True substrates: The exceptional resolution and unexceptional preservation of deep time snapshots on bedding surfaces

Abstract: Rock outcrops of the sedimentary–stratigraphic record often reveal bedding planes that can be considered to be true substrates: preserved surfaces that demonstrably existed at the sediment–water or sediment–air interface at the time of deposition. These surfaces have high value as repositories of palaeoenvironmental information, revealing fossilized snapshots of microscale topography from deep time. Some true substrates are notable for their sedimentary, palaeontological and ichnological signatures that provide windows into key intervals of Earth history, but countless others occur routinely throughout the sedimentary–stratigraphic record. They frequently reveal patterns that are strikingly familiar from modern sedimentary environments, such as ripple marks, animal trackways, raindrop impressions or mudcracks: all phenomena that are apparently ephemeral in modern settings, and which form on recognizably human timescales. This paper sets out to explain why these short‐term, transient, small‐scale features are counter‐intuitively abundant within a 3.8 billion year‐long sedimentary–stratigraphic record that is known to be inherently time‐incomplete. True substrates are fundamentally related to a state of stasis in ancient sedimentation systems, and distinguishable from other types of bedding surfaces that formed from a dominance of states of deposition or erosion. Stasis is shown to play a key role in both their formation and preservation, rendering them faithful and valuable archives of palaeoenvironmental and temporal information. Further, the intersection between the time–length scale of their formative processes and outcrop expressions can be used to explain why they are so frequently encountered in outcrop investigations. Explaining true substrates as inevitable and unexceptional by‐products of the accrual of the sedimentary–stratigraphic record should shift perspectives on what can be understood about Earth history from field studies of the sedimentary–stratigraphic record. They should be recognized as providing high‐definition information about the mundane day to day operation of ancient environments, and critically assuage the argument that the incomplete sedimentary–stratigraphic record is unrepresentative of the geological past

Unravelling Phanerozoic evolution of radial to rosette trace fossils

Feeding trace fossils, produced by either deposit or detritus feeders and showing radial to rosetted morphology, are all included in the same architectural category. These radial to rosette ichnofossils are widely recorded worldwide throughout the Phanerozoic and have attracted the attention of numerous ichnologists for decades. Construction of a database summarizing occurrences of radial to rosette trace fossils through the Phanerozoic shows that representatives of this category occurred for the first time during the Fortunian, which accounts for the appearance of at least 12% of the total number of ichnogenera in this category. Overall, 32% of all known rosette ichnogenera resulted from the Cambrian Explosion. A second ichnodiversity increase took place (20%) during the Ordovician. Subsequent to the Great Ordovician Biodiversification Event, this architectural category shows minor fluctuations in ichnodiversity resulting in a long-term plateau. The apparent decline in ichnodiversity by the end of the Cenozoic could reflect a taphonomic artefact resulting from the difficulties of identifying cumulative trace fossils in highly bioturbated modern sediments. Our data set indicates that several radial to rosette ichnogenera (e.g. Arenituba, Dactylophycus, Gyrophyllites, Phoebichnus, Volkichnium) occurred first in shallow-marine settings and then migrated to either deeper-water or marginal-marine environments, while others (e.g. Asterichnus, Cladichnus, Dactyloidites) apparently first occurred in deep-sea environments and then migrated to shallower waters.Fil: Muñoz, Diego Fernando. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Mangano, Maria Gabriela. University of Saskatchewan; Canadá. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Buatois, Luis Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. University of Saskatchewan; Canad