Trilobite biofacies reveal a ‘dual-slope’ paleogeographic pattern of South China during the Early–Middle Ordovician

During the Ordovician, the South China developed a platform–slope–basin paleogeographic framework from northwest to southeast: a chain of oldlands along its western margin, followed southeastward by the Yangtze Platform, the Jiangnan Slope, and the Zhujiang Basin. Previous studies focused mainly on the interior of the Yangtze Platform, whereas the deep-water facies of the South China, especially its northern margin, remain relatively poorly studied because of the limited exposure of Ordovician strata and insufficient fossil material. Notably, Early–Middle Ordovician trilobites in South China are abundant and markedly differentiated; in particular, the successive discoveries of deep-water trilobite faunas in recent years provide a new perspective for understanding the paleogeographic framework of the South China during this interval.

This study selected 62 valid trilobite genera and subgenera from 15 localities in the South China spanning the Early–Middle Ordovician (Floian–Dapingian); the material includes previously published data and specimens collected by the author. The data cover bathymetric gradients from the shallow-water platform to the deep-water slope, as well as substrate environments ranging from coarse siliciclastic facies through mixed siliciclastic–carbonate facies to fine siliciclastic facies. Cluster Analysis (CA), Principal Coordinate Analysis (PCoA), and Nonmetric Multidimensional Scaling (NMDS) were used to analyze the trilobite faunas, and seven trilobite biofacies were recognized: the Liomegalaspides, Neseuretus, Psilocephalina, and Taihungshania biofacies of the inner shallow-water platform; the Nileid Biofacies of the outer platform and platform margin; and the Cyclopygid and Olenid biofacies of the deep-water slope.

The Early–Middle Ordovician trilobite faunas of the South China exhibit pronounced ecological differentiation along bathymetric and substrate gradients. The deep-water Olenid and Cyclopygid biofacies support the existence of the Jiangbei Slope (northern slope) and the Jiangnan Slope (southern slope) of the South China, respectively, indicating a “dual-slope” paleogeographic pattern during this interval, that is, both the northern and southern flanks of the Yangtze Platform were bounded by slope environments. These results provide key paleontological evidence for the Early–Middle Ordovician paleogeographic reconstruction of the South China and demonstrate that deep-water trilobite biofacies are valuable for identifying paleoplate boundaries and slope settings.

Key findings include:

(1) Multivariate analyses of trilobites define seven biofacies from the platform to slope environments in South China;

(2) Cyclopygid and Olenid biofacies confirm a ‘dual-slope’ (Jiangnan and Jiangbei) paleogeographic pattern for South China.

Wei X. Trilobite biofacies reveal a ‘dual-slope’ paleogeographic pattern of South China during the Early–Middle Ordovician. Cont. Life Evol. 2026(3):0014, https://doi.org/10.55092/cle20260014.