Evolution of the Deep-Water
T. Peter Crimes
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 Pteridinium, 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 (e.g., 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.
The colonization of deep-sea environments appears to have been a slow process (Crimes 1974; Sepkoski and Miller 1985; Bottjer et al. 1988), and a high percentage