Hantkeninid planktonic foraminifera and eocene palaeoceanographic change.
The morphological and ecologicalevolution of middle-upper Eocene planktonic
foraminiferal family Hantkeninidae is investigated in the context of the dramatic
palaeoceanographic and climatic changes that marked the transition from Paleogene
"greenhouse" to Neogene "icehouse" climatic conditions.
Morphometric analysis proves that evolution in family Hantkeninidae was
gradual but complex in detail with periods of relative stasis. Multiple lines of evidence
demonstrate that Hantkenina evolved from planispiral clavate genus Clavigerinella and
not, as was previously believed, from Pseudohastigerina micra. The ancestor of
Clavigerinella was probably a low trochospiral form Paragloborotalia sp., which has
been recognized for the first time in this study at a number of sites. Trends in chamber
inflation, tubulospine angle and the position of the tubulospine on each chamber show
the most dramatic evolutionary changes, indicating that these are the most useful
characters for taxonomy. These morphological changes correlate well with known
palaeoceanographic changes as well as the shift in hantkeninid ecology from a deep to a
surface water habitat.
Hantkeninids underwent pronounced adaptive evolution in depth habitats during
the initial phase of the climatic transition. Lower middle Eocene forms lived in a cool
deep-water environment within or below the oceanic thermocline and shifted to warmer
surface waters in the late middle Eocene. They evolved in the low latitudes and were
primarily. a tropical-subtropical group. The occurrence of Hantkenma australis at
relatively high northerly and southerly latitudes during the middle Eocene may record a
temporary expansion of warmer water conditions into these regions, possibly
representing a hitherto unknown "hyperthermal" event. Clavigerinella is rare in middle
Eocene open-ocean sequences but occasionally occurs in relative abundance in other
localities (such as on continental margins and oceanic seamounts), suggesting that it was
specialized for living in upwelling regions.
A revised taxonomy of family Hantkeninidae is presented that reflects new
understanding ofhantkeninid evolution. The reconstructed phylogeny demonstrates that
the tubulospine-bearing genera Hantkenina and Cribrohantkenina represent a
monophyletic clade. Multivariate analysis suggests that more than one morphological
population existed at several times and that these may represent biological species.
The results demonstrate that the hantkeninids are not merely passive recorders
of ocean conditions but have instead evolved morphology and changed habitat in
response to climate change.