THE DIVERSITY AND STRATIGRAPHIC DISTRIBUTION OF PRE-DINOSAURIAN COMMUNITIES FROM THE TRIASSIC MOENKOPI FORMATION, UTAH

Recent discoveries in Torrey, Capitol Reef and San Rafael Swell, Utah has revealed new sites of terrestrial and subaqueous vertebrate traces and is the oldest and most laterally extensive track bearing horizons documented in North America. Ichnogenera (Chirotherium), (Rhynchosauroides) and (Rotodactylus) are the dominant forms. Rare fish fin drag marks (Undichna) and fish skeletal remains have been identified in the Torrey Member and equivalent strata of the Moenkopi Formation (Early/Middle? Triassic). Multiple vertebrate ichnostratigraphic units are distinguished in the Moenkopi based on the stratigraphic occurrences of track sites within the study area. Tracks are preserved as positive relief “casts” filling impressions in the underlying mudstones. Exposed traces occur on the undersides of resistant sandstone ledges where the mudstone has eroded away. The Torrey Member represents deposition on a broad, flat-lying coastal delta plain. Both nonmarine (fluvial) and marine (principally tidal) processes influenced deposition. Even-bedded mudstones, siltstones, claystones, and fine grained sandstones containing abundant ripple marks and parallel laminations dominate lithologic types. Ichnites indicating swimming/floating behavior are associated with the walking trackways. The water depth was sufficiently shallow to permit the vertebrates to touch the substrate with manus and pedes when moving through the water.

Tracks form locally dense concentrations of toe scrape marks which sometimes occur with complete plantigrade manus and pes impressions. occasional tail impressions or tail drag marks are present with the trackway sequences. Well preserved, skin, claw and pad impressions are common. Fish fin drag marks and fish skeletal material are preserved with tetrapod swim tracks. In addition to vertebrate ichnites, fossil invertebrate traces (Arenicolites, Paleophycus and Fuersichnus, are abundant within the track bearing units.

Lateral correlations of the ichnostratigraphic units identified in the Moenkopi Formation throughout central and southeastern Utah, western Colorado, and northern Arizona will aid interpretations about the paleoecology of the Western Interior during the Early/Middle? Triassic.