TRACES AND TRACKS FROM THE MIDDLE TRIASSIC MOENKOPI FORMATION OF DINOSAUR NATIONAL MONUMENT, UTAH

We report here the first tetrapod tacks from the Lower-Middle Triassic Moenkopi Formation of Dinosaur National Monument (DNM). The tracksites were discovered in 2010 by Geologist-in-the-Park interns in the “racetrack” exposures along the nose of Split Mountain in the Green River District of DNM. Two tracksites occur near the Sound of Silence trail (DNM 481, DNM 485) and another near the Desert Voices trail (DNM 486). The trackways preserve a variety of tracks and swim traces, generally in convex hyporelief on the underside of very fine-grained sandstones, many of which exhibit climbing ripples indicating westerly paleoflow. DNM 481 preserves manus- and pes prints broadly similar in dimensions and morphology to Synaptichnium, previously known from the Moenkopi Fm elsewhere, but with some resemblance to Parachirotherium, which has not previously been reported from the USA. Both taxa have been ascribed to archosauriform reptiles. The other two sites preserve a variety of swim traces representing at least three morphotypes. Swim traces are generally elongate, paired or tridactly traces, often with longitudinal striations and posterior overhangs. All of these sites also preserve invertebrate traces we have not indentified further.

All tracks are relatively high in the Moenkopi Formation and therefore likely of Middle Triassic age. Locally the Moenkopi Formation can be divided into several informal “members:” (1) a basal, rhythmically bedded red and gray silty interval that may be equivalent, in part, to the Dinwoody Fm; (2) a medial interval characterized by mm- to m-scale interbeds of gypsum in orange siltstone; and an upper interval characterized basally (3) by orange, silty beds lacking gypsum that grade upward into (4) thicker, grayish-red siltstone-dominated beds. The DNM trackways, and apparently others known from the region north of Vernal, UT, are all at or near the transition from (3) to (4). Previously reported Moenkopi Fm sections in the vicinity of DINO range in thickness from 475–970 ft (145–296 m), but most are not recorded in great detail, so higher-resolution correlations are lacking. As is the case with many younger units (e.g., Navajo/Nugget, Entrada eolianites), the presence of trace fossils demonstrates a more diverse and complex ecosystem than that indicated by the rather poor body fossil record.