Form-classification for microbially induced sedimentary structures

Abstract

Various ichnotaxonomic names have been proposed for microbially influenced sedimentary structures, but despite the historically broad scope of ichnotaxa allowed by the International Code of Zoological Nomenclature, recent opinion rejects these names for the Zoological Code. Nevertheless, such names are allowed by generous provisions for fossil genera (previously form genera) in the International Code of Nomenclature for Algae, Fungi and Plants. Fossil genera, such as Neantia, Rameshia and Rugalichnus, describe various undulose and striated textures of aquatic microbial mats. In contrast, the complex hackly, fissured and microtuffeted surfaces widely called ‘old elephant skin’ texture have been assigned to Rivularites repertus. The type specimen of that species has been lost since bombardment of Nancy in 1914. The locality exposing the Upper Triassic (Carnian), Grès à Roseaux Member of the Keuper Formation at Gemmelaincourt, France, is an abandoned coal mine, but has waste piles still available for collecting. This study describes a proposed neotype of the ichnospecies, and other fossils relevant to its environmental interpretation. The neotype of Rivularites came from coal measures with interbedded evaporites of a coastal plain, and is a non-marine texture as apparent from evidence of alternate desiccation, intergrowth and healing. Gregory J. Retallack [ gregr@uoregon.edu ], Department of Earth Sciences, University of Oregon. Eugene, OR 97403, USA