SECULAR PATTERNS IN MORPHOLOGICAL DISPARITY AND BODY SIZE OF ACRITARCHS THROUGH THE NEOPROTEROZOIC AND EARLY CAMBRIAN

Acritarchs are a group of phylogenetically heterogeneous microfossils that are interpreted as the resting cysts of eukaryotes. Their diversity through the Neoproterozoic and early Cambrian has been estimated by counting species. These estimates suggest that diversity increased steadily in the early Neoproterozoic and then decreased sharply between 700 and 600 Ma, a drop likely related to Neoproterozoic glaciations. An ephemeral diversification of process-bearing acritarchs (acanthomorphs) followed the glaciations, but many of these acanthomorphs disappeared in Ediacaran time. Acritarch species diversity increased again in the early Cambrian. Anecdotal evidence also suggests that acritarch size and complexity increased throughout the Neoproterozoic until the Ediacaran extinction, and that post-Ediacaran acritarchs reached the levels of morphological complexity, but not the large sizes, recorded for the Neoproterozoic.

This reconstruction of the evolutionary history of acritarchs has recently been brought into question, due to severe inconsistencies associated with acritarch taxonomy. We propose an independent strategy for investigating the evolutionary history of these early eukaryotes, by analyzing size and morphological complexity of acritarchs using quantitative data obtained from the literature. This method is not affected by the taxonomic problems of naming and counting species. Data are being assembled from monographs on Neoproterozoic to early Cambrian acritarchs. Size and morphologic data collected include: vesicle size and morphology, process size, morphology, and count, presence/absence of enveloping membranes, and excystment structures. Other data collected include: paleogeographic location, best estimated age, lithology, depositional environment, mode of preservation, and preparation technique. The resulting dataset is explored using multivariate and computer-intensive methods.

Preliminary data do not suggest any significant changes in acritarch vesicle size between 720 and 580 Ma. Moreover, Multidimensional Scaling based on morphological characters does not suggest any major shifts within the occupied morphospace through this time interval. These pilot results should be interpreted with caution until more data are available.