Paleoecology of lower and Middle Pennsylvanian (Middle Carboniferous)Chaetetes in North America

Chaetetes, a probable coralline demosponge, reportedly ranges from the Devonian to the Permian, but is particularly common in North America in limestone strata of the Middle Carboniferous (Pennsylvanian: Morrowan to Desmoinesian Series), occurring both as isolated colonies and as a major element of reef mounds and biostromes. Based on numerous descriptions and interpretations in the literature, the life habit ofChaetetes was from intertidal to below wave base in shelf and epicontinental areas of normal marine carbonate sedimentation. The inferred water energy ranges from “quiet” to “high”. A shallow, but locally protected setting has been proposed as well as moderate to high-energy based on the common occurrence of displaced and rolled colonies that probably reflect episodic storm action. InitialChaetetes colonization required a hard substrate, but evidence of host selectivity is lacking. Occurrences ofChaetetes described here are indicative of a wide range of very shallow subtidal conditions: capping a phylloid algal mound; closely associated with intraclast beds and a paleosol; colonizing oolite banks; and in biostromes that formed immediately below the sea surface. Within the narrow depth range represented by our examples, the optimum environment forChaetetes was in the shallowest water, approaching intertidal depths; and this is the probable bathymetric interpretation forChaetetes where present in abundance. Based on our observations,Chaetetes was rheophylic, favoring at least moderate water energy.Chaetetes is consistently associated with a stenohaline fauna, but euryhalinity cannot be entirely discounted. Circumstantial evidence does not contradict algal symbiosis inChaetetes but remains inconclusive. The morphology ofChaetetes colonies can be related to a number of environmental and biologic parameters: water energy, sedimentation, sea level as an upper limit to growth, and crowding. Because these may all exert influence concurrently, it may be difficult to determine specific aspects of the paleoenvironment solely from the morphology ofChaetetes colonies. Therefore our approach incorporates sedimentological criteria and facies relationships as well as morphologic data to elicit the primary paleoenvironment in whichChaetetes lived.