Bioturbating activities have played a vital role in shaping the marine ecosystem throughout metazoan history, influencing the abundance and preservation potential of body fossil-producing taxa and driving major environmental and geochemical changes. The earliest trace making behaviors arose during the late Ediacaran Period (∼ 560–541 Ma), disrupting the substrate previously occupied by dominantly sessile organisms. Simple dwelling and grazing behaviors exploited the organic-rich matgrounds, expanding into the underutilized microbial mat ecosystem. In the western United States, trace assemblages from Ediacaran–Cambrian boundary-spanning deposits document a thriving trace-maker ecosystem. One boundary-spanning deposit in this region, the lower member of the Wood Canyon Formation, crops out along the California-Nevada boundary and contains both trace and body fossil assemblages. The Chicago Pass section of the lower Wood Canyon Formation contains a suite of dominantly simple Ediacaran traces, which become commonplace in the upper part of the stratigraphic section, documenting the onset of prevalent trace-making behaviors in this region. While traces have been previously described from this locality, the addition of the complex trace Lamonte trevallis and quantification of trace fossil density of simple Ediacaran traces provides a more comprehensive ichnological view of the Chicago Pass section. Although Chicago Pass does not yield abundant tubicolous body fossils, as are found elsewhere in the region, the low diversity ichnoassemblages document both burgeoning surficial trace making groups and mat-targeted mining in the latest Ediacaran. The behaviors present at Chicago Pass are similar to those of the Dengying Formation in South China, and highlight the need for petrographic-based trace fossil studies. Additionally, studies of Nama Group trace fossils of the same age from Namibia report higher diversity and complexity in trace-making activities than what has been observed at Chicago Pass, but with similar, low Ediacara biota body fossil diversity. If Ediacara biota diversity is anticorrelated with trace-making behaviors, Chicago Pass represents a low-complexity end-member of the same phenomenon observed in Namibia. The effect of surface sediment disruption on the sessile Ediacaran communities may have been decoupled from complexity of the traces, more so influenced by the presence of general trace-making behaviors in aggregate, including simple traces.

Bioturbating organisms can dramatically alter the physical, chemical, and hydrological properties of the sediment and promote or hinder microbial growth. They are a classic example of “ecosystem engineers” as they alter the availability of resources to other species. Multiple evolutionary hypotheses evoke bioturbation as a possible driver for historical ecological change. To test these hypotheses, researchers need reliable and reproducible methods for estimating the impact of bioturbation in ancient environments. Early efforts to record and compare this impact through geologic time focused on the degree of bioturbation (e.g., bioturbation indices), the depth of bioturbation (e.g., bioturbation depth), or the structure of the infaunal community (e.g., tiering, ecospace utilization). Models which combine several parameters (e.g., functional groups, tier, motility, sediment interaction style) have been proposed and applied across the geological timescale in recent years. Here, we review all models that characterize the impact of bioturbators on the sedimentary environment (i.e., ‘ecosystem engineering’), in both modern and fossil sediments, and propose several questions. What are the assumptions of each approach? Are the current models appropriate for the metrics they wish to measure? Are they robust and reproducible? Our review highlights the nature of the sedimentary environment as an important parameter when characterizing ecosystem engineering intensity and outlines considerations for a best-practice model to measure the impact of bioturbation in geological datasets.

Throughout the history of life on Earth, sedimentary environments have placed controls on the trajectory of evolutionary innovations. To survive and thrive in newly colonized sedimentary environments, organisms have needed to develop novel behaviors: often evidenced in the rock record as architectural innovation and diversification in trace fossil morphology. This study focuses on ichnological diversification as a response to challenges presented by different sediment grain sizes during the late Silurian to Early Devonian colonization of the continents by invertebrate life. The ichnodiversity and ichnodisparity from this interval reveal details of the biological response to newly adopted sedimentary and environmental conditions.

Characteristics of ichnofaunas from terrestrial and emergent settings are compared across the Silurian-Devonian boundary, within both sand and mud dominated successions, to identify differences associated with different substrate compositions. Two trends are revealed: 1) Successions dominated by mudrock contain a lower ichnodiversity than sandstone-dominated successions of similar age, potentially due to the different challenges associated with burrowing in cohesive versus non-cohesive media. Alternatively, this could be due to preference of the tracemakers for the broader environmental conditions that lead to sand or mud deposition. 2) The maximum size of trace fossils within a given formation is larger in sandstone dominated strata than in mudrock dominated strata. Together, these suggest that the availability of substrates with different grain sizes was one factor determining the constitution of early animal communities and behavioral styles during the colonization of the continents.

Body-size distributions of organisms across environments in space and time are a powerful source of information on ecological and evolutionary processes. However, most studies only focus on selected parameters of size distributions (e.g., central tendency or extremes) and rarely take into account entire distributions and how they are affected by the collection style and facies. Here we analyze the impact of facies, region, taxonomy, and collection style over size distributions using diameter as a proxy of Late Devonian ammonoids in their entirety using non-metric multidimensional scaling and PERMANOVA based on Kolmogorov distance. The effects are then compared with effects on mean sizes. In all analyses, lithology was the dominant effect, with sizes greater by 59% in marls and by 33% in limestones, as compared to black shales. The effect of complete sampling style was a decrease in size by 11%. Kurtosis was an important parameter differentiating size distributions, with platykurtic distributions in marls and leptokurtic distributions in limestones, suggesting that this parameter may reflect different degrees of time averaging. Most size distributions were positively skewed, but most strongly in marls. Complete sampling led to skewness values close to zero (symmetrical distributions) and high kurtosis.

Samples from higher paleolatitudes were on average smaller, but contained outliers with the largest sizes, highlighting the need to analyze entire distributions. Lithology and collection differences need to be accounted for when evaluating size differences across space (polar gigantism) and time (Lilliput effect). Similarly, differences in facies may affect species determination.