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

<jats:p> The evolutionary origin of <jats:italic>Morozovella angulata</jats:italic> from its immediate ancestor, <jats:italic>Praemurica uncinata</jats:italic> , 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 ( <jats:italic>Praemurica uncinata</jats:italic> ) results in an allometry that becomes more pronounced upsection. At the origin of <jats:italic>M. angulata</jats:italic> , 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 <jats:sup>13</jats:sup> C signal relative to that of the juvenile, while the δ <jats:sup>18</jats:sup> O data display no temporal or size-related trends. The temporal increase seen in the slope of the δ <jats:sup>13</jats:sup> C/size relationship may reflect the evolution of an increased dependency on photosymbionts. </jats:p>