FIRST REPORT OF AN ARTHROPOD FROM THE PASSAIC FORMATION (LATE TRIASSIC) OF NEW JERSEY

The Passaic Formation (Late Triassic) represents the most widespread and thickest (>5000 m) deposits in the Newark basin. It comprises marginal fluvio-lacustrine and cyclical lacustrine reddish brown, sporadically gray, black, and tan mudstones, sandstones and conglomerates. Sedimentary structures include ripple marks, desiccation cracks, raindrop impressions, cross-bedding, and tool marks. In addition, vertebrate footprints, plants, fish, and conchostracans are also present. An arthropod-bearing slab was collected from a small talus pile of Passaic deposits north of Milford, New Jersey. The slab exhibited sedimentary features similar to an exposure that is located several meters upslope. Exposures exhibit alternating ledge-forming deposits of poorly-sorted siltstone and less resistant fine-grained units. The arthropod is impressed on a reddish brown siltstone overlain and underlain by claystone laminae exhibiting desiccation cracks on their upper surfaces. The arthropod, the first evidence of such found in the Passaic Formation, is 3.5 cm in length, 2 cm in maximum width, and occurs in epirelief. No evidence of body tissue was found. There is a lack of impressions of appendages which may possibly be due final positioning after death, or subsequent detachment and removal prior to burial. In addition to the arthropod, the trace fossils Helminthoidichnites and Spongeliomorpha are preserved in epirelief, while on the underside several examples of Helminthoidichnites are present. Associated slabs exhibit the trace fossils Scoyenia, Helminthoidichnites, Lockeia, Spongeliomorpha, and the reptile footprint Rhynchosauroides, as well as desiccation cracks, cross-bedding, raindrop impressions, and tool marks. The Passaic at this location comprises lacustrine, fluvial, and floodplain deposits. The trace fossils present allow assignment to a Scoyenia ichnofacies, which relates to low energy continental sites subject to periodic submergence or emergence. The combination of desiccation cracks and sporadic raindrop impressions denote shallow water conditions subject to periodic subaerial exposure. The well-preserved arthropod impression suggests a post-mortem body impression, though the lack of body tissue opens up the question if it is a body fossil mold or a very well preserved resting track.