DISTRIBUTION OF FOSSIL DECAPOD CRUSTACEANS FROM THE LA MESETA FORMATION (EOCENE) ON SEYMOUR ISLAND, ANTARCTICA: CHANGING TIMES AND CHANGING PATTERNS

The Eocene La Meseta Formation, which crops out on the northern part of Seymour Island, Antarctica, is a marine sequence deposited in a composite incised valley system. The 720 meter-thick sedimentary fill can be divided into three main facies associations. The coarsening-upward lower 370 m section, comprised of the Valle de las Focas and Acantilados allomembers, represents a tide and wave influenced delta system prograding within the drowned valley. The 230 meter-thick middle association; the Campamento, Cucullaea I, Cucullaea II, and the lowermost Submeseta allomembers; is composed of the most heterogeneous lithology, from coarse-grained shell beds to mudstones deposited in a wide range of estuarine to shallow marine environments. The sandy upper 120 meter-thick section, most of the Submeseta Allomember, mainly represents shallow sublittoral to littoral environments. Twelve species in eleven genera of decapods have been collected, and their localities have been plotted using GPS techniques. Diversity and numerical abundance of decapod crustaceans varies widely throughout the formation. Six species first appear in the Acantilados Allomember; one lobster and one mudshrimp are Cretaceous relicts. Lyreidus, a raninid crab, first appears in a sandier phase of this allomember and increases in abundance upward into the middle sequence. This crab, represented by several hundred specimens, dominates the decapod fauna in the fine sand and silty sand of the mid to upper part of the Cucullaea I Allomember and subsequently disappears from the section. At least three of the four species of crabs are known exclusively from the coarser sands of the Submeseta Allomember, but they are rare occurrences. In general, the generic diversity of decapods is highest in the lower and the upper facies associations whereas the numerical abundance of specimens, primarily Lyreidus antarcticus, is greatest in the middle facies association. Thus, more precise stratigraphic location of the decapods in the La Meseta Formation demonstrates that the decapods track environmental changes previously determined by refined sedimentological analysis. This work was supported by NSF grant OPP9909184 to Feldmann and Karen Bice. Logistic support was provided by the Instituto Antartico Argentino.