THE DIVERSITY AND STRATIGRAPHIC DISTRIBUTION OF VERTEBRATE TRACK HORIZONS, FROM THE TRIASSIC MOENKOPI FORMATION, UTAH, U.S.A

Recent discoveries in the Moenkopi Formation (Middle Triassic?) of central and southeastern Utah have revealed new terrestrial and subaqueous vertebrate track localities. These well-preserved tracks occur on multiple stratigraphic horizons and are the oldest and most laterally extensive track-bearing horizons documented in the Western U.S. 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.

Tracks are preserved either as positive relief “casts” filling impressions in the underlying mudstones or on plane bed surfaces as negative relief “impressions”. Exposed traces occur on the undersides of resistant sandstone ledges where the mudstone has eroded away and in finer grained sediments such as mudstones and siltstones. 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. Well preserved, skin, claw, and pad, impressions are common. Occasional, well developed, tail-drag marks frequently occur in many of the trackway sequences. 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 horizons.

Lateral correlations of the ichnostratigraphic units identified in the Moenkopi Formation throughout Utah will aid interpretations about the paleoecology, and diversity of the Western Interior during the Middle Triassic-“the dawn of the dinosaurs”.