ALBERT

Description: Prokaryota in natural environments form biofilms, which are benthic assemblages of a variety of microorganisms embedded within their extracellular mucilage. Biofilms are firmly attached to surfaces such as aquatic sediments. Quorum sensing by the many microbes in a biofilm is collective decision making and cooperation for responding to internal and external parameters affecting the community. This communication is based on chemical signaling affecting gene expression of the microorganisms. Microorganisms situated in a biofilm change behaviors and metabolic activities to comply with the requirements of the entire biofilm cooperative. Consequently, reconstruction of the evolution of prokaryotes in Earth history must consider the biofilm way of microbial life. Biogenic sedimentary structures might not represent certain microbial groups, but in fact may be relics of modified cooperative microbial activities. Future research should focus on detectable biosignatures caused by biofilm consortia as a whole instead of on the appearance or extinction of individual microbial groups. Such sedimentary structures as stromatolites and microbially induced sedimentary structures (MISS) are intrinsically controlled by biofilms, but also affected by extrinsic (environmental) conditions.

Description: 〈span〉〈div〉ABSTRACT〈/div〉Microbially induced sedimentary structures may help preserve unique glimpses of ancient shoreline habitats, but are little known from Mesozoic epicontinental settings. To help fill this knowledge gap, we describe a diverse suite of microbial structures from the Upper Cretaceous “J” Sandstone (South Platte Formation, Dakota Group) that are spectacularly exposed at Dinosaur Ridge in Morrison, Colorado, USA. Structures include “tattered” bed surfaces and ferruginous sand chips in supratidal flat facies. A large over-flip structure is preserved in a channel locally known as Crocodile Creek. In upper-intertidal facies, multidirectional ripple marks occur. Perhaps the most well-known microbial structures are exposed on extensive bedding surfaces known as “Slimy Beach,” where lower supratidal-flat facies are dominated by decimeter-scale erosional remnants and pockets. Morphologies and superposition of the structures allows identification of three generations of erosional pockets. Generation A of these erosional pockets exhibit size similarities to ornithomimid, sauropod, and ornithopod dinosaur tracks from adjacent bedding planes, raising the question of whether initial disturbance of the mat-bound surface could have been from track making. Generation B erosional pockets are older and record continuous erosion of the initial pockets until they were eventually overgrown and sealed by microbial mats. Generation C pockets are the oldest ones, exposing wide areas of barren sediment that could not be overgrown by microbial mats anymore. In concert, the microbial structures point to seasonally variable meteorological conditions along the coastline of the Western Interior Seaway and indicate that the “Slimy Beach” bedding plane represents a multi-year record of dinosaur locomotion.〈/span〉