Summary
Large cyanoliths reaching up to 12 cm in size and mainly prolate, discoidal and spheroidal in shape occur in early Serpukhovian rocks from the San Antonio-La Juliana Unit (Guadiato Area, SW Spain). The main component in the cyanoliths is Girvanella, but other important components are the problematic-algae Sparaphralysia, Calcifolium and crustose Fasciella. A diverse assemblage of invertebrates, algae and aoujgaliids also occur in the cyanoliths. The internal structure of cyanoliths shows discontinuous cortices with tabular and colummar growth of Girvanella domes, and bafflestones of Calcifolium. These cyanoliths were constructed in a quiet water, low-energy dysphotic environment, in relatively deep water facies, and on the upper part of the slope or deep outer shelf. The probably dominant factors necessary for their growth were bioturbation and gravity. The shape, size, architecture, depositional setting, multibiotic composition and lumpy structure of these cyanoliths are similar to those of Cenozoic-Recent rhodoliths. Similarities to Osagia-type oncoids are few, and mostly related to the cyanobacterial composition. Only some Mississippian cyanoliths from Britain (digitate growth of cyanobacteria) and NW Spain and Poland (multibiotic composition) show notable similarities with cyanoliths from San Antonio-La Juliana. The San Antonio cyanoliths can be readly distinguished from coeval shallow-water oncoids in the Guadiato Area.