DO MISSISSIPPIAN RUGOSE CORALS HAVE EVOLUTIONARY STASIS IN CORALLITE MORPHOLOGY LIKE DEVONIAN RUGOSE CORALS?

The Bangor Limestone, Lacon Quarry, northeastern Alabama, has several species of colonial and solitary rugose corals dating to the Mississippian period. In particular, Acrocyathus floriformus (formerly Lithostrotion), a colonial rugosan, and the solitary rugose coral, Zaphrenites spinulosum, are guide fossils to the Mississipian period. Previous researchers showed that Devonian colonial and solitary rugosans had limited septal number and for a colonial species (Acinophyllum), the number of septa was correlated with corallite diameter. Here, we test whether the two Lacon Quarry rugosans have evolutionary stasis in those features that were documented for Devonian corals and add other morphology useful for eco-evolutionary comparisons.

Septal number was counted and corallite septal thickness, surface diameter and area were measured with electronic calipers for 30 corallites from A. floriformus and Z. spinulosum. For the colonial species, the number of corallite side walls was also counted. Results indicate that colonial A. floriformus had higher variation in septal number, but the average number of septa (46) was not significantly different from the solitary coral, Z. spinulosum (40). Both species had low correlation between septal number and diameter (r = 0.25), but for surface area and septal number, the colonial coral was moderately positively correlated (r = 0.64), while the solitary Z. spinulosum was not (r = 0.41). However, Z. spinulosum had significantly thicker septa and a larger corallite surface area than the colonial species. For A. floriformus, corallite side walls ranged from four to six, with five sides the most common; five-sided corallites were located near the middle of the colony but did not occur at the highest point or at colony edges, where corallites with four or six sides occurred. Like the Devonian rugosans, septal number in rugose corals was limited and was not significantly different between colonial and solitary forms, indicating stasis in septal number. Unlike the Devonian corals, septal number did not correlate with diameter, but did vary with surface area for the colonial species. Variation in the number of corallite sides in A. floriformus and the robust septa and a larger surface area in Z. spinulosum are likely environmental adaptations to current swept shallow seas.