TRENDS IN BODY-SIZE AND MORPHOLOGICAL DISPARITY OF PROTEROZOIC THROUGH MIDDLE CAMBRIAN ACRITARCHS: A LITERATURE-BASED APPROACH

Previous estimates of acritarch diversity, based on species counts, suggested a history of steadily increasing diversity through the Neoproterozoic, punctuated by at least two extinction events, likely related to late Neoproterozoic glaciations and the appearance of the Ediacaran metazoans. Diversity recovered in the early Cambrian. Anecdotal evidence suggests that body-size and morphological disparity increased through the Neoproterozoic until the Ediacaran extinction and that the Post-Ediacaran acritarch assemblages regained the complexity, but not the large body-sizes, found in pre-Ediacaran assemblages. The reliability of these interpretations has been questioned recently due to taxonomic problems that may affect acritarchs.

We utilize a taxon-free literature-based approach to examine the history of body-size and morphological disparity of Proterozoic-Cambrian acritarch assemblages. Body-size was estimated by maximum vesicle diameter reported in species descriptions and by measuring figured specimens. Morphological disparity was estimated using twenty-three characters used in species descriptions.

Average maximum vesicle diameter (AMVD) increased significantly from the Mesoproterozoic Bylot Formation (1270 Ma; 155 µm) to the Neoproterozoic assemblages from the Svanbergfjellet Fm. (720 Ma; 250 µm), Doushantuo Fm. (580 Ma; 200 µm), and the Pertatataka and Bitter Springs Fms. (170 µm). AMVD decreased significantly in the late Neoproterozoic/Cambrian Lublin Slope (30 µm) and middle Cambrian Tempe Fm. (60 µm) assemblages. AMVD are significantly correlated with the average size of figured specimens, suggesting that the latter may be a useful source of data on secular trends in acritarch body-size.

PCO analysis and dissimilarity coefficients (DC) indicate concordantly low morphological disparity in the Mesoproterozoic Bylot Fm (PCO var.=6.8E-07; DC=0.012). The disparity increased significantly in the Neoproterozoic (Svanbergfjellet Fm. [PCO var.=0.01; DC=0.21], Doushantuo Fm. [PCO var.=0.01; DC=0.21], Pertatataka and Bitter Springs Fms. [PCO var.=0.008; DC=0.23]). A slight, but significant, decrease in morphological disparity occurred in the early Cambrian Lublin Slope (PCO var.=0.008; DC=0.17) and middle Cambrian Tempe Fm. (PCO var.=0.005; DC=0.16) assemblages.