File:Archaeocyathan limestone (Ajax Limestone, Lower Cambrian; Mt. Scott Range, South Australia) 17.jpg

English: Fossiliferous limestone in the Cambrian of South Australia.

The Ajax Limestone is a Lower Cambrian unit in parts of South Australia that has abundant archaeocyathans, formerly a mystery group of sessile, benthic, filter-feeding organisms that made a calcareous skeleton (Phylum Archaeocyatha). These fossils are now known to be an extinct group of sponges (Phylum Porifera). Except for one Upper Cambrian occurrence in Antarctica, archaeocyathans are only known from the Lower Cambrian. The classic localities for these fossils are Siberia and Australia. The Ajax archaeocyathan fauna is quite diverse, with over two dozen species (Taylor, 1910).

Stratigraphy: Ajax Limestone, Atdabanian and Botomian Stages, Lower Cambrian

Locality: ridge-hilltop outcrop south of Aroona Creek, Mt. Scott Range, west of the town of Leigh Creek, South Australia (vicinity of 30° 35.956’ South latitude, 138° 20.426’ East longitude)

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Reference cited:

Taylor, T.G. 1910. The Archaeocyathinae from the Cambrian of South Australia with an account of the morphology and affinities of the whole class. Memoirs of the Royal Society of South Australia 2(2): 55-188, 16 pls.