The late Middle Devonian (Late Givetian) Tully Formation records a period of accelerated biotic changes and community reorganization that is referred to as the Taghanic Bioevent. These changes, which ultimately terminated the diverse, long-standing Hamilton fauna actually commenced in deeper water, dysoxic facies of the uppermost Hamilton Group prior to Tully Formation deposition. In central New York, a key Tully faunal element Rhyssochonetes is found to occur with Hamilton taxa in the uppermost Windom Member while in Pennsylvania, several key Hamilton taxa recur in abundance above Rhyssochonetes-bearing beds of the basal Tully. This suggest that the Tully faunal "invasion" may actually have occured in a dynamic pattern involving stepwise incursions during a succession of transgressions culminating in Tully fauna invasion of shelf settings during lower Tully deposition.

Regional correlation of Tully-equivalent deposits in east-central New York and central Pennyslvania shows that the Tully can be divided into, at least, seven ascending faunal association divisions: a basal silty interval rich in Rhyssochonetes; a lower Tully outer shelf interval yielding Emanuella and "Leiorhynchus"; a "lower Laurens" division yielding Spinatrypa and Hypothyridina; an "upper Laurens" division yielding diverse Tully taxa; an upper-middle Tully barren transgressive interval; an upper Tully interval yielding a recurrent diverse Hamilton fauna association (West Brook Shale-equivalent level); and a top-Tully interval yielding a Hamilton association of modest diversity.

Efforts are currently being directed to constraining the timing of the Tully fauna-recurrent Hamilton fauna transition in the upper-mid Tully "barren interval". We believe that the transition from Tully biotas into the younger Ithaca fauna is preserved in oxic post-Tully deposits east of Oneonta. One idea currently being investigated is that the "Tully fauna" may be an offshore subfacies of the Ithaca fauna and that a shoreward Tully fauna-to-Ithaca fauna transition may be present in eastern New York.