TAPHONOMIC ANALYSIS OF THE LATE CRETACEOUS KAIPAROWITS FORMATION IN THE GRAND STAIRCASE-ESCALANTE NATIONAL MONUMENT, SOUTHERN UTAH

An essential key to evaluating the quality of the fossil record lies in understanding the taphonomic biases at work in a given fossil assemblage. The taphonomy of the richly fossiliferous fluvial and floodplain deposits of the Late Cretaceous Kaiparowits Formation in the Grand Staircase-Escalante National Monument, southern Utah was examined in detail. This taphonomic analysis integrates stratigraphic, sedimentologic and paleontologic data as a tool for better evaluating the paleoecology and paleoenvironments of the Kaiparowits vertebrate fauna.

Field investigations and analysis of >150 vertebrate fossils indicate important trends and biases in fossil preservation. Significant variations in the degree of time-averaging were observed, and an unusually high variety of bone modification features were identified, including tooth traces, trample traces, insect traces and bone weathering. The greatest abundance of fossil localities was observed between the ~175-500 m levels (lower to middle portion of the 850 m thick formation). Four main types of fossil concentrations were identified: 1) microsites, 2) isolated macrosites, 3) associated macrosites and 4) articulated macrosites. Microsites are the most common (45%), characterized by highly time-averaged assemblages showing rare bone modification. Isolated and associated macrosites are the next most common fossil concentrations (33% and 20%, respectively). Both are interpreted to be moderately to highly time-averaged and display common bone modification features. The least common fossil concentrations are articulated macrosites (5%), which are typically related to rapid burial events and generally characterized by minimal time-averaging and little bone modification. These findings suggest that a diverse array of taphonomic biases affect fossil preservation. Due to renewed paleontologic interest and a wealth of new vertebrate fossil discoveries in the formation, this analysis is particularly significant because it permits a greater degree of accuracy and confidence in studies dealing with the evolutionary and ecologic history of the Kaiparowits Formation. Similar integrated taphonomic approaches may be equally useful for evaluating other fossil bearing formations.