One of the limitations of paleoenvironmental reconstructions based on multiple bioindicators is a lack of knowledge on the differential preservation of hard parts, which may lead to biases in interpretation. This is particularly important when biological proxies differ in their intrinsic properties, such as skeletal mineralogy or size. We explored and compared the preservational patterns of siliceous (diatoms) and carbonate (mollusks) fossils during the late Holocene (ca. 4000 cal. years B.P.) in two lacustrine sedimentary successions from Argentina (Nahuel Rucá and Hinojales–San Leoncio). Fragmentation and fine-scale surface-alteration indices were calculated on two target species: the diatom Cyclotella meneghiniana Kützing and the snail Heleobia parchappii (d'Orbigny, 1835). The taphonomic data were smoothed along depth with a locally weighted regression (LOESS) and statistically compared using Spearman correlations. Additionally, past environmental conditions were inferred from the autoecology of the dominant taxa. Diatoms and mollusks displayed similar tendencies in fragmentation, characterized by a gradual decrease of breakage toward the top of the successions. On the other hand, trends in surface preservation were opposite. Diatoms exhibited higher alteration in the oldest sedimentary levels, characterized as saline and less-productive water bodies, while mollusks were more altered at the topmost levels, characterized as freshwater highly productive lakes. This contrasting response of both indicators can be interpreted as a consequence of the differential reaction of carbonate and silica to dissolution agents acting in fresh and saline water. Hence, the accuracy of the paleoenvironmental information provided by both indicators under these contrasting conditions probably would be affected by the taphonomic biases suffered, which highlight the relevance of including taphonomic traits in Quaternary paleoenvironmental or paleoclimatic studies.

A meandering fluvial channel body at Coal Mine Point in the Joggins Fossil Cliffs, Nova Scotia contains an unusual fossil assemblage. During an early stage of channel abandonment, a wrinkled surface attributed to microbial mats was traversed by large arthropleurids (Diplichnites cuithensis trackways). Closely associated are smaller Diplichnites gouldi trackways, probably made by myriapods, as well as tetrapod tracks (Pseudobradypus?, Dromillopus, Hylopus) and invertebrate traces (Cochlichnus, Gordia), collectively representing the Scoyenia ichnofacies. The mats stabilized the sediment surface, allowing excellent trackway preservation, and may have formed a food source, although no feeding traces were identified. Overlying strata yield Protichnites followed by a succession containing paired mud drapes and an impoverished Skolithos ichnofacies encompassing Skolithos, Arenicolites, Cochlichnus, and possible Rhizocorallium, collectively suggesting brackish influence. The channel deposits contain some logs and large plant axes, and were colonized in late stages by lycopsid and calamitalean trees. The assemblage indicates that Early Pennsylvanian channels on a vegetated coastal plain near the tidal limit had a diverse and interconnected aquatic and riparian ecosystem, with tetrapods and terrestrial arthropods entering the channel. Microbial mats may have been common components of Pennsylvanian channels, much as they are in modern fluvial and tidal channels.

The Frasnian–Famennian boundary is correlated with one of several Late Devonian extinction pulses that resulted in a significant decrease in diversity as well as ecological restructuring. This event is recognized within the globally correlated Upper Kellwasser interval that is well exposed and biostratigraphically well constrained in shale units of western New York State. The ichnological and geochemical signals of the interval stratigraphically below the Upper Kellwasser event at these localities provides insight into the onset of this important extinction event. Detailed analysis of ichnogeneric composition, relative size of burrow populations, amount of bioturbation, and trace metal concentrations vary in concert. Deep-penetrating, pyritized Skolithos burrows terminate abruptly at a thin, laminated black shale interval with enriched Mo levels, up to 31 ppm, and are overlain by an interval of gray-green bioturbated shales dominated by Chondrites. These textural and chemical shifts reveal that bottom-water oxygen levels decrease rapidly below the base of the Upper Kellwasser interval. Relative oxygen levels are interpreted to remain low through the Chondrites-dominated interval, with protracted stressed conditions followed by a gradual decrease to anoxic conditions within the Upper Kellwasser interval. These results suggest that, at least locally in the Appalachian Basin, bottom-water oxygen stress and/or fluctuating oxygen conditions were present leading up to the extinction event. This evidence does not support an instantaneous onset of anoxia as causal mechanism for extinction.

Paleoecological consequences of the global Triassic–Jurassic mass extinction (201.3 Ma) are poorly understood. Fossiliferous marine boundary records are rare, commonly condensed, and typically reveal facies changes previously attributed to eustacy. Sedimentology and biofacies analyses from stratigraphically expanded successions of the lowest Jurassic strata, New York Canyon, Nevada, were investigated with high-resolution paleoenvironmental observations, fossil surveys, and microfacies analysis. Following the collapse of the uppermost Triassic carbonate ramp, the lowest Jurassic Ferguson Hill Member of the Sunrise Formation records a midshelf habitat dominated by previously unrecognized siliceous sponges for approximately two million years. In addition, the earliest Jurassic strata from the Pucara Group, central Peruvian Andes, were examined and record a more greatly expanded stratigraphic succession of facies across the inner to middle shelf. Like Nevada, the lowest Jurassic Aramachay Formation is replete with intense concentrations of siliceous sponges. The revelation of widespread, ecologically dominant siliceous sponges has been overlooked despite detailed biofacies studies in both depositional systems. Sponges expanded across shallow environments with sparse benthic biocalcifier populations, and were likely aided by increased ocean silica concentrations from the weathering of the Central Atlantic Magmatic Province. Facies changes previously attributed to sea-level change are thus interpreted to result from the collapse of the carbonate factory concomitant with the mass extinction, with transition to an alternate state dominated by siliceous sponges before a return to carbonate platform development in the Sinemurian. Our study highlights the need to separate biofacies from paleoenvironmental analysis during mass extinction times when nonactualistic assemblages may dominate and deviate from expected environments (e.g., siliceous sponges as indicators of deep paleoenvironments).