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SPIROPHYTON IN ALLUVIAL-TIDAL FACIES OF THE CATSKILL DELTAIC COMPLEX: POSSIBLE BIOLOGICAL CONTROL OF ICHNOFOSSIL DISTRIBUTION

The trace fossil Spirophyton occurs in rocks of the alluvial-tidal facies of the Devonian Tully clastic correlatives in east-central New York. It is absent from nearshore and offshore deposits in the Tully equivalents, although the similar form Zoophycos is abundant in these facies and Spi- rophyton is found in offshore siltstones of the underlying Hamilton Group. In areas other than New York Spirophyton occurs in nearshore and offshore facies. The fact that Spirophyton is found only in alluvial-tidal siltstones and very fine grained sandstones of the Tully clastic correlatives and is absent from equivalent rock types in nearshore and offshore facies suggests that energy and substrate conditions did not entirely determine the distribution of the Spirophyton producer. Kitchell (1979) and Kitchell and Clark (1979) suggested that the distribution of deep-sea traces may be controlled by ecological factors such as competition and predation, and the same may have been true for the Devonian Spirophyton. Facts consistent with the interpretation of its producer as a r-selected opportunist competitively excluded from more benign deeper water environments include: 1, it is absent from its normal facies where the similar Zoophycos is abundant and it occurs profusely locally in the marginal marine facies where Zoophycos is absent; 2, it is restricted to rocks which, by analogy with the present, were deposited in harsh unpredictable environments which favor r-selected species; and 3, the pattern of its occurrences and its densities fit previously delineated characteristics of opportunistic fossil species.