Chaetetids

The chaetetids compose a small group of organisms that was most often presumed to be among the anthozoan corals, more specifically allied to the Tabulata. However, with the discovery of a living representative, it was learned that chaetetids are actually sponges. Previously, chaetetids were known from the Ordovician to the Miocene. The overall body form varied from flat encrusting to upright columns. Some achieved a size of over 3 meters. Their fossil record suggests that they were mostly warm shallow water organisms often associated with photosynthetic algae. They apparently required hard substrates for settlement and the onset of growth. They contributed to the development of reefs during the Paleozoic and Mesozoic, both overgrowing and serving as substrate for other reef-associated organisms.

The general chaetetid growth form can be described as a cluster of closely packed tubes with floors or tabulae. The rigid skeleton is calcareous. The living chaetetid, Acanthochaetetes wellsi possesses siliceous spicules and soft parts that clearly place it within the demosponges. Spicules are rarely preserved in fossil chaetetids. It is unknown if chaetetids form a monophyletic grouping, although the hypothesis of monophyly has not been falsified at this point in time.