VERTEBRATE AND INVERTEBRATE TRACE FOSSILS IN THE UPPER JURASSIC MORRISON FORMATION, BIG HORN BASIN, WYOMING: LINKING ICHNOFOSSILS AND SUBSTRATES IN AVULSION AND FLOODPLAIN DEPOSITS

The middle and upper parts of the Morrison Formation in the study area contain 1.5 to 3 m thick intervals, and consist of mudstones interbedded with ribbon and thin sheet sandstones. These intervals are intercalated with moderately to well-developed paleosols. Invertebrate and vertebrate traces are preserved in the paleosols, at the paleosol-sandstone contacts, and within the sandstones and mudstones. Invertebrate traces include crayfish burrows, soil bug and beetle burrows, orthopertan burrows, termite nests, bivalve and gastropod resting traces, and gastropod and annelid trails. Most of these traces are found in pedogenically modified deposits. Vertebrate traces include lobate, indistinct to pentadactyl prints interpreted as sauropod tracks with rare pad and skin impressions. Some tracks appear to record multiple, overprinted trackways that penetrate to different depths. At another locality, less abundant monodactyl to tridactyl footprints likely represent theropod locomotion. In most cases, only the distal phalangeal prints appear to be preserved, suggesting digitigrate locomotion, perhaps while running, wading, or swimming. The apparent bias toward preservation of digits II and III resulted from the preferential distribution of the weight of the track-maker during the weight-bearing and push-off stages of locomotion. Two or three parallel striations from 10 to 40 cm in length indicate swimming traces. Weak to no pedogenic modification of the sandstone and mudstone beds is indicated by the lack of bioturbation and pedoturbation, drab colored mudstones, and presence of relict bedding. The intervals are interpreted as channel avulsion deposits on alluvial floodplains that represent a relatively short amount of time.