Resumen de Peripheral structure in chambers of Globorotalia scitula praescitula and some descendants
Globorotalia scitula praescitula (lower Miocene) from New Zealand has a weakly delimited zone at the chamber periphery in which density of pores is very low. This semiperforate area is better defined and extends into the umbilical surface of chambers in G. miozea miozea (lower-middle Miocene). In G. miozea conoidea and G. miotumida miotumida (middle-upper Miocene) a keel is present at the spiral margin of a wide semi-perforate area. Taxa with semi perforate areas and keels also arise from G. scitula praescitula in Trinidad strata. There is a major trend in taxa descendant from G. scitula praescitula towards greater axial compression of chambers formed in late ontogeny: this seems to be a technique to improve form resistance against passive sinking. I suggest that axial compression of chambers causes the periphery to become a site at which bending stress may develop. The semiperforate area reduces stresses in moderately compressed chambers while the keel is a reinforcing structure in strongly compressed chambers.
