Neoichnology of siliciclastic shallow-marine environments: Invertebrates, traces, and environmental conditions

Abstract

Biogenic sedimentary structures, or traces, on the modern seafloor (neoichnology) record organism behaviors in response to physico-chemical conditions; consequently, they, act as analogues to similar structures preserved in the sedimentary record. Neoichnological insights are fundamental to the broad fields of paleobiology, paleontology, and sedimentology. This paper summarizes the current state of knowledge regarding animal-sediment interactions in siliciclastic shallow-marine environments with the aim of helping workers link their observations of biogenic structures to trace-making organisms and to predict the environmental conditions under which those traces were produced. This knowledge base is useful for reconstructing paleo-ecosystems and evolutionary trends through Earth history, as well as for constructing integrated facies models that explain the nature and distribution of sedimentary strata.

Annelids, bivalves, crustaceans, echinoderms, sponges, and sea anemones produce most marine bioturbation. These organisms respond in predictable ways to major physical and chemical stressors in their environment. Such stressors include sediment texture, substrate consistency, sedimentation rate, subaerial exposure, temperature, turbidity, oxygenation, and water salinity. Invertebrates prefer to burrow in sand and firm mud, and generally avoid water saturated mud and coarse-grained sediment. Loosegrounds and softgrounds display the most diverse trace assemblages, compared to stiff, firm, and hard substrates. Low sedimentation rates result in high bioturbation intensity and diverse trace assemblages. Increasing sedimentation rate decreases the intensity of bioturbation. Subaerial exposure shifts faunal populations towards trophic generalists who produce simple structures. Highly turbid water causes deposit-feeding behaviors to predominate. Water salinity controls endobenthos and their burrowing assemblages. Normal marine conditions have diverse ichnofauna, whereas brackish water settings display low diversity but sometimes high-density bioturbation. Hypersaline waters contain low diversity ichnofaunas. Low dissolved oxygen manifests in low abundances, diversities, and densities of trace assemblages. With increasing latitude, burrowing organisms shift to deeper water, burrowing by crustaceans decreases, polychaete-generated structures are more abundant, and the size of burrows increases. Despite the current knowledge base of invertebrate neoichnology in siliciclastic shallow marine environments, the link between physico-chemical environmental stressors and burrowing remain mostly qualitative.

Introduction

In the same way that physical sedimentary structures record hydrodynamic conditions and grain size availability within sedimentary environments, biogenic sedimentary structures are manifestations of the behaviors employed by organisms to cope with the physico-chemical conditions present in their habitats. Since Lyell (1832) set forth his actualistic view of Earth's history based on the notion of vera causa – more widely known as “the present is key to the past” – physical geology has benefitted from observing modern-day processes operating in real time and using these observations as analogues to understand the geological record. However, despite Darwin (1881)’s early recognition that animal behavior impacts sedimentary fabric, actualism in the field of ichnology took another 40 years to become widely adopted (Richter, 1920, Richter, 1921, Richter, 1924a, Richter, 1924b, Richter, 1926a). Today, ichnological concepts are routinely applied to understand depositional environments and conditions from Earth's past, but the foundations of this ichnological toolbox draw heavily on direct comparison between modern trace-making organisms (i.e., neoichnology) and their analogous biogenic structures in the sedimentary rock record (e.g., Dashtgard, 2011a; Ekdale and Berger, 1978; Frey, 1968, Frey, 1970; Gingras et al., 1999; Häntzschel, 1939; La Croix et al., 2019; Nathorst, 1881; Schäfer, 1972).

In the broadest sense, ichnology – the study of tracks, trails, tubes, and burrows, as well as bioturbation, biostratification, biodeposition, and bioerosion – is an interdisciplinary science that bridges biology, paleontology, and sedimentology. Each discipline views animal-sediment interactions differently. Biological studies focus on the relationship between bioturbating organisms and ecosystem functions (Needham et al., 2011; Solan et al., 2004) and emphasize how animals eco-engineer their environments (Meysman et al., 2007); these studies are best described as “organism centric”. Ichnology, as a paleontological tool, provides insights into the evolution of animal behaviors and changes in faunal communities through time (Bottjer and Ausich, 1986; Buatois et al., 2005; Buatois and Mángano, 2012; Buatois et al., 2022; Buatois et al., 2017; Droser and Bottjer, 1989; Jablonski et al., 1983; Minter et al., 2017), and is best described as “structure centric”. Sedimentology seeks to illuminate the relationship between biogenic structures and the physical and chemical conditions that characterize depositional environments (Dashtgard and Gingras, 2012; Gingras et al., 2011; MacEachern et al., 2007, MacEachern et al., 2007; McIlroy, 2008; Taylor et al., 2003) resulting in “context centric” ichnology. Each point of view provides valuable and unique information, but they all build on the foundational principle that bioturbation reflects the behaviors of sediment-dwelling organisms.

Studies of present-day sediments and the trace makers that inhabit them are essential to parameterize environmental conditions and related animal behaviors (Baucon, 2021; Clifton and Thompson, 1978; Dafoe et al., 2008; Dashtgard et al., 2008; Gingras et al., 2008c; Hauck et al., 2008; Howard and Frey, 1975; La Croix et al., 2015; Scott et al., 2019). This is especially important because animal behavior remained consistent through the Phanerozoic (Buatois et al., 2005; Buatois and Mángano, 2011), and therefore, burrow-behavior linkages derived from modern studies should be applicable theoretically across geological timescales.

This paper summarizes the current state of knowledge regarding invertebrate neoichnology of siliciclastic marginal- and shallow-marine environments. We review the main groups of trace-making organisms and range of burrow morphologies they produce, examine how physico-chemical conditions affect trace-making organisms and the functionality of their burrows, and unite possible aspects of neoichnology that are relevant to applications in biology, paleontology, and sedimentology. The end goal is to build a holistic picture of how invertebrate animals live in and move through sediment in siliciclastic marginal- and shallow-marine environments.