In the clastic Genesee Group of the Catskill delta, lateral changes of the fauna are believed to reflect onshore-offshore physicochemical gradients. A shoreward increase of infauna is interpreted as adaptation to increased environmental stress. Free immobile taxa were concentrated offshore, while vagile forms, presumably able to cope with shifting substrata, are dominant nearshore. A shorewards replacement of brachiopods by bivalves reflects the eurytopy and infaunal habits of the bivalves.
In Gencsee time, progradation was first rapid, then slow. The sequence is reversed in the superjacent Sonyea Group and the accompanying reversal of faunal patterns is strong evidence of faunal control by the rate of progradation. This indicates the hazardous nature of attempts to trace ‘community evolutioneditor’ using only a few studies from each period.