Diverse earliest Triassic ostracod fauna of the non-microbialite-bearing shallow marine carbonates of the Yangou section, South China

Abstract

Since diverse ostracod faunas in the immediate aftermath of the latest Permian mass extinction are mainly found within Permian–Triassic boundary microbialites (PTBMs), the idea of an ostracod ‘microbial‐related refuge’ has been proposed. Here, we report a diversified earliest Triassic ostracod fauna from the Yangou section in South China, where no PTBMs were deposited, providing evidence inconsistent with this ‘microbial‐related refuge’ hypothesis. In addition, a significant ostracod extinction is recorded, corresponding with the earliest Triassic mass extinction (ETME). This ETME of ostracods is associated with size increases and a length/height ratio (L/H) decrease, indicating varied evolutionary patterns of shape and size of ostracods through the Permian–Triassic (P‐Tr) extinction events. Although the nature of these biotic changes is somewhat unclear, the temporally varied ‘refuge zone’ scenario provides us with a window to reconstruct the environmental dynamics of ecosystem changes during the P‐Tr transition.

Decoupling of morphological disparity and taxonomic diversity during the end-Permian mass extinction

“…Not only that, the filter-feeding ostracods, occupying a larger morphospace in the microbialite facies, preferred oligotrophic conditions (Forel et al 2011; Horne et al 2011), which is inconsistent with the eutrophic conditions during the extinction interval (Schobben et al 2016). Even for the deposit-feeding ostracods, there are no grazing traces, such as Lamonte trevallis (O'Neil et al 2020), recorded in the microbialites (Forel et al 2013b; Qiu et al 2019). Therefore, the view that microbial mats served as a food source for ostracods is still equivocal, and more supporting evidence is required.…”

“…Hautmann et al (2015) observed that the bivalve and gastropod species found in microbialites did not differ from those in non-microbialite facies, and therefore, microbial mats cannot truly be considered as refugia. Likewise, relatively diverse, postextinction metazoan communities have been reported from non-microbialite successions (Twitchett et al 2004; Beatty et al 2008), including ostracod assemblages from South China and Iran (Qiu et al 2019; Gliwa et al 2020a,b). Further, the anoxic–dysoxic signal associated with microbialite successions has been reinterpreted to represent reducing conditions within the sediment rather than in the water column (Heindel et al 2018; Kershaw et al 2018), which remained well mixed and oxygenated by wave-induced currents in shallow-marine settings (Tang et al 2017; Heindel et al 2018).…”

“…Ostracods show a significant turnover due to the end-Permian event, yet a broad range of Permian survivors and newly evolved species dominated a “survival stage” into the earliest Griesbachian and a subsequent “poverty phase” from the Griesbachian Isarcicella staeschei conodont zone into the Dienerian (Crasquin and Forel 2014). Furthermore, the ostracods from the survival stage are typically associated with microbialite facies (Crasquin-Soleau et al 2004; Forel 2012, 2014; Forel et al 2013a,b, 2015; Crasquin and Forel 2014; Wan et al 2019), with only a few exceptions (Foster et al 2019b; Qiu et al 2019; Gliwa et al 2020a, b). The co-occurrence of microbialites and ostracods led Forel et al (2013b) to suggest that microbial mats provided favorable settings, or refuges, during the extinction interval that could support diverse and abundant ostracod communities.…”

A diverse gastropod fauna from the shallow marine carbonate platform of the Yangou section (south China) in the immediate aftermath of the Permian–Triassic mass extinction

“…Although this hypothesis does explain why certain diverse benthic invertebrates during the Griesbachian are associated with microbialites, it has been contested by numerous recent investigations of fossil ostracod and other invertebrates. Results showed that these taxa also occur in sections where no microbialites were deposited, suggesting that benthic life in the end‐Permian ocean was not necessarily restricted to microbial mat ecosystems (e.g., Foster, Lehrmann, Yu, & Martindale, 2019; Qiu et al, 2019). Recent ichnological studies also suggest similar features (Luo et al, 2019).…”

“…Qiu et al (2019) reported a distinct Early Triassic Mass Extinction (ETME) associated with ostracod faunal turnover from a highly diverse and abundant fauna to a unique fauna with extremely low diversity and low abundance in the topmost H. parvus Zone at the Yangou section. Coinciding with this horizon, a faunal turnover of gastropods could also be identified (Figure6), characterized by an abrupt transition from a fauna with low abundance and moderate diversity to a fauna with high abundance and high diversity, showing no evidence of impacts from the ETME.…”