THE EVOLUTIONARY ORIGIN OF HANTKENINA (PLANKTONIC FORAMINIFERA) IN THE MIDDLE EOCENE AND COMMENTS ON ITS BIOSTRATIGRAPHIC SIGNIFICANCE

Hantkenina is a highly distinctive planktonic foraminiferal genus from the middle and upper Eocene characterized by a single robust hollow tubulospine on each adult chamber. Various possible ancestral forms have been proposed but careful dissection of the early ontogenetic stages shows a close relationship with Clavigerinella. The evolutionary transition between Clavigerinella and Hantkenina has now been described from sections in Tanzania, Austria, Spain, and Italy. Here we review the Clavigerinella – Hantkenina transition in Tanzania using 145 specimens from 23 stratigraphic intervals in a single core and show how it encompasses several morphological varieties that we assign variously to Clavigerinella caucasica, Hantkenina singanoae, and Hantkenina mexicana. We discuss the morphogenetic constraints involved in the evolutionary transition and propose an ecological / adaptive model for the selective pressures involved in the evolution of tubulospines. The first occurrence of Hantkenina has traditionally been associated with the base of the Lutetian stage and the base of the middle Eocene. However evidence from all the localities in which the transition has been described indicate a much younger age than previously thought. The stratigraphic ranges of all planktonic foraminiferal index taxa in the lower middle Eocene are reviewed.