ORIGIN AND MORPHOLOGY OF THE EOCENE PLANKTONIC FORAMINIFER HANTKENINA

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Abstract
Study of the origin and early evolution of the tubulospine-bearing planktonic foraminiferal genus Hantkenina reveals that it evolved gradually from the clavate species Clavigerinella eocanica in the earliest middle Eocene and is unrelated to the genus Pseudohastigerina. Clavigerinella eocanica and the lower middle Eocene species Hantkenina nuttalli share many morphologic features and show similar developmental patterns but differ significantly in these aspects from P. micra. Rare, transitional Clavigerinella-Hantkenina forms from the Helvetikum section of Austria bridge the gap between clavate and tubulospinose morphologies, providing direct, stratigraphically-ordered evidence of the evolutionary transition between Hantkenina and Clavigerinella. Clavigerinellid ancestry is traced to a previously undescribed low-trochospiral species, Parasubbotina eoclava sp. nov., at Ocean Drilling Program Site 865. We speculate that Hantkenina originated through competition for limited food resources in a deep, oxygen-deficient habitat below the thermocline. The tubulospines may represent a structural adaptation to this new trophic strategy, allowing the organism to harvest a greater volume of water at minimal metabolic cost. The abrupt occurrence of Hantkenina in pelagic sediment cores from the central Pacific and other regions of the world ocean may represent immigration into these areas following speciation within the hydrographically evolving Tethys Seaway. Alternatively, cladogenesis may have occurred over a wider area, but due to a contemporaneous global hiatus the fossil record of this bioevent is poorly preserved.

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