Colony morphologies and missed opportunities during the Cincinnatian (Late Ordovician) bryozoan radiation: examples from Heterotrypa frondosa and Monticulipora mammulata

Renewed interest in Paleozoic Bryozoa is focusing upon zoarial architecture and colony morphologies. A reassembled zoarium of Heterotrypa frondosa (d’Orbigny) measuring 13 cm high and 26 cm wide has provided basis for study of functional morphology, habitat tolerances and growth strategies. Morphologies of H. frondosa and sympatric Monticulipora mammulata d’Orbigny in the same Upper Ordovician (Cincinnatian: Maysvillian) strata of the lower Bellevue Formation provide evidence that fair-weather wave resistance was achieved by multiple trepostomate species during this depositional sequence. A growth sequence from the reassembled H. frondosa includes erection of a triad of vertical coalescing blades forming archways and windows, then coalescence of adjoining blades to produce chambers which extended into fronds as the cycle repeated. Such heavily buttressed, wave-resistant colonies were numerous for a brief time interval in shallow waters above storm wave base. Their presence suggests that creation of biogenic structures analogous to modern cnidarian reefs lay briefly within the ecological “grasp” of trepostomes. Only a self-replicating community structure, or serai succession, leading to deposition of an interlocking bioclastic framework failed to evolve. That this opportunity was not seized appears, regionally, to have been the result of sea level or tectonic instability’, shelf-ramp morphology, climatic instability, sediment influx or a combination of these extrinsic factors. Thus, environmental instability, not biological inability, prohibited evolution of a stable seral succession of a local fringing-reef type. The terminal Ordovician extinction ended this “experiment” before its global application.