PEDOMORPHOSIS AND THE ORIGIN OF THE PALEOGENE PLANKTONIC FORAMINIFERAL GENUS MOROZOVELLA

The evolutionary origin of Morozovella angulata from its immediate anc estor, Praemurica uncinata, is preserved in Paleocene sediments from t he Gulf of Mexico. This event represents the beginning of the morozove llid radiation and marks the first appearance of keeled planktonic for aminifera after the Cretaceous/Tertiary extinction. Parallel biometric and isotropic analyses were performed on size-segregated specimens fr om a succession of stratigraphic horizons. The biometric data reveal a temporal pattern of variation consistent with paedomorphosis. The app earance of angulose juvenile chambers in the otherwise rounded ancestr al form (Praemurica uncinata) results in an allometry that becomes mor e pronounced upsection. At the origin of M. angulata, the juvenile mor phology of the ancestor is retained throughout the entire ontogeny. Is otopic analysis of this sequence reveals the gradual acquisition of an increasingly heavy adult delta(13)C signal relative to that of the ju venile, while the delta(18)O data display no temporal or size-related trends. The temporal increase seen in the slope of the delta(13)C/size relationship may reflect the evolution of an increased dependency on photosymbionts.