DEEP-SEA TRACE FOSSILS: WHAT WE KNOW AND WHAT WE WOULD LIKE TO KNOW

A good deal of what we know about evolutionary history and ecology of deep-marine endobenthic animals comes from trace fossils preserved in turbidites and pelagic limestones exposed on land. The patterns are different from the records of shallow-marine and continental trace producers. Although diversity and distribution are controlled by many of the same general kinds of factors as in other environments, the deep-ocean record is truly unique in several ways. (1) The fossil record of deep-sea metazoans consists almost entirely of burrows made by detritus-/deposit-feeders and possible microbe-farmers (because of preservation processes and also because trophic resources are mostly imported from the upper water column or adjacent continents/islands). (2) The associated epibenthic and pelagic organisms typically are not preserved in deep-ocean sedimentary rocks (with some notable exceptions). (3) Evidence of alternation of pre- and post-disturbance ecologic regimes is an obvious feature of many ichnoassemblages (reflecting depositional processes and environmental factors controlling both composition and development of ecosystems and preservation potential of endobenthic structures). (4) And the most familiar components of ichnofaunas—complex meanders, spirals and network burrow systems known as graphoglyptids—are widespread and under certain conditions diverse, and have been collected in modern deep-ocean samples, but we still do not know identity of the producers and their roles in deep-marine ecosystems. Together with certain physical and chemical properties, geologists use trace fossils to interpret deep-marine facies, yet many of the ichnogenera have unknown biologic properties because modern producers have never been observed and bodies are not preserved with structures.