A molecular clock for coccolithophores and implications for selectivity of phytoplankton extinctions across the K/T boundary

A molecular clock has been constructed for the haptophyte algae using the 18S rDNA gene and calibrated using the fossil record of the coccolithophorid algae, which have the best fossil record of any microalgal group. There is high consistency between the molecular genetic estimates of relative timing of divergence and palaeontological estimates of divergence times, so ages can be inferred for undated nodes in the tree with a reasonable degree of confidence. The placement of the K/T boundary across the tree strongly supports the palaeontological model that extant coccolithophorid algae diversified after this event and are the survivors of a few lineages that survived this major extinction. In contrast, the non-calcifying haptophytes are diverse before and after the extinction, with no evidence of bottlenecking associated with the event. This result is surprising, because it has been assumed that ability to produce resting stages was a key determinant of phytoplankton survival across the K/T boundary, but in this regard the coccolithophores and non-calcifying haptophytes are similar. The adaptation of non-calcifying haptophytes to eutrophic coastal environments and their ability to switch modes of nutrition from autotrophy to mixotrophy are discussed as possible explanations for their survival during this abrupt global change event.