Evolution and Dispersal of Deepsea Traces

Shallow water late Precambrian and Early Cambrian sequences have yielded not only shallow water trace fossils but also many examples of traces typical of later deep water deposits. Significant colonization of the deep oceans by trace making animals was, however, delayed until the Early Ordovician. There followed a gradual increase in ichnogeneric diversity through much of the Phanerozoic, with an acceleration from the Cretaceous through the Tertiary. The apparent migration of deep water forms from their origins in shallow water out to the deep sea was accompanied by their virtual disappearance from shallow water, thereby providing an early example of "retreat'. There is no consistent evidence to indicate that behavioral optimization of deepsea traces developed gradually during the Phanerozoic. Indeed, most lineages of deepsea traces reached a high level of behavioral development by the early Cambrian or even late Precambrian. Some shallow water traces appeared temporarily in the deep ocean during the Lower Paleozoic but more significant onshore-offshore migrations of ichnogenera such as Ophiomorpha and Thalassinoides took place from the Cretaceous onwards. Detailed investigations of Mesozoic and Tertiary deepsea sand fans in the last fifteen years have revealed a relationship between the ichnofaunas and depositional sub-environments. Submarine canyons and the inner parts of fans commonly contain a low diversity ichnofauna with a preponderance of shallow water types such as Ophiomorpha and Thalassinoides. Middle fan sub-environments have a mixed ichnofauna of shallow and deep water types, with particularly high diversity in interchannel areas. Outer fan and fan fringe sub-environments rarely have shallow water traces but deep water forms are abundant and diverse.