COLOGNATHUS OBSCURUS CASE, AN UNUSUAL VERTEBRATE INDEX FOSSIL OF ADAMANIAN (ST. JOHNSIAN: LATEST CARNIAN) TIME, FROM THE UPPER TRIASSIC OF THE AMERICAN SOUTHWEST

Over the past 90 years workers have collected approximately 25 specimens of the unusual gnathostome Colognathus obscurus Case from Upper Triassic strata in the American Southwest. The vast majority of these occurrences are from the Tecovas Formation of West Texas, and most Texan localities are in classic collecting areas near Kalgary in Crosby County. Other localities include Palo Duro Canyon in West Texas, a microvertebrate locality from the Los Esteros Member in the Santa Rosa Formation of central New Mexico, and the "Dying Grounds" in the Blue Mesa Member of the Petrified Forest Formation in eastern Arizona. At these localities, Colognathus co-occurs with age-diagnostic nonmarine archosaurian tetrapods, including the aetosaur Stagonolepis, "Rutiodon-grade" phytosaurs, and the putative ornithischian Krzyzanowskisaurus. Thus, all of these localities bear index taxa of the St. Johnsian sub land-vertebrate faunachron (lvf) of the Adamanian lvf, are therefore of early Late Triassic (latest Carnian) age. Identifiable Colognathus specimens thus far consist of dentigerous fragments, the most complete of which include a conical anterior tooth and an elongate molariform tooth. This latter tooth is readily identified as a shed crown, rendering Colognathus as an index fossil of the St. Johnsian sub-lvf. Colognathus' biostratigraphic utility is important because its actual affinities remain unclear. Case originally considered it an osteichthyan, although more recent workers have typically considered it an amniote, perhaps allied with procolophonids. Detailed study of tooth implantation and enamel microstructure should clarify this question, but were not feasible until recently due to a lack of specimens. With the advent of microvertebrate methods, the number of Colognathus specimens has risen dramatically, demonstrating its potential as an index fossil by its widespread occurrence, restricted stratigraphic range, and easy recognition, even if its affinities remain enigmatic.