Ichnostratigraphy/Tr-J Conference

Ichnite record of terrestrial tetrapod abundance and diversity through a critical period in Earth history, Jacksonwald syncline, Newark basin, Pennsylvania

Abstract--The study area, in the Jacksonwald syncline of the Newark basin, Pennsylvania, contains numerous intervals of abundant terrestrial tetrapod footprint-bearing horizons that occur mainly in the regressive portions of Van Houten lacustrine sedimentary cycles. These 20,000-year cycles represent lake transgression, highstand, and regression driven by climatic changres. Thus Van Houten cycles provide a precise time control against which rates of terrestrial tetrapod radiation and extinction can be measured. The study area also contains the palynologically-determined Triassic-Jurassic boundary. A continuous drill core through section correlative to field sites, coupled with paleomagnetic data for both core and field sites, allows unambiguous stratigraphic placement of strata from field sites and therefore precise stratigraphic placement of ichnites.

Extremely abundant ichnofaunal horizons from latest Triassic strata reveal a rich and diverse assemblage of terrestrial tetrapod footprints. Ichnites present include Atreipus milfordensis and Atreipus spp. produced by small herbiverous quadrupedal dinosaurs; Batrachopus deweyii and Batrachopus spp., crocodilian footprints; Brachychirotherium parvum and Brachychirotherium spp., rauisuchid and stagonolepid tracks; Chirotherium lulli, chirothere footprints; Grallator spp., the tracks of a small carnivorous dinosaur; Gwyneddichnium majore and Gwyneddichnium spp., produced by minute amphibians; Rhynchosauroides brunswickii and Rhynchosauroides spp. the tracks of small sphenodontids; and a new, currently unnamed taxon.

The earliest Jurassic assemblage in the study locality is much reduced and composed predominantly of latest Triassic survivors, mainly larger-sized Grallator, and extremely rare Rhynchosauroides, with the addition of Anomoepus.

Previous work showed the diverse latest Triassic assemblage present to within 400,000 years of the Triassic-Jurassic boundary. Recent research at new exposures in the Jacksonwald syncline area reveals another new Late Triassic taxon, and allows the extension of several ichnite ranges.

Stratigraphic analysis of new field sites shows: 1) the local fauna was abundant and increasing in diversity in the very latest Triassic, and 2) faunal turnover from a rich latest Trassic assemblage to a sparse Jurassic assemblage occurs within a single Van Houten cycle, or within ~20,000 years.

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