Cyanophycean nature of stromatoporoids

Abstract

STROMATOPOROIDS are calcareous fossils abundant in many Palaeozoic (mainly Silurian and Devonian) shallow-water carbonate sediments. Forms ascribed to stromatoporoids have also been reported from Mesozoic epicontinental and tethyan carbonate deposits¹. The nodular, tabular or cylindrical and, generally, vertically laminated calcitic skeletons of stromatoporoids (Fig. 1a), ranging in size from a few mm to > 1.0 m, are internally formed of curved plates, short or long pillars and vertical walls. A characteristic feature of many stromatoporoids are stellate structures, a few mm to 6–7 cm across, termed astrorhizae. In spite of 150 yr of investigation the systematic position of stromatoporoids is still controversial. They have been variously interpreted as foraminifers, sponges and hydrozoans^2,3 with discussion on the latter two groups bearing particularly on the interpretation of the skeletal morphogenesis^4,5. New discoveries⁶ of peculiar modern sponges with mixed calcareous and spicular siliceous skeletons (Sclerospongiae) reopened discussion on the relationship of this group to stromatoporoids, initiated at the beginning of the century by Kirkpatrick^7,8. Hartman and Goreau's suggestion⁹ has been taken up and supported by Steam^5,10 and Wendt¹¹. Stearn argued that stromatoporoids should be recognised as a separate subphylum of the Porifera. Wendt similarly regarded stromatoporoids as a conservative order of the Calcispongiae closely related to the Pharetronida. Spicules, however, are quite unknown in stromatoporoids and the discovery of fossil (Cretaceous) sclerosponges with preserved spicules^12,13 shows that their absence cannot arise from diagenetic dissolution as Hartman and Goreau suggested. Interpretation of the astro-rhizae is critical to any discussion of stromatoporoid affinities. As poriferans, the astrorhizae become traces left by the sponge exhalent canals, or as hydrozoans they are structures homologous to coenosarcal stolons (hydrorhizae). Kazmierczak¹⁴ rejected both interpretations and argued that astrorhizae are traces of foreign organisms probably commensal or mutualistic with stromatoporoids. The new observations reported here modify but confirm this.