The record of Scalichnus ichnofabrics in ancient fluvial settings and its paleoecological significance

Abstract

Bivalves are a dominant group of benthic invertebrates in freshwater ecosystems playing significant ecological roles in fluvial and marginal lacustrine environments. Despite being active burrowers, the traces left by freshwater bivalve mollusks are less studied and documented than those of their marine counterparts, both in modern and ancient records. In this study, we provide a detailed description of a bivalve ichnofabric primarily characterized by Scalichnus found in fluvial deposits of the Cretaceous Exu Formation, Araripe Basin, northeastern Brazil. Most preserved traces exhibit menisci or irregular concave-upwards laminae at the base of the oval chamber, indicating upward displacement in response to sediment aggradation. Additionally, the presence of stacked Scalichnus in the sandbars suggests that bivalves repositioned themselves upward in the substrate during episodic or seasonal sediment accumulation. In some cases, stacked Scalichnus extend over 60 cm, indicating multiple aggradation events under low deposition rates, which likely allowed the animals to survive these depositional changes. In this context, the preservation of Scalichnus ichnofabrics is associated with variations in sedimentation rates driven by climatic seasonality. These fluctuations likely required the upward repositioning of the bivalves in response to substrate aggradation.