Possible pelagic lifestyle of the Silurian trilobite Deiphon evaluated through three-dimensional modeling

Abstract Many euarthropods have evolved pelagic lifestyles, but evaluating this in extinct taxa relies on documenting a broad distribution in diverse lithologies, occurrence across multiple paleobiogeographic regions, functional morphology, and morphological comparisons to modern analogues. Trilobites living in the water column have been proposed to take two forms, well streamlined and poorly streamlined, reflecting different lifestyles and behaviors. Both forms went extinct at the end of the Ordovician. A pelagic lifestyle was suggested for the Silurian trilobite Deiphon based on its highly inflated, spherical glabella and reduced body, but this was not supported by recent workers. The sedimentological and geographic information for this animal is poor, making morphology the only proxy available to test its life mode. We use a 3D model of Deiphon to estimate volume and density of different tissue types (exoskeleton, gut, body, limbs) to calculate its specific gravity. To investigate the impact of the bubble and its contents, we tested the impact on buoyancy by varying presence of the inflated glabella, content of the bubble, and exoskeletal thickness. Exoskeletal thickness has a large impact on the buoyancy of Deiphon , suggesting pelagic trilobites may have required thin exoskeleton to remain buoyant in the water. Our results support a pelagic lifestyle for Deiphon , if its bubble was filled with lipids, which we consider the most likely of the possibilities when the rest of the animal’s morphology is considered. If a lipid-filled bubble is common for deiphonines as a whole, this group of inferred armored hoverers would have become pelagic as mesozooplankton in the Middle Ordovician and survived the Late Ordovician Mass Extinction (LOME), with the Deiphon lineage becoming the first macrozooplankton representative in the Silurian.