The 140 m thick Wirrealpa Limestone of South Australia is an open-shelf marine deposit of Early–Middle Cambrian (Botomian or Toyonian) age. Bioherms preserved within this unit have a distinctive Epiphyton thrombolitic stromatolite framestone core colonized by a low-diversity archaeocyathan–radiocyathan biota of Ajacicyathus sp., Archaeocyathus abacus n.sp., and Girphanovella gondwana n.sp., together with other skeletal invertebrates and macroborers. The largest bioherm, some 3 m in thickness and 36 m in length, is enclosed within an interval of thin-bedded to nodular silty bioclast limestone, but rests directly on a substrate of bioclast, intraclast, and onkoid packstone–floatstone–rudstone. The archaeocyathan–radiocyathan biota represents the youngest known Australian occurrence of these groups.

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