MORPHOLOGY AND DIAGENESIS OF DIMORPHOSIPHON TALBOTORUM N. SP., AN ORDOVICIAN SKELETON-BUILDING ALGA (CHLOROPHYTA: DIMORPHOSIPHONACEAE)

Abstract

Dimorphosiphon talbotorum n. sp. from the Upper Ordovician Bighorn Dolomite of Wyoming is an ancient representative of the erect skeleton-building green algae. Circumstantial evidence indicates that the original skeletal material was aragonite. If so, D. talbotorum and its Ordovician relatives in eastern North America, Europe, and Kazakhstan are reminders that even morphologically simple aragonite producers could thrive during one of Earth's “calcite sea” intervals.Unlike previously described Ordovician members of its family, the new species is represented by selectively silicified thalli showing three-dimensional details of internal tubes and a highly variable external form. It differs from a similar taxon, D. rectangulare, in having branched as well as unbranched thalli and in morphology of radial tubes. The new species is the first thoroughly documented record of Dimorphosiphon in western North America. Its stratigraphic position in the Richmondian part of the Bighorn Dolomite correlates with previously reported occurrences of the genus in the Red River Formation of North Dakota and Manitoba.The Wyoming fossils typically occur as scattered components of wackestone and include both silicified and calcitic individuals. The change in skeletal composition from aragonite to calcite apparently took place before tubes decayed, thus preserving tube morphology. Subsequent silicification varied in degree among specimens, probably reflecting differences in permeability of the secondarily calcitic skeletons.