The earliest Spongeliomorpha from the upper Silurian of South China: insights into the paleobiology and paleoecology of early marine arthropods

Spongeliomorpha is a common component in Mesozoic and younger strata. However, Paleozoic records of Spongeliomorpha are rare. Here, we report widespread distribution of Spongeliomorpha from the upper Silurian shallow marine siliciclastic succession of South China, which is by far the oldest record of this ichnogenus. The trace fossil displays consistent characteristics along a depositional stretch of more than 900 km. Specimens show Y- or T-shaped bifurcations and bundles of fan-shaped scratches converging to form a plaited pattern, allowing assignment to S. aff. sudolica. Comparative study regarding neoichnological observations and functional morphology analysis indicates that early crustaceans, or other arthropods with rigid appendages were plausible candidates. These burrows most likely functioned as dwelling structures of a detritus feeder that obtained food from the sediment surface. The occurrence of Spongeliomorpha in the late Silurian demonstrates that complex side-burrowing behavior, previously associated primarily with Mesozoic and Cenozoic decapod crustaceans, was already well established in Paleozoic arthropods. This finding highlights behavioral convergence across distantly related arthropod lineages and cautions against the routine assignment of Spongeliomorpha to a single producer such as decapods.