Evolutionary trends of Antarctissa in the Quaternary using morphometric analysis

Abstract

Three piston cores from a latitudinal traverse across the Polar Front in the southeast Indian Ocean have been used as the basis for a morphological analysis of the Antarctissa complex (Radiolaria). All three cores penetrate sediments deposited during at least the last 0.5 Ma B.P. The radiolarian genus Antarctissa has been selected to look for variations in size and shape through time. The method used is an interactive outline detection utilizing a cubic b-spline technique. Shape change is evaluated by phi function analysis. Size variability is measured by the area of the test. The results of this study indicate that Antarctissa tests seem to oscillate in size and shape according to prevailing oceanographical/climatological conditions, and thus they may be employed as indices of paleoclimatological fluctuations in the Southern Ocean. The size pattern for Antarctissa shows larger forms during glacial periods and smaller forms during interglacial periods. For the shape change, the study is differentiated between an allometric component and one growth-free component. The allometric component shows variation closely related to the size pattern, i.e. an oscillating pattern close to the changes in glacial/interglacial patterns. The growth-free shape component on the other hand shows a long-term variation in shape that may be interpreted as change due to evolution. The use of the computerized measuring technique employed here, enables measurements of a large number of specimens per sample and provides information of use in obtaining a high-resolution stratigraphy.