PALEOECOLOGY OF ALASKA'S JURASSIC PARK

A trackway on the Alaska Peninsula represents the oldest known occurrence of dinosaurs in Alaska. Although the tracks were discovered in the 1970s, recent palynological, sedimentological and paleontological investigations provide the first paleoenvironmental reconstructions for this site. The prints are preserved as true tracks, exposed on a near-vertical bedding plane within the Upper Jurassic Naknek Formation. Complete prints are tridactyl, with a maximum length of 17 cm. The digits are relatively long and narrow, and impressions of digital pads and claws can be discerned. Based on overall size and morphology, the tracks are attributed to a small- to medium-sized theropod dinosaur. The tracks occur at the upper surface of a trough cross-bedded sandstone unit that caps a series of coarsening-upward packages interpreted as shallow marine offshore to upper shoreface successions. Overlying the track-bearing sandstone is a succession of coastal plain deposits consisting of sandstone, siltstone, mudstone and coal.

Biostratigraphic control is provided by the presence of the bivalve Buchia mosquensis (Buch) in shallow marine strata above and below the trackway, indicating a late Kimmeridgian to middle Tithonian age. Palynological assemblages from siltstone and coal units overlying the trackway contain the trilete spore genera Deltoidospora, Osmundacidites and Cyathidites, consistent with a Late Jurassic age. Dominance of spores in the siltstone facies suggests that ferns may have colonized less stable substrates on beaches or fluvial bars. Bisaccate and monosaccate pollen grains are more common in the coal facies, indicating the presence of coniferous forests. Prior estimates of Late Jurassic paleolatitude for this portion of the Alaska Peninsula vary from middle to high latitudes, but marine invertebrate assemblages from the upper Naknek Formation indicate relatively cool settings consistent with a high paleolatitude.