'Autochthonous' bivalve assemblages and palaeoecologic interpretation in the upper Jurassic-Lowermost Cretaceous La Caja Formation from the Canon de San Matias (Zacatecas, Mexico)

The present research approaches the precise palaeoecologic interpretation b ased on the recognition of 12 'autochthonous' bivalve assemblages among 52 registered from the Upper Jurassic-Lowermost Cretaceous La Caja Formation i n the Canon de San Matias section (Mexican Altiplano). Fine-grained deposit s together with the near-exclusive record of suspension-feeders are conside red the evidence for low-energy and nutrient-rich environments. The presenc e of oxygen-dependent bivalves, mainly Buchia and Lucina, supports the inte rpretation of oxygenation as a major factor controlling ecology and therefo re the composition of the 'autochthonous' bivalve assemblages studied. Sign ificant changes in the composition of bivalve assemblages show no relation between fluctuations in ecological factors and background depositional cond itions. (C) 2000 Academie des sciences / Editions scientifiques et medicate s Elsevier SAS.