Decapod crustaceans from the Eocene La Meseta Formation, Seymour Island, Antarctica: a model for preservation of decapods

Thirteen species of fossil decapod crustaceans collected from 43 localities in the La Meseta Formation (Eocene), Seymour Island, Antarctica, have been located precisely using global positioning system techniques and accurately placed in their stratigraphic context using an allostratigraphic scheme. Three decapod species are congeneric with extant species known from outer shelf and slope habitats documenting an onshore-offshore pattern of ecological preference. Functional morphological analysis of the decapods confirms the environmental interpretations based upon sedimentological studies. Species diversity is greatest in the lower and upper parts of the section, in the Acantilados and Submeseta allomembers, respectively. However, numerical abundance of specimens is greatest near the top of the coarsening-upward sequence in the Cucullaea I Allomember. This allomember and the underlying Campamento Allomember are overwhelmingly dominated by a single species, Lyreidus antarcticus. The relatively high diversity of decapods in the lower and upper parts of the La Meseta Formation is interpreted as resulting from favourable preservational environments in delta-front and nearshore environments where burial was rapid and sediment reworking was minimal. In the middle part of the La Meseta Formation, sediment reworking was probably high and fragile arthropod cuticular material was destroyed, except in the case of Lyreidus, which is a burrower and was probably preserved within its burrow. The pattern of diversity documented by the arthropods differs markedly from that of the molluscs, probably because of the presence of more durable shell material in the pelecypods and gastropods. The La Meseta Formation record of decapods may serve as an important model for arthropod preservation against which other occurrences can be tested.