Analysis of trends of morphologic variation within faunal assemblages is significant for paleoecologic interpretation, and complements paleoenvironment interpretations based on analogy with Holocene sedimentary environments. The Rondout Formation (Late Silurian), Hudson Valley, New York, demonstrates the importance of such analysis.

The Glasco Limestone Member (12 ft) records offshore subtidal deposition. It is overlain and underlain by supratidal mudflats--the Whiteport Dolomite (1-7 ft) and Rosendale Dolomite Members, respectively.

Within the Glasco, 5 units in vertical sequence from base to top show morphologic variation of halysitids, coenitids, stromatoporoids, and various algae, coincident with shallowing: Unit I (1 ft), "amoeboid" stromatoporoids in wackestone matrix; Unit II (4 ft), thin, laminar stromatoporoids, "head-shaped" halysitids, encrusting algae, and small-diameter branching coenitids in packstone and grainstone matrix; Unit III (1 ft), halysitids change to "blade" morphology; Unit IV (4 ft), no halysitids, bloom of larger diameter branching coenitids, branching and encrusting algae, and laminar and "amoeboid" stromatoporoids in packstone and grainstone matrix (rare wackestone); Unit V (2 ft), massive domal stromatoporoids in calcareous shale.

Transitions from "heads" to branches, from "heads" to "blades," from smaller to larger diameter coenitids, and from encrusting to branching algae are interpreted as adaptation to increased turbulence. Halysitid establishment and laminar stromatoporoid morphology correlate with "firm" substrate. A "soft" substrate supported "amoeboid" stromatoporoids. A decrease of turbulence permitted vertical expansion of stromatoporoids to domal morphology.

Paleoecologic interpretations based on single morphologic occurrences, as contrasted with trends, must be made with reservation.

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