Rosselia ichnofabrics from the Lower Ordovician of the Alborz Mountains (northern Iran): palaeoecology, palaeobiology and sedimentology

Ordovician matground and mixground ecosystems in shoreface–offshore and barrier-island environments from Central Iran, northern Gondwana

“…The unstable conditions in these substrates forced the tracemakers to adjust their burrows, which resulted in the development of stacked segments in Rosselia. According to Bayet-Goll et al (2021d), in the lower to middle shoreface areas, characterized by thick, amalgamated storm beds because of highly energetic conditions and high sedimentation rate as well as short-lived fair-weather conditions between storm events, only a very small amount of mud was deposited. In such situations, infaunal suspension-feeding animals, capable of readjusting their dwellings after sedimentation-erosion events, were able to survive.…”

“…In the host deposits of CT ichnofabric, the close association of Trichophycus with muddy, organic-rich substrates points to the activity of tracemakers in stable habitats and muddy substrates above the SWB with a high nutrient content, low-flow regimes and well-oxygenated conditions, and hence, to a wider colonization window. According to Bayet-Goll et al (2021d), Trichophycus resulting from resident fair-weather tracemakers is a common trace fossil in the archetypical Cruziana ichnofacies in the Shirgesht Formation, and reflects the activity of populations demonstrating a K-selected or climax strategy. Toward the shallower areas, near the FWWB (lower-shoreface / offshore-transition zone), because of short fair-weather conditions between storm events, the mud layers are thin and limited in lateral extent.…”

“…A Bayet-Goll activity and barrier destruction, the high rate of washover sand accumulation and the creation of unstable depositional substrates in the washover fan prohibited mat-building microorganisms and most tracemakers (e.g. filter-and suspension-feeding benthos) from colonizing the substrate, except for the Rosselia maker, which appears to have endured highly energetic conditions and sedimentation rates (Bayet-Goll et al 2021d). The presence of a highly stressed depositional substrate in washover sands forced the tracemaker of Rosselia to adjust its burrows to the conditions, resulting in the development of stacked segments.…”

“…These adjustments prevented the organism from being buried by washover sands and maintained its connection with the sediment-water interface. Short-term colonization and stressed environments in host facies resulted in the development of CR ichnofabric (Nara, 2002;Netto et al 2014;Bayet-Goll et al 2021d). High sedimentation rate and high turbidity levels in the water column near the sea-floor prevented mat-building microorganisms from affecting the substrate and enabling MISS to develop.…”

“…The occurrence of equilibrium-adjustment behaviours of infaunal terebellid populations (synchronous upward movement by Rosselia producers) related to the CR and RoSk, ichnofabrics has also been considered responsible for the increase in progressive ecospace utilization (Desjardins et al 2010;Bayet-Goll et al 2021d) that took place in the shallow-marine ecosystems of this formation. The importance of feeding structures of detritus-feeders (CR and RoSk ichnofabrics) and deposit-feeders (CT ichnofabric) is that such structures demonstrate complex morphological/behavioural patterns of trace fossils with systematic burrowing and sophisticated feeding behaviours, exploiting the infaunal ecospace.…”