New insight on architecture and microstructure of Ellipsactinia and Sphaeractinia (Demosponges) from the Gargano promontory (Southern Italy)

Ellipsactinia and Sphaeractinia are two groups of organism which represent very important stratigraphic markers, especially for the Jurassic and the Cretaceous, becoming the principal constituents of carbonate platform margins of these periods. Furthermore, because they lived exclusively in the Mesozoic Tethys sea, we consider them as good palaeogeographic and palaeoclimatic fossils. They were very abundant in Late Jurassic and Valanginian in the Adriatic and Apulian carbonate platforms. The architecture and microstructure of these two groups are completely different. Ellipsactinia has a simpler architecture than Sphaeractinia, which exhibits an organization of lamellae and radial channels termed "dry-stone wall", and the presence of astrorhizae-like structures. Also the microstructure is different. In Ellipsactinia there is a "water jet" pattern of fibers, whereas in Sphaeractinia the fiber arrangement is spherulitic. In both the genera, rare, isolated and scattered, monaxon spicules are present. The skeletal organization and the types of microstructures allow us to ascribe these two genera to Demospongiae.