Abstract:
Various facets of the Truncorotalia juanai-Truncorotalia crassaformis evolutionary transition are considered here. These planktonic foraminifera transitioned over the Miocene/Pliocene boundary. This pair offers an excellent opportunity to assess phyletic gradualism verses punctuated equilibrium in the context of planktonic foraminifera. Other topics studied using these species are evolutionary rates, modes and drivers. To study these topics, morphometric data from 1449 Truncorotalia specimens were obtained. These data facilitate the quantitate analysis of shape change during the transition.
A principal components analysis (PCA) and cluster analysis of the morphometric data suggested that the transition from Truncorotalia juanai to Truncorotalia crassaformis illustrates gradualism. This result agrees with studies that suggest planktonic foraminiferal evolution is anagenetic. Studying this gradual transition prompts the suggestion of using the morphometrically defined intermediate forms’ highest abundance as a species boundary.
Rates and modes of the Truncorotalia evolution were statistically determined using PCA scores as traits. Both the random walk and driven modes were determined using these PCA scores. The rate values were much lower than expected, possibly an artefact of using PCs as traits.
Possible drivers of transition are studied by comparing sea surface temperature and δ¹⁸O curves to a species abundance plot. The proposed species boundary is coincident with large fluctuations in the sea surface and ice volume records. Late Miocene paleoceanographic changes are therefore possible evolutionary drivers.
Finally, a phylogeny for nine members of Truncorotalia is constructed using stratocladistics. This application illustrates that stratocladistics can successfully be applied to planktonic foraminifera and useful phylogenies can be derived. This application suggests that Truncorotalia juanai is a more appropriate ancestral species for Truncorotalia than Hirsutella cibaoensis, given the employed characters.
