Diversity, Stratigraphic Distribution and Paleobiogeography of the Echinocaridina (Phyllocarida, Crustacea) in the Middle Palaeozoic

The suborder Echinocaridina represents the most diverse and widely distributed group of Devonian phyllocarids. Their evolutionary history is crucial for understanding the macroevolution of Paleozoic crustaceans and Devonian biotic-environmental events. This study systematically compiled stratigraphic and geographic distribution data for echinocaridinans, integrating 86 species from 29 genera and 105 morphological characters to conduct a phylogenetic analysis. Combined with marine environmental data and hierarchical clustering analysis, we reconstructed the diversity evolutionary sequence, paleogeographic dispersal pathways, and environmental response patterns of this group. The results indicate that the diversity peaks of echinocaridinans were closely associated with low sea-level and warm climates, whereas their extinction events were tightly coupled with rapid sea-level rise and anoxic events. Echinocaridinans first appeared in the Baltica–Avalonia region during the Wenlock, and subsequently dispersed to the Rhenish–Bohemian region in the Pridoli. Aristozoids underwent two independent radiations during the Early Devonian, corresponding to deep-water shelf environments and shallow-water reef environments, respectively. The Middle Devonian marine transgressions and anoxic events led to the decline of aristozoids and ptychocaridids, with echinocaridids replacing them as the dominant group. Paleogeographic analysis reveals that Middle Devonian echinocaridinans dispersed westward to the Laurentia and eastward to the northeastern margin of Gondwana, initially establishing an initial trans-plate distribution pattern. The global dispersal of echinocarididines during the Late Devonian was driven by dual factors: the persistent transgression during the Frasnian provided corridors and time windows for trans-oceanic dispersal of groups with planktonic larvae; the sea-level fall during the Famennian caused the expansion of epicontinental seas, creating extensive new shallow-marine habitats that facilitated high-diversity radiations in local regions. The Hangenberg Event at the Devonian–Carboniferous transition significantly reduced echinocaridinan diversity, with only a single echinocaridinan species persisting into the early Tournaisian. These findings provide new biostratigraphic and paleobiogeographic evidence for utilizing phyllocarids in high-resolution stratigraphic correlation, paleogeographic reconstruction, and paleoenvironmental analysis.