EXTANT SOFT BODIED DEEP-SEA DENIZENS: WHAT DO THEY TELL US ABOUT ANCIENT FAUNAS?

Today’s deep-sea fauna includes soft-bodied organisms from approximately 17 of 30 phyla that do not readily fossilize. Some scattered but intriguing fossil evidence shows that soft-bodied animals were well represented in ancient deep-sea faunas, but with a few exceptions these have seldom been given their due in discussions of ancient faunal composition and origin. Some examples, drawn mainly from the Phylum Echinodermata, Class Holothuroidea, of ecologically important extant soft-bodied forms are discussed, and their role in today’s deep-sea environments is assessed. Over extensive tracts of the deep-sea floor, holothurians can comprise approximately 95% of the total weight of epibenthic megafauna, and they can reach population densities in excess of 500,000 individuals per hectare. Burrowing species in the Orders Molpadiida and Apodida form a significant component of the mega- and macro-infauna as well. The sketchy fossil record, mostly microfossils, of sea cucumbers indicates a long history of the group. The largest extant deep-sea group, the Order Elasipodida, dates back at least to the middle Devonian, and the Order Aspidochirotida, Family Synallactidae, to the Upper Ordovician. A species-rich burrowing macrofaunal group, the Family Myriotrochidae, has existed since at least the Early Silurian. The very few whole body holothurian fossils suggest that there seems to have been a conservative persistence of body types over very long time periods. Some authors have suggested that extant deep-sea holothurians may have colonized and re-colonized deep-sea habitats at various times, such as the latest Mesozoic or early Cenozoic, from shallow-water regions, as suitable deep-sea habitats became available. Others propose, based upon echinoderm microfossil data, that the modern deep-sea fauna is considerably older. Regrettably, the known fossil record provides no detailed information on the role of these organisms in the general economy of ancient deep seas.