Coelophysis, Rioarribasaurus, and the stratigraphy and biochronology of the Upper Chinle Group in north-central New Mexico

An abstract describing the stratigraphy of dinosaur localities at and near Ghost Ranch.

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We demonstrate by a combination of measured stratigraphic sections and paleontologic, topographic, and historical evidence that the original Coelophysis fossils collected by David Baldwin in 1881 and named by E.D. Cope in 1887 were almost certainly collected from what is now known as the Painted Desert Member of the Petrified Forest Formation (Chinle Group). In contrast, the Whitaker quarry at Ghost Ranch, which has produced dozens of skeletons of the theropod dinosaur Rioarribasaurus, is located within the overlying Rock Point Formation (Chinle Group). Furthermore, our results support the hypothesis that "Coelophysis" and Rioarribasaurus are two distinct, valid taxa, and as such indicate the first clearly superposed Chinle dinosaur taxa.

The topotypic material of "Coelophysis" was collected from a low-lying, easily accessible suite of bentonitic mudstones and very thin sandstones and conglomerates cropping out as low badlands approximately 2/5 km SSE of Ghost Ranch in a horizon correlative to the Canjilon phytosaur quarry. Tetrapod fossils from the Canjilon quarry, particularly the phytosaur Pseudopalatus and the aetosaur Typothorax, support an early-mid Norian age for the Painted Desert Member of the Petrified Forest Formation in this region. The Whitaker quarry is topographically and stratigraphically higher in a series of interbedded, nonbentonitic sandstones and siltstones that we assign to the Rock Point Formation. Vertebrate fossils, particularly the phytosaur Redondasaurus, from the Rock Point and correlative units indicate that this unit is of latest Norian or Rhaetian age. This marks the only known occurrence of two unambiguously superposed dinosaur genera in the Chinle Group, re-emphasizing that dinosaurs remain remarkably poor for use in correlation within the Chinle because of their great rarity.