New insights on ichnofacies distribution and behavioral adaptability of Diplocraterion in the Geirud Formation (Upper Devonian, Iran): Response to environmental dynamics and relative sea-level changes

The trace-fossil Diplocraterion , abundant in the Upper Devonian Geirud Formation, was analyzed, revealing significant variations in density, size, spatial distribution, and orientation across different systems tracts and sequence stratigraphic surfaces. The findings highlight the diverse preservational variants of Diplocraterion , including complete U-shaped structures, paired circular openings, elliptical marginal tubes, and partially eroded remnants, negative-relief endichnia, and positive-relief hypichnia, offering valuable insights into the sedimentary dynamics, orientation, and adaptive behaviors of the tracemakers. D. parallelum preservation varies across different facies, reflecting the control by both sea-level fluctuations and environmental factors (substrate characteristics, hydrodynamic conditions, sedimentation rates, and preservation potential). Despite the early evolutionary development of upward and downward adjustments in Diplocraterion since the Cambrian, the preference for simpler, less energy-intensive downward adjustments in younger Devonian samples underscores the selective use of behaviors based on environmental conditions, with upward adjustments employed only when necessary to avoid burial. The behavioral adaptability of Diplocraterion tracemakers encompasses habitat creation, nutrient acquisition, and maintaining balance with environmental conditions. Their ability to construct and modify burrow systems allows them to thrive in varying sedimentary regimes, ensuring survival and stability. Diplocraterion tracemakers predominantly operate in offshore and lower shoreface environments due to fluctuating hydrodynamic conditions favorable for habitat creation. In mid-upper shoreface to foreshore environments, high sedimentation rates and erosion reduce the frequency of these traces, leading to abandonment and shallower burrows. This explains that D. parallelum is typical of the distal Skolithos ichnofacies in lower shoreface and offshore transition environments. However, in shoreface environments with archetypal Skolithos ichnofacies, this trace fossil has shallower penetration depths and smaller size, making it a subordinate element of this ichnofacies. The analysis of D. parallelum burrows in the Geirud Formation reveals that their preferred orientations align with bidirectional and basinward currents, optimizing feeding efficiency. This alignment, consistent with other studies and modern analogs, underscores the ecological strategy of tracemakers to enhance nutrient acquisition by aligning burrows with water currents. The findings highlight the importance of understanding both the physical and biological factors that influence the distribution and morphology of Diplocraterion under different environmental conditions. • Diplocraterion in the Geirud Formation is shaped by various environmental factors. • Diplocraterion adjusts burrows downward to survive erosion and maintain niches. • Burrow orientations align with sustained currents, optimizing feeding efficiency. • Diplocraterion -dominated Glossifungites surfaces are classified into autocyclic and allocyclic surfaces based on their origin. • Patchy burrow distribution due to environmental changes and sediment supply variations. • Small burrows in offshore areas suggest juvenile habitats; larger ones in shoreface areas. • D. parallelum is typical in distal Skolithos ichnofacies and a subordinate element of archetypal Skolithos ichnofacies.