A new marrellomorph arthropod from southern Ontario: A rare case of soft tissue preservation on a late Ordovician open marine shelf

Ordovician open marine lagerstätten are relatively rare and widely dispersed, producing a patchy picture of the diversity and biogeography of non-mineralized marine organisms, and challenging our understanding of the fate of Cambrian groups. Here, for the first time, we report soft-bodied fossils, including a well-preserved marrellomorph arthropod, fragmentary carapaces, and macroalgae, from the late Ordovician (Katian) Upper Member of the Kirkfield Formation near Brechin, Ontario. The unmineralized elements and associated exceptionally preserved shelly biota were entombed rapidly in storm deposits that smothered the shallow, carbonate-dominated shelf. The marrellomorph, Tomlinsonus dimitrii gen. et sp. nov., is remarkable for its ornate, curving cephalic spines and pair of hypertrophied appendages, suggesting a slow-moving, benthic lifestyle. Re-evaluation of marrellomorph phylogeny using new data favours an arachnomorph affinity, though internal relationships are robust to differing o...