Paedomorphosis and the origin of the Paleogene planktonic foraminiferal genus <i>Morozovella</i>

Abstract

The evolutionary origin of Morozovella angulata from its immediate ancestor, Praemurica uncinata , is preserved in Paleocene sediments from the Gulf of Mexico. This event represents the beginning of the morozovellid radiation and marks the first appearance of keeled planktonic foraminifera after the Cretaceous/Tertiary extinction. Parallel biometric and isotopic analyses were performed on size-segregated specimens from a succession of stratigraphic horizons. The biometric data reveal a temporal pattern of variation consistent with paedomorphosis. The appearance of angulose juvenile chambers in the otherwise rounded ancestral form ( Praemurica uncinata ) results in an allometry that becomes more pronounced upsection. At the origin of M. angulata , the juvenile morphology of the ancestor is retained throughout the entire ontogeny. Isotopic analysis of this sequence reveals the gradual acquisition of an increasingly heavy adult δ 13 C signal relative to that of the juvenile, while the δ 18 O data display no temporal or size-related trends. The temporal increase seen in the slope of the δ 13 C/size relationship may reflect the evolution of an increased dependency on photosymbionts.