Sclerobionts on soft-bottom, free-living Stylomaeandra Fromentel corals from the Lower Cretaceous Agrio Formation, Neuquén Basin, Argentina: palaeobiological implications for umbrella-shaped colonies

Abstract

Several beds hosting corals are found within the marine members of the Agrio Formation (Lower Cretaceous, Neuquén Basin). In one of them, free-living, soft-bottom pedunculate colonies of Stylomaeandra sp. occur commonly; their palaeoecology and sclerobiont community were surveyed. The best-preserved coral specimens were statistically analysed; their upper and lower surfaces were mapped. Stylomaeandra sp. morphology suggests that it lacked automobility. Sclerobionts were dominated by thecideid brachiopods followed by oysters; other sclerobiont taxa comprise tubicolous polychaetes, cyclostome bryozoans, calcareous sponges and encrusting corals. Sclerobionts are more speciose and abundant in the coral’s lower surfaces, the upper ones being scarcely colonized, indicating likely in vivo encrustation. Serpulid tubes were often embedded within the coral’s skeleton, an example of in vivo interaction by bioimmuration. On most undersurfaces sclerobionts were clustered, half or more of such surfaces remaining unoccupied. This suggests part of the colony undersurface rested on the seafloor, agreeing with the small pedicle and lack of automobility of Stylomaeandra sp. In the coral meadows, Stylomaeandra sp. dwelled among larger massive and ramose corals and in their vicinity; but unlike the latter, that coped better with sedimentation, the umbrella-shaped Stylomaeandra sp. were eventually smothered.