The organisms had shells, and were subject to predation by boring and other means from the Ordovician onwards. Many survived attacks by predators. Several cornulitids were endobiotic symbionts in the stromatoporoids and tabulates.
Their affinity is unknown; they have been placed in many phyla, and have been considered worms, corals, molluscs and more. They appear to be closely related to other taxa of uncertain affinity, including the microconchids, trypanoporids and tentaculitids.