WHEN TRACE FOSSILS ARE ACTUALLY BODY FOSSILS: MEDUSOID (CNIDARIA) TENTACLES FROM THE POTSDAM GROUP OF NEW YORK

Study of tidal flat, beach and bar facies of the white, laminated, oscillation-rippled, fine sandstone, now recognized as the Keeseville Fm. in the Potsdam Group in northern NY and adjacent SE Quebec, Ca., has produced new understanding of the regional Late Cambrian biota. During examination of tidal flat facies, a cluster of indeterminate fossils assumed to be burrow traces (Planolites-like) was recognized, but was disregarded. A recent return to the outcrop gave opportunity for reinterpretation of this group of elongate, bifurcating features draped in epi-relief over a set of oscillation ripple marks on a bedding plane exposure of ~ 1.5 square meters.

A more thorough inspection of these features shows them to have properties unlike any burrows known from the Potsdam (Bjerstedt and Erickson, 1989) or elsewhere. Rather, they are recognized as preserved body fossils for the following reasons: 1) they reduce in width systematically as they branch; 2) they show no lebensspuren textures either concentric or linear; 3) they appear draped on the deposurface like a cord let down onto the bed; 4) portions appear to be isolated fragments several centimeters long, terminated abruptly. Kelp-like algal thalli could drape over beach sand, but they would be expected to preserve some evidence of budding; there is no evidence of any plant-like morphology. These features are interpreted as the oral tentacles of a large scyphozoan medusoid or colonial branches of a once-rooted hydrozoan cnidarian that stranded on this Cambrian tidal flat. A depression of the bed, around which tentacles are scattered, vaguely suggests a buried medusan bell. Wind-blown sand adhered to, and buried, the tentacles as it also covered a trackway elsewhere on the same bedding plane. When entire, the tentacles were >1 meter in length. No such structures are known from the Potsdam, although many presumed scyphozoan bells lacking oral tentacles are known (Hagadorn and Belt, 2009).

Among modern scyphomedusoids only oral tentacles of the Order Rhizostomae branch, 4 split to 8 at the manubrium and more beyond. Such tentacles have multiple mouths, but no oral structures can be interpreted in the fossils. No hydromedusae branch; sessile hydroid colonies with polypoid members do so today. No polyp structures are preserved. This is a new paleobiotic record for the Potsdam Group.