REGIONALLY CONSERVED SWIM TRACK MORPHOTYPES FROM THE EARLY TRIASSIC OF WESTERN NORTH AMERICA

Vertebrate body fossils reported from the Early Triassic (late Olenekian) Moenkopi Formation of the Colorado Plateau are dominated by occurrences of temnospondyl amphibians whereas reptiles are rare. No vertebrate body fossils have been reported in coeval strata from the Red Peak Formation of Wyoming. In contrast, a medium-high diversity vertebrate ichnofossil assemblage dominated by reptile tracks and trackways is known from both formations. A considerable component of those assemblages is scratch marks commonly interpreted as swim tracks. Swim tracks from the Moenkopi and Red Peak formations may be categorized into three regionally conserved morphotypes distinguished by size, shape, and individual characteristics. Each morphotype has a different interpretation regarding the possible trackmakers and their aquatic behaviors. Morphotype I tracks are one-, two-, or three-digit footmarks with little or no interdigital spacing and disc-shaped posterior margins which project posteriorly to produce overhangs. Individual digits may display longitudinal striations on each digit mark. These striations suggest that the trackmaker had a hard nail/claw on the end of each digit, some sort of scaly/textured skin, or both. Morphotype II tracks are single-digit footmarks which are intertwined or reflexed to form a z-shaped trace. This morphotype may also display longitudinal striations and a disc-shaped posterior margin. Similarities in the individual characteristics of morphotype I and II swim tracks (e.g., striations, disc-shaped margins) and traceways which are composed of both morphotypes indicate that they can be attributed to the same or very similar trackmakers. Morphotype III tracks are wholly different from other morphotypes in being consistently much smaller with relatively thinner digits and larger interdigital spacing. There is little or no divarication of the digits and the posterior margins of complete tracks are straight. Some footmarks show evidence of webbing between digits. Due to the paucity of reptilian body fossils from the Moenkopi Formation and their absence from the Red Peak Formation, vertebrate trace fossils from these formations, especially swim tracks, provide unique and important insights into the geographic distributions and aquatic behaviors of Early Triassic reptiles.