Micro- and Small Vertebrate Biostratigraphy and Biochronology of the Upper Triassic Chinle Group, Southwestern Usa

A long-term goal of vertebrate biostratigraphers is to construct a microvertebrate biostratigraphy for lower Mesozoic rocks that complements and extends the mammal-based biostratigraphy in place for Upper Cretaceous-Recent deposits. Here we report substantial progress toward achieving this goal based on the distribution of microvertebrate fossils in the Upper Triassic Chinle Group of the western USA. This biostratigraphic hypothesis independently corroborates the existing macrovertebrate biostratigraphy, recognizing four temporally successive assemblages—the Otischalkian, Adamanian, Revueltian, and Apachean, with subdivisions of the Adamanian (St. Johnsian and Lamyan) and Revueltian (Barrancan and Lucianoan). Many microvertebrate taxa, unfortunately, have long stratigraphic ranges and thus, while widespread and easily recognized, are not reliable index fossils. These taxa include the chondrichthyans Lissodus and Reticulodus synergus, most lepidosauromorphs (including sphenodontians), and various microvertebrate archosauromorphs. However, other taxa have relatively short stratigraphic ranges and occur in multiple localities and thus have some utility as index taxa. Adamanian index taxa include the procolophonid(?) Colognathus obscurus and the putative ornithischians Tecovasaurus murryi, Crosbysaurus harrisae and Krzyzanowskisaurus hunti. Revueltian index taxa include the chondrichthyan Reticulodus synergus, the crurotarsan Revueltosaurus callenderi and the cynodont Pseudotriconodon chatterjeei. Sphenodontian taxa appear to have relatively little utility as biostratigraphic markers at this time scale, although similar forms cooccur at the Placerias quarry in Arizona and correlative strata in the Tecovas Formation of West Texas. The best candidates for index taxa are those that, like therian mammals, have complex teeth and thus are identifiable to the genus or species level based on isolated teeth or tooth fragments. Many of these taxa are rare in the macrovertebrate record but common in some microvertebrate assemblages.