Taphonomic analysis of modern and fossil mollusk assemblages from Tierra del Fuego using the venerid Tawera gayi (Hupé) as a target species.

GORDILLO, S., CARDENAS,J., MARTINELL, J.

Valdivia, Chile

Congreso; VII Congreso Latinoamericano de Malacología; 2008

Living Tawera gayi is a typical element of shallow marine waters in southern South America. In Tierra del Fuego, it occurs infaunally near the sediment surface. In the same region, fossil T. gayi was recovered from Holocene raised marine terraces along the Beagle Channel and the Magellan Strait. These terraces (previously dated between 2,000 and 6,000 yr BP) represent sea level fluctuations in the recent past. The objective of this work is to evaluate the use of the taphon T. gayi in paleoenvironmental reconstructions. The choice of a target species (and not pooled species) was based on the possibility to eliminate preservational differences among taxa, associated to shell properties, life habits and habitat. Besides, T. gayi was selected because it has a thick and solid shell, and it is a very common species, typically less fragmented than mytilids, which were also collected in the fossil assemblages. Both fossil and modern mollusk assemblages were sampled. Fossil Tawera was separated from bulk samples and hand-picked (to prevent artificial disarticulation) samples taken from Holocene raised marine terraces. Modern samples were collected using quadrats. Whole Tawera shells were measured in shell height and length. Taphonomic features included disarticulation, fragmentation, abrasion, dissolution, color and bioerosion. The degree of taphonomic features were studied for individual shells (N=981) in one sample, and then were averaged over the entire sample. Results indicate that shell preservation in T. gayi varies widely among sites. These differences might be associated to sedimentation rates, hydrodynamic energy and freshwater input, among others. Some sites represent authochtonous or parauthoctonous fossil assemblages deposited under low to moderate energy marine conditions, and provide tools for the reconstruction of paleocommunities. It is concluded that T.  gayi is a good paleobiological and paleoenvironmental indicator, and it can be useful to evaluate fossil mollusk assemblages in the recent past.