Abstract:
We present a detailed quantitative account of morphological evolution in the benthic foraminifer genus Parkiella throughout the last 10 Myr of the Cretaceous (late Campanian to Maastrichtian). The studied evolutionary lineage was endemic to the low-latitude central Pacific Ocean; so far it was found only in two deep-sea sequences (DSDP Sites 465 and 171) from the Hess Rise and the Mid-Pacific Mountains. The sequences at both sites overlap in a complementary pattern facilitating the recovery of the entire evolutionary history of the lineage. Altogether 437 specimens from 53 samples were collected and the shape of their apical outlines, chamber arrangements, shell sizes and proloculus sizes were quantified. The size and the stratigraphic resolution of this database are not ultimately satisfactory, yet the sequential occurrence of morphologically intermediate paleopopulations observed at both sites suggests that the lineage was not in the state of morphological stasis for the major part of the studied interval. The distribution of all morphological variables in the analysed samples appears to be normal or at least unimodal, supporting the classification of the observed evolutionary pattern as an anagenetic trend in a single non-branching lineage. This finding seems to confirm the predictions of the "Plus ca change" model, which states that gradual evolution is expected to occur in stable or narrowly fluctuating environments, such the pelagic realm or the deep seas. Yet, at about the same time as the onset of the gradual evolution of the lineage, there was a major paleoceanographic event--the Maastrichtian deep-water reversal, which influenced the benthic faunas worldwide. The question remains, why this environmental perturbation did not cause a punctuated evolutionary event. Perhaps, the gradual evolution did not result from the environmental perturbation itself. Instead, it may represent a reaction on the conditions established after this event. To conclude, this study shows that gradual evolution may indeed occur in the deep-sea environment, as predicted by the "Plus ca change" model. However, the understanding of the underlying causes and processes generating such an evolutionary pattern is still poor.