MICROFOSSILS PRESERVED IN CHERT FROM THE LOWER MIDDLE DEVONIAN ONONDAGA LIMESTONE, FINGER LAKES REGION OF NEW YORK, USA

Shallow, tropical seas covered much of North America during Devonian time (419-359 Ma). The lower Middle Devonian (Eifelian, 398-392 Ma) Onondaga limestone represents the broad carbonate bank environments that dominated the northern end of the Appalachian basin during this time. The Onondaga was populated by a rich variety of marine macroinvertebrates; we know much less about the diversity of the marine microflora and microfauna from this formation. We used transmitted light microscopy of thin sections to investigate the microfauna and microflora preserved in chert nodules from two localities in the Finger Lakes region of New York.

Acritarchs dominate the assemblage (69% of specimens); we noted more than 20 morphotypes, including Veryhachium, Micrystridium, Stellinium, and Villosacapsula. Fungi (7% of specimens), chitinozoans (6%), meiospores (5%), scolecodonts (polychaete teeth, 3%), possible arthropod cuticle (3%), green and brown algae (<1%) and a single meiofaunal bryozoan colony, likely Orthopora regularis, represent most of the rest of the material.

Preservation in the Onondaga chert is of very high quality. Acritarchs and other organic remains are preserved in three dimensions; most show little or no post-depositional crushing, decay, or other damage, suggesting very early chert diagenesis. The Thermal Alteration Index (TAI) is approximately 3+/4-, which is consistent with regional thermal history. No calcite or aragonite is present. Any organisms with calcite skeletons preserve as organic “ghosts” – without mineral material.

The presence of meiospores (likely of terrestrial plant origin) may indicate the proximity of land. In addition, an ovoid accumulation of meiospores, possibly in different developmental stages, may represent the sporangium of a terrestrial plant.