THE “HIDDEN” BIODIVERSITY AND ECOLOGY OF EARLY TRIASSIC ORGANISMS REVEALED BY TRACE FOSSILS FROM THE MOENKOPI FORMATION OF CENTRAL UTAH

The Lower-Middle Triassic Moenkopi Formation in central Utah almost completely devoid of body fossils. However, tracks attributed to reptiles have long been known from the Torrey Member which was deposited by a large, prograding delta during the Early Triassic. Recently discovered vertebrate track sites from this unit and the invertebrate trace fossils associated with them are beginning to reveal the “hidden” biodiversity of this ancient deltaic ecosystem. Track sites are dominated by fish swimming traces (ichnogenus Undichna) and larger elongate traces interpreted as swim tracks produced by reptiles. Occurrences of reptile footprints (Rhynchosauroides and ?Protochirotherium) are rare. Recently, a commonly occurring track morphotype has been recognized which represents a new ichnotaxon characterized by four triangular digits of equal length arranged in a crescentic pattern with a total track width of about 2 cm. Invertebrate trace fossils associated with the vertebrate tracks represent a range of possible tracemakers and include ichnogenera commonly attributed to polychaete worms (Arenicolites and Skolithos), horseshoe crabs (Kouphichnium), crustaceans (Thalassinoides), and conchostracans (Lockeia). These trace fossils also record a range of behaviors (ethologies) and can be grouped into several different ethological classes which include feeding traces (Rhizocorallium), locomotion traces (Planolites), permanent dwellings (Diplocraterion), and predation activities (rusophycid traces). Although ichnotaxonomic diversity is relatively high and trace fossils can be locally dense, an overwhelming majority of the beds within the Torrey Member completely lack bioturbation. This is most likely a result of the protracted ecologic recovery following the end-Permian mass extinction and reflects the activity of opportunistic organisms under stressed, brackish conditions. As a whole, trace fossils from the Torrey Member provide evidence for the presence of fish, at least three types of reptiles, several arthropods, conchostracans, and multiple other invertebrates as well as insights into the life habits for many of them. They reveal a previously unknown biodiversity and ecology of Early Triassic organisms which would have remained hidden were it not for the traces they left behind.