CHAPTER THIRTEEN Evolution of the Deep-Water Benthic Community Megascopic life evolved in the Archean with the buildup of stromatolitic mounds in shallow-water environments. By the Proterozoic, stromatolites had already extended down to well below fair-weather wave base. | 13-C1099 8 10 00 2 12 PM Page 275 CHAPTER THIRTEEN T. Peter Crimes Evolution of the Deep-Water Benthic Community Megascopic life evolved in the Archean with the buildup of stromatolitic mounds in shallow-water environments. By the Proterozoic stromatolites had already extended down to well below fair-weather wave base. During the late Vendian there was an increase in megascopic biota in shallow water with both soft-bodied fossils and trace fossils becoming relatively abundant. Some of the soft-bodied forms such as Pteri-dinium were large and preserved three-dimensionally with remarkable detail in high-energy medium-to-coarse-grained sandstones. This style of preservation resembles that of trace fossils which were produced within similar sequences during the Phanerozoic and may suggest that some of these early life-forms grew through already deposited sediment as a unicellular protoplasmic mass. Some Ediacaran body fossils . Charniodiscus Ediacaria Pteridinium may have survived into the Cambrian by migrating into deeper water where many of the reported body fossils were exceptionally preserved soft-bodied forms. There was also a slight increase in trace fossil diversity in deep water during the Cambrian and this too may reflect the activity of a dominantly soft-bodied fauna. There was a major progressive colonization by hard-bodied forms of the outer shelf by the Early Ordovician and of the slope toward the end of the Middle Ordovician. In contrast there is a significant increase in trace fossil abundance and diversity in deep-water flysch sequences as early as the Early Ordovician. It appears that soft-bodied animals including those which produced trace fossils were involved first in the onshore-offshore migration and were generally well established in deeper-water niches before the arrival of faunas rich in skeletal forms. INTRODUCTION The colonization of deep-sea environments appears to have been a slow process Crimes 1974 Sepkoski and Miller 1985 Bottjer et .
