Heterogeneous palaeo-ecogeography of brachiopods during the Late Ordovician mass extinction in South China

Following the first phase of the Late Ordovician Mass Extinction (LOME), the globally distributed Hirnantia brachiopod fauna exhibited homogeneity both latitudinally and longitudinally. The uniform global paleobiogeography of the Hirnantia Fauna, solely based on occurrence data, might obscure its heterogeneous nature during glaciation. In this study, leveraging diversity and abundance data from well-sampled collections in South China, along with NMDS using the Raup-Crick measure, Network Analysis (NA), and abundance models, we unveil the paleo-ecogeography of the Hirnantia Fauna for the first time. The distribution of the Hirnantia Fauna in South China is categorized into three areas: the deep water area, the shallower water area, and the very shallow water area based on NMDS and NA. Drastically different benthic assemblages among these areas reveal diverse environmental settings, with a rough westward shallower trend and a deeper trend extending both northward and southeastward. Analysis of species-abundance models for eight well-sampled collections demonstrates different best-fitted models, suggesting competitive ecology operates in varied contexts despite common BA 3 environmental settings and close geographic proximity. While a substantial global sea level falling and climate cooling around the Katian-Hirnantian boundary plays important roles in the LOME, the paleo-ecogeography of the Hirnantia Fauna in South China is predominantly influenced by the expansion of the Cathaysian oldland and the Qianzhong uplift. The interval between the two LOME phases, marked by kinetic conditions, witnessed heterogeneous thriving of the Hirnantia Fauna.

Methods
There are two datasets: one based on occurrence information and the other on abundance data.For the occurrence dataset of Hirnantian brachiopod fauna from South China, we aggregated adjacent localities into a unified palaeogeographical analysis unit to balance the distribution of the occurrence data. Consequently, our analysis involved a total of 22 distinct units.The abundance data is derived from 9070 specimens collected from 8 representative sections of the Hirnantia Fauna.

We conducted the palaeobiogeographic study using NMDS with PAST (version 4.01) and network analysis with Gephi (version 0.10) for the 22 occurrence data units. Additionally, we used rarefaction analysis with PAST (version 4.01) to test the sampling sufficiency of the 8 representative sections. We also built species-abundance models using R programming with the Vegan package to test the abundance models for these sections.