The Laluzia armini (gen. et spec. nov.) ecosystem: Understanding a deeper-water rudist lithosome from the Early Maastrichtian of Mexico

In the Lower Maastrichtian Cardenas Formation exposed at La Luz (State of San Luis Potosí, east-central Mexico), a shallowing-upwards mixed-clastic-carbonate sequence is exposed. The sequence passes from marls with thin siltstones, through lower and upper hippuritid-rudist-dominated intervals, and into a Durania-dominated interval. This succession shows an increase in grain size upwards and a progressive reworking of rudists upwards (preserved in life position, other than in distinct tempestites, in the lower part; invariably reworked/toppled in the upper part). Epibionts show a change from a serpulid-bryozoan assemblage in the lower sequence to a red algal-rhodolith assemblage in the upper part. Using these data, we argue that the sequence shows a change from a low-energy, relatively deep-water, nutrient-rich environment with low-light intensity in the lower part, to a high-energy, well-lit environment in the upper part. Two rudist species are present: a new multifold hippuritid rudist with cellular outer shell layer, Laluzia armini, that lacks pallial canals in its free valve and has a unique myocardinal pillar arrangement, in the lower part; and Durania in the upper part. Laluzia was adapted to low-energy, low-light, soft-bottom environments that were abundant within the lower part of the sequence exposed in the La Luz section - a very unusual environment for rudists. © Springer-Verlag 2009.