TAPHONOMY OF THE DALTON WELLS DINOSAUR QUARRY, A DEBRIS FLOW HOSTED VERTEBRATE ASSEMBLAGE (EARLY CRETACEOUS, CEDAR MOUNTAIN FORMATION)

The Dalton Wells (DW) fossil assemblage is the most diverse Early Cretaceous terrestrial vertebrate fauna known from the western hemisphere and is a source of significant paleobiological and paleoecological data. At DW several bonebeds occur in a stacked succession of debris flow deposits at the base of the Yellow Cat Member of the Cedar Mountain Formation, a 10-20 m thick sedimentary package deposited in a seasonally-dry, alluvial-lacustrine setting. More than 4200 fossil elements have been collected from the bonebeds and the assemblage is dominated by the remains of at least 57 individuals representing nine dinosaur taxa. The assemblage includes fewer than 10 elements from four non-dinosaur vertebrate taxa, and a few plant fragments. The dinosaur fauna is sauropod-dominated (MNI 20) and includes a brachiosaurid, a camarasaurid and a basal macronarian. Utahraptor, three other theropods, a large iguanodontid and the ankylosaurid, Gastonia, comprise the balance of the dinosaur fauna. Patterns of skeletal articulation and clustering, ontogenetic representation, and bone surface modification all indicate that the DW assemblage accumulated during a year or more as the result of multiple mass-mortalities that preferentially killed subadults and juveniles. Patterns of tooth marks, sorting, breakage and scratch marks indicate that the assemblage was scavenged by theropods, hydraulically winnowed and trampled by large dinosaurs. Subsequently, elements were entrained and transported a short distance in a succession of at least three debris flows that converged on hyperconcentrated flow conditions. At the site of deposition some elements were again trampled (shown by in situ breaks) and scavenged by insects (shown by sediment-filled grooves in bone that continue into the matrix).

Geologic data aside, there are few taphonomic clues that the DW assemblage was actually reworked by debris flows. Data from DW and a review of published taphonomic data from ancient debris flow deposits suggest that this absence of a debris-flow taphonomic signature probably reflects the short time that elements are exposed to reworking in debris flows, and underscore that the taphonomic signatures in debris-flow hosted assemblages are commonly established prior to debris flow interment.