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ABSTRACT

The scanning electron microscope was used to examine over 1,000 specimens of Neogloboquadrina pachyderma (Ehrenberg) and related forms from different water masses throughout the late Cenozoic to study the ancestry, evolution, and environmental relations within this species and phylogenetic relations with several other forms. Two principal surface types are distinguished: reticulate microcrystalline ultrastructure which predominates in Arctic and subant-arctic populations, and crystalline ultrastructure which dominates in populations from other areas. In subtropical populations, crystalline forms are distinguished from those of high latitudes by thinner walls, higher pore concentration, and a lack of rosette-patterned crusts.

These ultrastructural differences reflect the degree of secondary calcification with reticulate microcrystalline ultrastructure representing an earlier stage. Differene in dominance of the ultrastructural types presumably reflects environmental differences associated with the various water masses. Overall similarity in ultrastructure within N. pachyderma links subtropical populations with temperate populations from the late Miocene to the Recent as one phylogenetic species that evolved from “Globorotalia”continuosa in the late middle Miocene and early late Miocene.

Neogloboquadrina acostaensis is considered to be a tropical to warm subtropical phenotypic variant of a late Miocene to early Pliocene cline with the temperate to polar variant represented by N. pachyderma. Identical surface ultrastructure and morphological intergradation between N. pachyderma and N. dutertrei dutertrei populations in subtropical sequences suggest that the two forms are genetically linked as a cline.

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