Ichnostratigraphy of the Jacksonwald syncline: the last 7 million years of the Triassic
Abstract--Reptile footprint-bearing lake-margin facies are especially well developed in the Jacksonwald syncline of the southwestern Newark basin. Footprint-bearing intervals occur in the regressive portions of most Van Houten cycles, which are caused by the 21,000 year cycle of the precession of the equinoxes. Sampling of footprint assemblages from this common ichnofacies through the upper Passaic Formation in the syncline, documents the large-scale ecologically important change in reptile lake-margin communities towards and through the Triassic-Jurassic boundary.

The oldest well-sampled assemblage comes from a prominent black and gray lacustrine sequence in Douglasville, Pa. Most abundant is the quadrupedal dinosaurian ichnite Atreipus; other forms present include Grallator (Grallator) sp., Rhynchosauroides brunswickii, and Gwynnedichnium sp. This assemblage is almost identical to those from the basal Lockatong Formation, some 16 million years older. However, also present is the oldest [earliest] occurrence of the crocodilian track Batrachopus sp., an ichnotaxon abundant in the Jurassic. Younger Triassic horizons record an apparent increase in the abundance of Grallator spp., however, these layers are as yet poorly sampled.

Footprints from between the two pollen-bearing horizons which define the Triassic-Jurassic boundary comprise a Grallator-dominated assemblage with abundant Rhynchosauroides sp. and Batrachopus. This assemblage is identical to several from just below the Orange Mt. Basalt near Montclair and Clifton, N.J. Post-Orange Mt. Basalt assemblages from many localities differ from these by the addition of Anomoepus and a great reduction in the abundance of Rhynchosauroides.

While as yet insufficiently sampled, the footprint-bearing intervals of the Jacksonwald syncline do show the elimination of the Lockatong-lower Passaic Atreipus -dominated association and its replacement by a Grallator -dominated one, composed of survivors. Further sampling of these new localities should refine our knowledge of the rate and time at which this transition took place.

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