MICROVERTEBRATE BIOSTRATIGRAPHY OF THE UPPER TRIASSIC CHINLE GROUP, SOUTHWESTERN U.S.A
A long-term goal of vertebrate biostratigraphy 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 on this goal based on the distribution of microvertebrate fossils in the Upper Triassic Chinle Group. This biostratigraphic hypothesis independently corroborates the existing macrovertebrate biostratigraphy, recognizing four temporally successive assemblages: the Otischalkian, Adamanian, Revueltian, and Apachean. 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 ÒXenacanthusÓ moorei, 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. The archosauromorph T. buettneri is an index taxon of Otischalkian-Adamanian age, and its sister species, T. jacobsi similarly marks an interval of Adamanian-early Revueltian time. Other Adamanian index taxa include the procolophonid(?) Colognathus obscuris, the ornithischians Tecovasaurus and Revueltosaurus hunti and several unnamed taxa. Revueltian index taxa include the chondrichthyan Reticulodus, the ornithischian 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 co-occur 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. Many of these taxa are rare in the macrovertebrate record but not uncommon in microvertebrate assemblages.