Microbial communities developing on modern clastic sedimentary surfaces of arid lands are dominated by phototrophic microorganisms that form a variety of characteristic “microbially induced sedimentary structures” (MISS) through their interactions with detrital sedimentary grains, aided by secretions of extracellular polymeric substances and other organic materials. In this study, we describe modern MISS from unvegetated arid topsoils and compare them with fossil MISS found within decimeter- to meter-thick sedimentary sequences of Mesoproterozoic siliciclastic outcrops of the Dripping Spring Quartzite formation of the Apache Group in central Arizona, USA. These sequences contain numerous bedding plane exposures with desiccation surfaces including polygonal cracks, curls, and chips. Repetition of these structures within stratigraphic sequences indicates recurring episodes of subaerial exposure. Some of these MISS contain cellular microfossils that exhibit morphological adaptations for surviving desiccation. The strong similarities between modern and ancient MISS in this study provide additional criteria for recognizing morphological biosignatures of terrestrial microbial communities in ancient deposits. Our results provide compelling evidence for the presence of land-based microbial communities by the Mesoproterozoic (∼1200 Ma). The association of MISS features further suggests that the primary producers that had colonized Mesoproterozoic land surfaces were likely desiccation-adapted photosynthetic microbes, similar to modern desert soil crust communities.

In order to evaluate taxonomic and environmental control on the preservation pattern of brachiopod accumulations, sedimentologic and taphonomic data have been integrated with those inferred from the structure of brachiopod accumulations from the easternmost Lower Jurassic Subbetic deposits in Spain. Two brachiopod communities (Praesphaeroidothyris and Securina communities) were distinguished showing a mainly free-lying way of life in soft-bottom habitats. Three taphofacies are discriminated based on proportion of disarticulation, fragmentation, packing, and shell filling. Taphofacies 1 is represented by thinly fragmented, dispersed brachiopod shells in wackestone beds. Taphofacies 2 is spatially restricted to small lenses where shells are poorly fragmented, rarely disarticulated, usually void filled, and highly packed. Taphofacies 3 is represented by mud or cement filled, loosely packed, articulated brachiopods forming large pocket-like structures. Temporal and spatial averaging were minimally involved in taphofacies 2 and 3. It is interpreted that patchy preservation implies preservation of primary original patchiness of brachiopod communities on the seafloor. The origin of shell-rich taphofacies (2 and 3) is related to rapid burial due to episodic storm activity, while shell-poor taphofacies 1 records background conditions. The nature and comparative diversity of these taphofacies underscores the importance of rapid burial for shell beds preservation. Differences in preservation between taphofacies 2 and 3 are mainly related to environmental criteria, most importantly storm energy and water depth. In contrast, the taxonomic-specific pattern of the communities is a subordinate element of control, controlling only minor within-taphofacies differences in preservation.

The stratigraphy, paleobathymetry, and paleontology of the Cincinnatian strata have been studied extensively, providing an excellent framework within which to explore spatial and temporal variations in encrusting sclerobiont assemblages. Recently, the relative paleobathymetry of Upper Ordovician Cincinnatian strata has been assessed using assemblages of light-sensitive microendolithic ichnotaxa, resulting in the application of a zonation based on light intensity: shallow euphotic, deep euphotic, and dysphotic–aphotic. This paper assesses differences in sclerobiont communities observed on brachiopod shells (primarily Rafinesquina) among shallow euphotic-, deep euphotic-, and dysphotic-zones from the Upper Ordovician strata of the Cincinnati Arch region. Sclerobiont assemblages can thus successfully define a sclerobiofacies sensitive to interpreted paleobathymetric photic zonation. Ordovician sclerobionts show predictable declines in sample richness, frequency of encrustation of host shells, and areal coverage of host with relative depth and/or photic zonation. Bryozoans predominate in all environments, although notably sheetlike trepostome bryozoans and inarticulate brachiopods were particularly dominant in the shallow euphotic zone, and paleotubuliporid bryozoans, microconchids, and cornulitids were more successful in the deep euphotic zone. If sclerobiont assemblages can be correlated with relative light intensity zonation throughout the Phanerozoic, this work may be an important first step in delineating depth-related sclerobiofacies that could provide an important tool for paleobathymetric reconstruction throughout geologic time.