Inferred pseudo-cryptic speciation in the coccolithophore species Braarudosphaera bigelowii (Gran and Braarud) during the Early Paleocene (Danian)

Pseudo-cryptic speciation occurs commonly in living oceanic plankton. In the coccolithophores, recent genetic studies have revealed that Braarudosphaera bigelowii has a number of unique genotypes that correlate well with pentalith size, indicating that this coccolithophore “species” actually contains multiple pseudo-cryptic taxa. In order to look for evidence of pseudo-cryptic speciation in the past, we measured the radii of 2800 B. bigelowii pentaliths from the Braarudosphaera-rich limestones that contain the GSSP for the Lower/Middle Paleocene (Danian/Selandian) boundary at Zumaia, Spain. Analysis of the radius measurements using the Expectation-Maximization algorithm yielded two size groups, herein termed form X (< 4.5 μm) and form Y (≥ 4.5 μm). This result suggests the possibility of at least two pseudo-cryptic species in the Early Paleocene taxon B. bigelowii. In addition, there is a sharp increase in overall B. bigelowii abundance near the Paleocene Magnetochron C27n/26r reversal in the Zumaia section, forming an acme zone that spans approximately four meters. Isotopic analyses of organic matter show that an abrupt negative excursion of 3.5‰ in δ13Corg coincided with the onset of the B. bigelowii acme. Biostratigraphic correlation between this section and the Danian/Selandian boundary section at Qreiya, Egypt confirmed that this isotopic shift correlates with a hyperthermal event (Latest Danian Event), and suggests that B. bigelowii abundance may have been affected by changing climatic conditions.