and the umbilicus is small or closed; the dutertrei group has an open umbilicus, and the apertural edge has only a smooth band (temperate form such as the subspecies subcretacea) or toothlike flanges (tropical forms such as the typical dutertrei).
Preferential coiling ratios occur almost at the base of the range for T. pachyderma, derived from its ancestor T. continuosa. Sinistral populations adjusted to cold polar waters whereas dextral populations adapted to temperate waters, providing one of the better planktonic indices of paleoceanography for the late Neogene.
Upper Miocene and Pliocene forms have mostly 4 to 4½ chambers in the final whorl; the aperture varies from being almost a closed slit to more of an open arch. In the glacial Pleistocene, additional variants developed sometimes with 5 chambers in the final whorl; other variants show the progressive restriction of the aperture, especially the development of a highly thickened wall in polar waters.
In wall thickening, the early stage is that with little thickening ("incompta" of Cifelli), a second stage is that in which some thickening is added ("pseudopachyderma" of Cita, Silva, and Rossi), and finally a third stage is the typical thick-walled form. Wall thickening progressively reduces the depressions between the chambers until it is very difficult to recognize the individual chambers in extreme cases. This transition occurs as the populations settle progressively deeper in the water column.
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