Origin of the Eocene planktonic foraminifer Hantkenina by gradual evolution

Abstract

Hantkenina is a distinctive planktonic foraminiferal genus characterized by the presence of tubulospines (robust hollow projections) on each adult chamber, from Middle and Upper Eocene marine sediments worldwide. Here we illustrate its evolutionary origin using c. 150 specimens from 30 stratigraphic intervals in two sediment cores from Tanzania. The specimens, which span an estimated time interval of 300 ka, show four intermediate steps in the evolution of the tubulospines that amount to a complete intergradation from Clavigerinella caucasica, which does not possess them, to Hantkenina mexicana, which does. Stable isotope analyses indicate that the transitional forms evolved in a deep planktonic habitat not occupied at that time by other species of planktonic foraminifera. We discuss the morphogenetic constraints involved in the evolutionary transition and propose an ecological/adaptive model for the selective pressures that resulted in the evolution of tubulospines. We compare our record with similar, recently described assemblages from Austria and Italy, and we update the biostratigraphy and systematic taxonomy of the key morphospecies involved in the transition.

The impact of Cenozoic cooling on assemblage diversity in planktonic foraminifera

“…By considering time-slices through the Eocene it is possible to interrogate the effects of the cooling trend from Early Eocene conditions of Cenozoic peak warmth through to Late Eocene climates, immediately prior to the onset of the present ‘icehouse’ climate state [ 75 ]. The Mid- to Late Eocene saw changes in the distribution of ocean fronts and regions of productivity [ 76 , 77 ], as well as in the ocean nutricline and the structuring of planktonic niches [ 78 , 79 ]. By the Late Eocene, global cooling had produced biogeographically distinct high-latitude communities [ 80 ].…”

“…Most species with the ‘muricate’ wall structure, characteristic of the acarininids, went extinct in the Eocene, and this morphology was finally lost in the Oligocene [ 29 ]. The Hantkenina lineage initially occupied a unique warm deep-water niche [ 79 ], which was lost when it migrated permanently into a shallower habitat [ 83 ]. Previous studies of Eocene diversity have mainly focused on the global picture (e.g.…”

Demise of the Planktic Foraminifer Genus Morozovella during the Early Eocene Climatic Optimum: New Records from ODP Site 1258 (Demerara Rise, Western Equatorial Atlantic) and Site 1263 (Walvis Ridge, South Atlantic)

“…These authors hypothesize that intermediate water temperatures probably rose significantly during the EECO, thus becoming too warm for this genus. In addition, elevated ocean temperatures could have enhanced the rate of bacterial respiration and remineralisation significantly, thus resulting in more efficient recycling of nutrients higher in the water column [71,72,100]. This would have resulted in a restriction of food supply at depth and subsequently cut out the deeper dwelling niche of chiloguembelinids.…”

Massive and permanent decline of symbiont bearing morozovellids and δ<sup>13</sup>C perturbations across the Early Eocene Climatic Optimum at the Possagno section (Southern Alps of northeastern Italy)

“…Most of the new species will characterize the thermocline of the middle and late Eocene oceans: Subbotina corpulenta, S. eocena, S. hagni, S. senni, S. yeguanesis, Parasubbotina griffinae, and P. pseudowilsoni. The appearance of the radially chambered Parasubbotina eoclava, which is considered to be the precursor of the truly clavate chambered Clavigerinella (Coxall et al, 2003;Pearson and Coxall, 2014) occurs at 19.8 m (Luciani and Giusberti, 2014). Clavigerinella is the ancestor of the genus Hantkenina that successfully inhabited the sub-surface middle through late Eocene oceans.…”