Nearshore warm-water biota development in the aftermath of the Late Ordovician Mass Extinction in South China

Abstract

The Late Ordovician Mass Extinction (LOME) occurred between two significant biotic diversifications: the Great Ordovician Biodiversification Event and the Silurian–Devonian Nekton Revolution. Upper Hirnantian carbonate deposits therefore are critical archives for understanding warm-water biotas, biodiversity resurgence and trophic chain development in the immediate aftermath of the LOME. In this study, we use microfacies analysis to investigate composition and biodiversity of the upper Hirnantian carbonates developed in northeastern Guizhou Province of China. These carbonates lay along the northern margins of the Dianqiangui landmass and are composed of intraclastic pack- to grainstone, mud- to wackestone, and bioclastic wacke- to packstone. Depositional age of the carbonates is estimated to be late Hirnantian, chiefly indicated by their coincidence with the Hirnantian Carbon Isotope Excursion and the brachiopods Cathaysiorthis and Eospirigerina. The development of carbonates containing diverse warm-water fossils at this transitional time reflects glacio-eustatic changes in the aftermath of the maximum Hirnantian glaciation and LOME, and the consequence of regional Kwangsian Orogeny in South China. Supposed autotrophic organisms in the carbonates are represented by calcimicrobes Allonema and Girvanella, and calcareous algae Amsassia, Dasyporella and Dimorphosiphon. Heterotrophic organisms are also abundant, including bryozoans (Atactoporella, Dekayia, Eridotrypa, Hallopora, Moorephylloporina and Ptilodictya?), trilobites (Dicranopeltis, Eoleonaspis, Mucronaspis (Songxites), Niuchangella and an aulacopleurid-like trilobite), rugose corals (Axiphoria, Eurogrewingkia, Meitanolasma and Grewingkia), tabulate corals (auloporid coral, Catenipora, Halysites, Paleofavosites and Propora), stromatoporoids (Camptodictyon, Ecclimadictyon and Cystostroma), conodonts (Ozarkodina and Walliserodus), benthic graptolites (Desmograptus), gastropods (at least two genera), and ostracods. Such a high biodiversity is evidence of a complex trophic web after the extinction pulses in South China. The biotic development of the warm-water benthic community might have been triggered by a short-lived localized favourable environment that facilitated the survival of warm-water biota during the Hirnantian, which is postulated to have pioneered further Silurian reef recovery in South China.