Shell beds as paleoecological puzzles: A case study from the Upper Permian of the Parana Basin, Brazil

Microstratigraphic, sedimentological, and taphonomic features of the Ferraz Shell Bed, from the Upper Permian (Kazanian-Tatarian?) Corumbatai Formation of Rio Claro Region (the Parana Basin, Brazil), indicate that the bed consists of four distinct microstratigraphic units. They include, from bottom to top, a lag concentration (Unit 1), a partly reworked storm deposit (Unit 2), a rapidly deposited sandstone unit with three thin horizons recording episodes of reworking (Unit 3), and a shell-rich horizon generated by reworking/winnowing that was subsequently buried by storm-induced obrution deposit (Unit 4). The bioclasts of the Ferraz Shell Bed represent exclusively bivalve mollusks. Pinzonella illusa and Terraia aequilateralis are the dominant species. Taphonomic analysis indicates that mollusks are heavily time-averaged (except for some parts of Unit 3). Moreover, different species are time-averaged to a different degree (disharmonious time-averaging). The units differ statistically from one another in their taxonomic and ecological composition, in their taphonomic pattern, and in the size-frequency distributions of the two most common species. Other Permian shell beds of the Parana Basin are similar to the Ferraz Shell Bed in their faunal composition (they typically contain similar sets of 5 to 10 bivalve species) and in their taphonomic, sedimentologic, and microstratigraphic characteristics. However, rare shell beds that include 2-3 species only and are dominated by articulated shells preserved in life position also occur. Diversity levels in the Permian benthic associations of the Parana Basin were very low, with the point diversity of 2-3 species and with the within-habitat and basin-wide (alpha and gamma) diversities of 10 species, at most. The Parana Basin benthic communities may have thus been analogous to low-diversity bivalve-dominated associations of the present-day Baltic Sea. The 'Ferraz-type' shell beds of the Parana Basin represent genetically complex and highly heterogeneous sources of paleontological data. They are cumulative records of spectra of benthic ecosystems time-averaged over long periods of time (10(2)-10(4) years judging from actualistic research). Detailed biostratinomic reconstructions of shell beds can not only offer useful insights into their depositional histories, but may also allow paleoecologists to optimize their sampling designs, and consequently, refine paleoecological and paleoenvironmental interpretations.

A new monohedral hexamorphic prototile

In this paper we apply the fitting method and a result due to Martin (see [''Polynominoes-A Guide to Puzzles and Problems in Tiling,'' the Mathematical Association of American, New York, 1991] to get a hexamorphic prototile different from the only one we have known, presented by Fontaine and Martin in [J. Combin. Theory Ser. A 34 (1983), 119-121]. (C) 1997 Academic Press.

Can entanglement explain black hole entropy?

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Produção de material didático para o ensino de solos

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Índices de erotismo no discurso machadiano

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

A utilização dos jogos pedagógicos como ferramenta auxiliar no processo de aquisição de escrita

Pós-graduação em Letras - FCLAR