EARLY PALEOZOIC SPONGE-BEARING INTRASKELETAL CAVITIES: IMPLICATION FOR METAZOAN INVASION INTO CRYPTS

Marine cavities harboring cryptic organisms were ubiquitous throughout the Phanerozoic. However, our knowledge of early cryptic communities are yet insufficient and how and when metazoans began to utilize such habitats remain unsolved. In this report we document demosponge remains within intraskeletal cavities imbedded in micritic carbonates from Late Ordovician succession of southeast China, where numerous demosponge remains have been recently found from surrounding sediments. Mollusks and corals commonly contain patches of “spicular” demosponge remains within their intraskeletal spaces, mostly ranging 1.6–25.6 mm² in area. These cryptic sponges occur in 17% of mollusk shells (n=63/363), including 12% of gastropods (n=16/138), 20% of bivalves (n=25/127), 30% of nautiloids (n=16/53), and 13% of unidentified mollusks (n=6/45). They also occur in 12% of corals (n=7/58) of solitary rugosan Tryplasma (n=4/55; 5/173 intraskeletal spaces) and colonial agetolitids (n=3/3; 301/1979 intertabularial spaces). It is interpreted that the sponges mainly exploited small and short-lived cryptic habitats and could have inhabited both exposed and cryptic habitats. Prevailing occurrence of the sponges within shells and skeletons in the Xiazhen Formation indicates that vacant intraskeletal spaces elsewhere could have been occupied by the sponges. Simplicity of the Late Ordovician non-reef cryptic communities is comparable to middle Cambrian to Early Ordovician reef cryptic ecosystems, which have evolved toward more complex and diverse community structures in the Middle Ordovician onwards. It suggests that an early Paleozoic cryptic ecosystem might have been “decoupled”, and their evolutionary pathways were different within various environmental conditions.