CrowdedTrichophycusichnofabrics in the early Ordovician successions of central Iran: insight into the Ordovician radiation

Abstract

The Tremadocian‐to‐Floian siliciclastic deposits of the Shirgesht Formation in the Kalmard Block of central Iran show abundant occurrences of the mid‐tier Trichophycus venosus, a common ichnotaxon in the archetypical Cruziana ichnofacies. This trace fossil records a considerable increase in exploitation of offshore infaunal ecospace in comparison with older formations. Here, Trichophycus is relatively long and wide, with numerous and deep successively stacked causative burrows, which form dense burrow systems building the Crowded Trichophycus ichnofabric (CTI). Such development of CTI and the increase in depth of bioturbation had a negative effect on preservation of shallow‐tier traces, for example arthropod burrows and trackways. This resulted in decreasing abundance of trilobite‐produced trace fossils towards the top of the succession. The palaeobiological and palaeoecological interpretation of CTI points to stable habitats in muddy substrates above the storm wave base with high content of food and oxygen in pore waters, low‐energy hydrodynamic regimes and a long colonization window. Moreover, the architecture and the morphological features of Trichophycus suggest a combined dwelling‐feeding activity of the resident fair‐weather producer showing the K‐selected/climax strategy. The ethology of the producer, palaeoecological interpretations and taphonomic signatures revealed that it preferred dewatered, compacted muddy substrates (firmgrounds) in offshore settings, which might be related to minor hiatuses or short‐lived discontinuities. Although the appearance of Trichophycus is concomitant to the earliest Cambrian agronomic revolution, it was uncommon until the early Ordovician, when locally it records infaunalization related to the Great Ordovician Biodiversification Event.

The interplay of environmental constraints and bioturbation on matground development along the marine depositional profile during the Ordovician Radiation

“…The Lashkerak and Ghelli Formations provide an integrated ichnologic, sedimentologic, and geobiologic record of the Ordovician Radiation, which represents a changeover point in the history of marine ecosystems (Mángano et al, 2016; Mángano & Droser, 2004; Sepkoski, 1995; Servais & Harper, 2018; Servais et al, 2010; Stigall et al, 2019). The increase in ichnodiversity and degree and depth of bioturbation that took place in marine environments during the Ordovician Radiation had major geobiologic impacts, including most notably the further disruption of microbially stabilized surfaces (Bayet‐Goll, Knaust, et al, 2021; Bayet‐Goll, Uchman, et al, 2021; Buatois & Mángano, 2012; Buatois et al, 2016; Buatois et al, 2020; Mángano et al, 2016; Mángano & Droser, 2004). In particular, bioturbation by large, relatively deep, highly mobile deposit feeders, including the so‐called sediment bulldozers, was particularly effective in sediment mixing, being detrimental to the formation of matgrounds.…”

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

“…A comprehensive litho-ichnofacies analysis of the Shirgesht Formation in the Kalmard Block of Central Iran was been recently carried out by Bayet-Goll et al (2016) and Bayet-Goll et al (2021c). These studies show that the Shirgesht Formation formed a mixed carbonate-siliciclastic depositional system that consisted of a carbonate ramp (member 3), barrier-island (member 1) and shoreface-offshore (member 2) complexes (Fig.…”

“…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. Despite its Early Cambrian appearance, Trichophycus remained uncommon until the Early Ordovician, and when compared to other burrowers, Trichophycus tracemakers may have only been local contributors to the GOBE infaunalization (Bayet-Goll et al 2021c). However, due to the Ordovician radiation, the producer of Trichophycus exerted an important role in the Shirgesht marine environment by increasing the complexity of tiering and the depth of bioturbation.…”