The 'living-fossil community' of the cyrtocrinid Cyathidium foresti and the deep-sea oyster Neopycnodonte zibrowii (Azores Archipelago)

摘要

From the steep submarine slope of the southern Faial Channel (Azores Archipelago), a conspicuous archibenthic community of two 'living fossils', comprising the up to 30 cm sized deep-sea oyster Neopycnodonte zibrowii and the sessile cyrtocrinid Cyathidium foresti, is described. Both thrive in particularly high densities concealed underneath bedrock overhangs in 420-500 m water depth under comparatively stable environmental conditions (12.3 +/- 0.25 degrees C and open marine salinity). Other faunal elements include various sponges, bryozoans, serpulid worms, and large barnacles. Submersible investigations showed that the direct association of C foresti and N. zibrowii is not obligatory. The crinoid is commonly encountered settling on dead as well as live oysters, which provide benefits for the crinoid in terms of substrate availability and/or participation in the oysters' active feeding current. The upside down position of both species provides protection from background sedimentation and possibly shelter from predators. After death and partial disarticulation, the crinoids are commonly overgrown by other oysters or subsequent crinoid generations, mirroring limited substrate availability. Both genera, represented by the type species Cyathidium holopus and Pycnodonte sp., respectively, are also found as fossils more than 60 million years old in the Middle Danian (Paleocene) of Faxe in Denmark. Associated with coral limestone, C holopus was found occasionally in impressive numbers either isolated or settling on various substrata, such as scleractinian corals, other Cyathidium calyxes, and - analogous to the Azorean occurrence - the inner surface of pycnodontine oysters. Previous palaeohabitat models for C holopus such as the pseudoplanktonic attachment to drifting wood or attachment to cave ceilings in a photic reef, are rejected in favour of an interpretation of this association thriving on small hardground cliffs and under overhangs in a deep-shelf palaeoenvironment, associated with the cold- and deep-water coral bioherms proposed by recent palaeoenvironment assessments. We assume a habitat-shift of the cyrtocrinids from presumable shallow palaeoenvironments in the Late Cretaceous toward a deep shelf setting in the Danian, which reflects the escape from enhanced predation pressure during the 'Mesozoic Marine Revolution'. Sessile crinoids almost completely vanished from shallow waters and were replaced by mobile and often strictly nocturnal comatulids. The Cyathidium/(Neo)pycnodonte association did not only survive the Cretaceous/Palaeogene mass extinction, but the whole of the Cenozoic and can today be studied as a 'living-fossil community'.