PATTERNS OF WALL TEXTURE CHANGE DURING THE EARLY EVOLUTIONARY STAGES OF A NEOGENE PLANKTONIC FORAMINIFERAL (GLOBOCONELLA) CLADE

Following a prolonged period of relatively low diversity, the planktonic foraminifera underwent a major diversification during the Neogene. A critical milestone in this diversification was the evolution of the Globoconella clade during the Early Miocene. Previous studies have established that the earliest stages of globoconellid evolution are encapsulated by the transition from a morphologically conservative ancestral species (Paragloborotalia nana) possessing round, globular chambers to a descendant species (G. zealandica) with a test having a flattened spiral side and angulo-conical chambers. These two morphological end-member species are linked via an intermediate species called G. incognita. Use of scanning electron microscopy (SEM) to examine the tests of dozens of specimens from various stages of this morphoseries reveals that wall texture patterns underwent a significant degree of evolutionary change as well. In short, the wall textural patterns evolve from being coarsely cancellate (honey comb-like) to a smooth, “microperforate” condition. This pattern of wall texture evolution is strikingly similar to that seen in other phylogenetically removed clades of planktonic foraminifera from other time periods and, therefore, conforms to a recurrent theme seen throughout the evolutionary history of the planktonic foraminifera; that being, ancestral species tend to have coarse wall textures while their descendants possess smoother wall textures. At present, it is unclear whether this recurrent theme in wall texture evolution is driven by cyclical changes in marine carbonate chemistry or iterative evolutionary changes in the calcification process. Nevertheless, documenting the changes in wall texture that paralleled the morphological evolution of the globoconellid clade is important because wall texture is generally considered to be a conservative morphocharacter that is widely used to reconstruct phylogenetic relationships within the planktonic foraminifera.