Tabulaconus Handfield: microstructure and its implication in the taxonomy of primitive corals

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Abstract
Numerous specimens of Tabulaconus Handfield, 1969, have been collected in carbonate buildups within the Adams Argillite (Early Cambrian, Tatonduk area, Alaska). The wall structure of this form has been investigated, along with contemporaneous archaeocyaths and algae, through the use of polished ultra-thin sections (2–3 μm thick) and scanning electron microscopy. The results of this microstructural comparison indicate that despite diagenetic alteration Tabulaconus has a skeleton that is unlike any presently known and is quite distinct from associated algae or archaeocyaths. It is more elaborate than that found in the archaeocyaths but has not reached the stage of complexity seen in the primitive coral Cothonion Jell and Jell, 1976. The presence of some elongated units may represent an initial step towards the fibrous skeleton typical of Paleozoic corals.This study shows that even though diagenesis alters the original microstructure of calcareous skeletons, the resultant fabrics and detailed structures can be useful in systematic descriptions.Tabulaconus is removed from the Gastroconidae Kordae due to the presence of rudimentary septa and constitution of the tabularium. A number of species assigned to the genus Bačatocyathus Vologdin and included within the Archaeocyatha appear to be examples of Tabulaconus or very close relatives. An emended description of Tabulaconus kordae, the type species, is proposed.