Foraminifera are protozoans with a hard calcareous shell. There are two major groups: benthic and planktic. Benthic organisms live on the sea floor whereas planktic are buoyant and live in the upper water layer as part of the plankton. Their shells, although mostly microscopic, are much studied because they are found in profusion in oceanic sediments. The geological history of the benthic forams goes back to the Early Cambrian, but planktic species first appear in Mid-Jurassic sediments.
"Traditionally, all planktic foraminifera have been seen as monophyletic [Suborder Globigerinina], descended from a single Early-Middle Jurassic ancestor, similar to the monophyletic origins of other planktic groups."
Streptochilus globigerus (credit: Kate Darling, source here)
Six years ago, Hart and colleagues reviewed evidence bearing on the ancestor of the planktic forams and suggested that its evolution was triggered by an oceanic anoxic event in the Early Jurassic. They introduce their paper with an acknowledgement of the problems:
"In a recent review of the earliest planktic Foraminifera (Globigerinina) Simmons et al. (1997) report that the origins of the group are '. . . still shrouded in uncertainty'."
The new research by Darling and colleagues has found that a planktic species, Streptochilus globigerus, is genetically the same as a benthic species, Bolivina variabilis. This is the first time such a lifestyle has been recognised in foraminifera. The technical term is tychopelagic.
"The word "tychopelagic" is used to describe organisms that usually live as benthos but can survive and grow in fairly large numbers as plankton and may be advected well offshore into open ocean assemblages. Such a lifestyle is known from diatoms, but until now has never been documented for foraminifera."
The implications are many. First, the sharp line drawn between benthic and planktic forams needs to be erased. The issue of buoyancy suddenly becomes secondary. The previous stance - that planktic forams were a radical evolutionary innovation - needs to be discarded. "Interestingly, buoyancy is generally assumed to be one of the major constraining evolutionary traits on the passage from benthos to plankton."
Secondly, the monophyletic radiation of planktic forams, with examples of both punctuated and gradual changes, needs to be re-examined. If tychopelagic forams are common, everything goes into the melting pot.
"The Cenozoic planktic foraminiferal phylogeny of microperforates, the group containing biserial and triserial forms, has generally presented taxonomists with problems. Many of these genera and species show discontinuous stratigraphic records, making ancestor-descendant patterns difficult to reconstruct. This could be the result of a lack of observation of the small forms, in a size fraction that commonly is not included in study. In our view, however, such ancestor-descendant relations simply do not exist."
[. . .]
"Appearances of biserial and triserial planktic forms in the geological record should therefore not be considered as necessarily discrete punctuated evolutionary events but as a series of excursions of expatriated tychopelagic microperforates into the planktic domain."
Thirdly, the concept of recolonisation has been underplayed by evolutionary biologists. They get as far as colonisation, but perceive this as a process of gradual adaptation under the influence of natural selection. They have given little thought to inbuilt capabilities of rapid adaptation to new environments - perhaps because this could be understood as a designed capability. Nevertheless, the authors of the new research recognise the ecological advantages possessed by tychopelagic organisms.
"The ability to survive in both planktic and benthic habitats should be seen as an extraordinary ecological adaptation for long-term survival. After mass extinctions in the plankton, e.g., as caused by bolide impacts and oceanic anoxic events, tychopelagic species are able to repopulate the pelagic realm and evolve into purely planktic forms."
[. . .]
"We thus argue that radiation and repopulation of the empty niche in the plankton after the end Cretaceous mass extinction may at least in part have occurred from benthic tychopelagic species rather than from nerito-planktic ones."
This research provides another angle on the hypothesis outlined in a previous blog that the fossil record has more to do with ecology and the colonisation/recolonisation of habitats than it has to do with evolutionary transformation.
Surviving mass extinction by bridging the benthic/planktic divide
Kate F. Darling, Ellen Thomas, Simone A. Kasemann, Heidi A. Seears, Christopher W. Smart and Christopher M. Wade
Proceedings of the National Academy of Sciences USA, online before print July 2, 2009, doi: 10.1073/pnas.0902827106 (abstract)
Abstract: Evolution of planktic organisms from benthic ancestors is commonly thought to represent unidirectional expansion into new ecological domains, possibly only once per clade. For foraminifera, this evolutionary expansion occurred in the Early-Middle Jurassic, and all living and extinct planktic foraminifera have been placed within 1 clade, the Suborder Globigerinina. The subsequent radiation of planktic foraminifera in the Jurassic and Cretaceous resulted in highly diverse assemblages, which suffered mass extinction at the end of the Cretaceous, leaving an impoverished assemblage dominated by microperforate triserial and biserial forms. The few survivor species radiated to form diverse assemblages once again in the Cenozoic. There have, however, long been doubts regarding the monophyletic origin of planktic foraminifera. We present surprising but conclusive genetic evidence that the Recent biserial planktic Streptochilus globigerus belongs to the same biological species as the benthic Bolivina variabilis, and geochemical evidence that this ecologically flexible species actively grows within the open-ocean surface waters, thus occupying both planktic and benthic domains. Such a lifestyle (tychopelagic) had not been recognized as adapted by foraminifera. Tychopelagic are endowed with great ecological advantage, enabling rapid recolonization of the extinction-susceptible pelagic domain from the benthos. We argue that the existence of such forms must be considered in resolving foraminiferal phylogeny.
Tiny marine organism lives double life to survive extinction, Planet Earth Online, 1 July 2009.
Hart, M.B., Hylton, M.D., Oxford, M.J., Price, G.D., Hudson W. and Smart, C.W., The search for the origin of the planktic Foraminifera, Journal of the Geological Society, 160, 2003, 341-343.
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