Db_abstract

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

Documents the lake beds under the main Ghost Ranch Coelophysis bonebed, and argues that flood events washed the Coelophysis carcasses into a topographic low.

Item Details

Abstract

Prior to preparation, New Mexico Museum of Natural History's Apachean age Whitaker Quarry block (C-8-82) was turned over. Preparation of the ~12,000 pound block has proceeded from the "bottom-up" thus permitting careful microstratigraphic study of bedding 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, that likely indicate 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.


To date, the fauna from the block comprises the above invertebrates; the redfieldiid fish Synorichthyes, the coelacanth fish Chinlea, the archosauromorph Vancleavea, a planocephalosaur-like sphenodontid, a phytosaur, probably pertaining to Redondasaurus gregorii, and the theropod dinosaur Coelophysis bauri. Additionally, an indeterminate redfieldiid-like fish, several isolated enigmatic scutes, and teeth of a possible sphenosuchian or ornithischian dinosaur are 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. The invertebrates and the sphenodontid constitute the only previously undocumented fauna from the Whitaker quarry.


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