TAPHONOMIC IMPLICATIONS OF THE MICROSTRATIGRAPHY AND FAUNA OF A SINGLE BLOCK FROM THE LATE TRIASSIC WHITAKER (COELOPHYSIS) QUARRY (ROCK POINT FORMATION: CHINLE GROUP), NORTH-CENTRAL NEW MEXICO

The Whitaker Quarry in the Upper Triassic Rock Point Formation of the Chinle Group at Ghost Ranch, New Mexico, is one of the most extensive Late Triassic dinosaur assemblages known. Prior to preparation, the New Mexico Museum of Natural History’s block (C-8-82) from the Whitaker Quarry was turned over. Preparation proceeded from the “bottom-up,” thus permitting careful microstratigraphy for >1m under the main bonebed. The block shows two fining upward sequences above a basal sandy siltstone. The base of sequence one contains the ostracode Darwinula and the conchostracan Lioestheria, indicating a topographic low containing an ephemeral pond. The basal coarse silt and very fine sand of sequence one fines up to very fine silt with clay lenses. Sequence two fines up from a basal, matrix-supported sandy conglomerate to very fine silt. All beds above the invertebrates contain elongate rip-up mud clasts that are closely aligned and trend WNW-ESE. The dinosaur skeletons show general alignment with the mud clasts. The block stratigraphy indicates two flood events of increasing energy.

The fauna from the block comprises the above invertebrates; the redfieldiid Synorichthyes, the coelacanth Chinlea, the archosauromorph Vancleavea, a sphenodontid, a phytosaur, and the theropod Coelophysis bauri. An indeterminate redfieldiid-like fish, several isolated archosauromorph scutes, and teeth of a possible sphenosuchian or ornithischian dinosaur are also present. In general, the fossil material coarsens up throughout the block. Sequence one contains (ascending order) invertebrates, fish scales and bones, whole fish, non-dinosaurian tetrapods and very small juvenile dinosaurs. Sequence two contains the sphenodontid, and larger juvenile and adult dinosaurs.

Complete articulation of the dinosaur skeletons is the norm in the block, including sclerotic ossicles in life-position in the orbits. Given this amazing preservation and lack of evidence of scavenging or weathering, the most parsimonious taphonomic interpretation is that a large congregation of Coelophysis was overcome by a sequence of flood events, washed into a topographic low, and immediately buried.