Insights into the morphology of Sphenothallus (Cnidaria): New features identified in an old genus, from the Carboniferous (Pennsylvanian, Bashkirian) of western Ireland

Abstract: The recent discovery of well-preserved fragments of Sphenothallus in the Central Clare Group (Pennsylvanian) of western Ireland provides new insights into the morphology of this enigmatic, putative cnidarian. The specimens demonstrate a morphological plasticity, including features not previously described for Sphenothallus , such as the presence of flat sides and angular lateral margins, non-bilateral symmetry in sigmoidal cross-sections suggestive of growth torsion, and a new type of bifurcating branching (new term 'rail-track junction branching' introduced herein), not related to known budding. We propose that these structures were all growth-related features during life, and not post-mortem artefacts of collapse or compaction. The flat sides and growth torsion are compared with similar features in conulariids, supporting the previously proposed phylogenetic linkage between the two groups, which may be an example of atavism. However, the bifurcating 'railtrack junction branching' is unknown in conulariids or other similar organisms.