From shallow to deep water: an ecological study of the Hirnantia brachiopod Fauna (Late Ordovician) and its global implications

Abstract

A large brachiopod fauna collected through the upper Wanyaoshu Formation (middle Hirnantian), West Yunnan, China displays, for the first time, a vertical transition within the Hirnantia fauna. The inner to mid‐shelf Fardenia–Hirnantia association segues into the deeper water Aegiromena–Anisopleurella association during the Metabolograptus persculptus marine transgression. This transition parallels the pattern apparent in onshore–offshore gradients within the Hirnantian, imitating Walther’s Law for facies, instead predicting the seaward transitions between community types within a single transgressive sequence. In addition, large‐sized brachiopods such as Hirnantia and Eostropheodonta become rarer and small‐sized brachiopods, for example, Aegiromena and Anisopleurella more common. It is a rare opportunity to view transformations in community composition and structure within the Hirnantia fauna together with changes in both the size of individuals and populations (from large to small) during the transgression.

The latest Ordovician Hirnantian brachiopod faunas: New global insights

“…It is largely based on the data from Baltica, Sibumasu and South China, where a depth gradient occurs across the regions from shallow-water to deep-water facies. Along the gradient, the diversity of the fauna evidently changed, even in a single section, e.g., Mangshi, western Yunnan J o u r n a l P r e -p r o o f Journal Pre-proof (Huang et al, 2020c). A very similar picture was suggested by Rasmussen et al (2012) More examples came from Estonia-Latvia (Hints and Harper, 2015) and Algeria (Popov et al, 2019).…”

“…New biota (e.g., the shallow water Edgewood-Cathay Fauna) took over the eco-spaces left empty by the extinction of the Hirnantia-Mucronaspis Fauna. Clearly, these biological events marked the second Hirnantian phase of the end-Ordovician mass extinctions (Brenchley, 1989;Brenchley et al, 2003;Harper and Rong, 1995;Sheehan, 2001;Rong et al, 2002;Benedetto et al, 2013;Rasmussen, 2014;Finnegan et al, 2016;Huang et al, 2020c). A more traditional double pulsed Hirnantian extinction model, rather than a single pulse (for South China in Hallam and Wignall, 1997;Wang et al, 2019) is supported by the new dataset presented herein.…”

“…7). Whereas other siliciclastic facies are marked by the Anisopleurella-Aegiromena Community in an off-shore, deepening upwards Hirnantian succession (from BA3 to BA4) in southwestern Yunnan (Huang et al, 2020c). The two communities are interpreted to occur in the interval of transition from the later to latest stage of the glaciation and the start of global sea-level rise.…”

“…To date, the diversity ranges from as few as a single taxon up to 26 genera in particular associations. A high diversity is documented in South China, Perunica, Sibumasu and Baltica, such as Yichang, Hubei (26 genera, Rong, 1984;Zeng et al, 2016); Bohemia (25, Marek and Havlíček, 1967); Mandalay, Myanmar (23, Rong et al, 2020) and western Yunnan (23, Huang et al, 2020c); Changning and Gongxian (22, this paper), Nanjiang and Xiushan (21, this paper), South China;and Latvia (20, Harper and Hints, 2016), all within low latitudes, and developed in deeper water settings, usually in higher stratigraphic levels. The Keisley brachiopod fauna for northern England has a high diversity, associated with many deeper water species (Temple, 1968).…”

“…Another example in Chu-Ili, a terrane in the tropical zone, where some elements like Anisopleurella, J o u r n a l P r e -p r o o f Journal Pre-proof Rugosowerbyella and Templeella are associated with the common constituents of Hirnantia Fauna, which may indicate a more flexible pattern of communication facilitated by oceanic currents (Huang et al, 2020c).…”