OSTRACODES AND CONCHOSTRACANS FROM THE UPPER TRIASSIC WHITAKER QUARRY, ROCK POINT FORMATION, CHINLE GROUP, NORTH-CENTRAL NEW MEXICO

New Mexico Museum of Natural History’s Whitaker (Coelophysis) Quarry block (C-8-82) from the Upper Triassic (Apachean) Rock Point Formation of the Chinle Group at Ghost Ranch, NM, contains an invertebrate fauna previously undocumented from this locality. The 1.1-m-thick block is primarily siltstone with mudstone and intraformational conglomerate lenses and contains rip-up mud clasts throughout. Ostracodes and conchostracans are found 0.65 m below the main (Coelophysis) bonebed and are the lowermost components of two fining-upward sequences of sediments in the block. They probably represent an autochthonous assemblage that existed in a pre-flood pond. The fossil progression from bottom up is: ostracodes, conchostracans, fish scales and bones, whole fish, isolated bones and scutes, small and/or juvenile tetrapods, large and/or adult tetrapods.

The ostracodes are assigned to Darwinula sp. based on characteristic features of the genus, including ovate valves with anterior end smaller than posterior end, right valve larger than left valve, and a complete lack of ornamentation. Darwinula is known from the homotaxial Redonda and Sloan Canyon formations of the Chinle Group, but is a temporally long-ranging genus of no precise biostratigraphic significance.

The conchostracans are assigned to the ubiquitous Late Devonian to Late Cretaceous genus Lioestheria. Valves are ovate, pellucid, and show ~20 irregular growth rings. The growth rings are double, and intra-ring ornamentation consists of anastomosing hachure marks superimposed on a finely punctate background. Measurements of 38 specimens shows a multimodal distribution of sizes where the mean valve height is ~3 mm, mean length is ~4.3 mm, allometric h/l constant is ~0.63, and umbones are at ~0.35 of length.

The conchostracans indicate a quiet, probably ephemeral, freshwater environment, but the block sediments and fossils shows strong flow indicators throughout. This, taken together with the generally coarsening up fossil material, may indicate a series of flood events of gradually increasing flow.