MORPHOMETRIC AND SURVIVABILITY EXPERIMENTS ON RANUNCULUS (BUTTERCUP) ACHENES RELATED TO THEIR OCCURRENCE AS FOSSILS IN THE MEYER DESERT FORMATION, SIRIUS GROUP, ANTARCTICA

A morphometric study of fossil achenes of Ranuculus from the Neogene Meyer Desert Formation (Sirius Group), Antarctica, showed a significant linear relationship (Cheng, 2003). On the basis of moderate to high r² values (0.539 and 0.745) for plots of area against short and long axes, respectively, Ashworth and Cantrill (2004) suggested that the achenes were from a single species of Ranuculus. Further, variation in size and shape was attributed to position in the flower head with additional modification resulting from wear in the depositional environment.

To test for variation in the flower head, we measured the long and short axes and the areas of 110 randomly selected achenes from a single extant flower head of Ranuculus. The paleoenvironment of the Meyer Desert Formation, determined by litho- and biofacies, was a braided outwash plain in which patches of tundra plants grew on the interchannel gravel bars. To simulate the abrasion that achenes deposited in this environment would be subjected to, we tumbled split subsets (15-20 achenes in each experiment), both dry and wet, in coarse sand (0.0 φ), fine sand (2.0 φ) and silt (4.0 φ). The size and shape variation in achenes expressed in a single flower head is similar to that observed in the fossils from the Meyer Desert Formation. This supports the hypothesis that the linear relationship could result from them belonging to a single species rather than multiple species. The experiment producing the most wear on the achenes was with dry, coarse sand. Even so, the wear was minimal and fits within the variation of the original non-tumbled dataset. The relative lack of wear helps to explain why achenes of Ranunculus were able to survive in a potentially abrasive depositional environment. Further, the greater degree of wear on the fossil achenes demonstrates that they were repeatedly reworked and transported, probably in both air and water, before final burial. This REU research was supported by NSF grant OPP-0230696.