Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 3
Presentation Time: 8:40 AM


EATON, Jeffrey G., Geosciences, Weber State Univ, 2507 University Circle, Ogden, UT 84408-2507,

The traditional method of discovering microvertebrate localities has been dependent on surface prospecting of outcrops for accumulations of fossil materials. For several years I have been randomly sampling fine-grained organic-bearing mudstones and processing the recovered matrix by wet screen-washing. This "blind washing" method successfully produced significant microvertebrate materials (along with ostracods and other invertebrates) for about 50% of the units sampled and has produced the first microfossils from the Henefer and Frontier formations and for the Dakota Formation in Cedar Canyon. The advantages of this method are: 1) it permits sampling in steep sandstone dominated sequences or in areas with poor outcrops; 2) it has produced microvertebrates from units that previously had been considered unfossiliferous; 3) the recovered delicate microfossils are often less fragmentary than in localities where fluvial transport has concentrated fossils; 4) it permits continuous sampling throughout the stratigraphic section increasing biostratigraphic resolution; 5) it provides broader possibilities for sampling across the geographic extent of a unit; and 6) taxa are less paleoecologically mixed than those recovered from hydraulically concentrated accumulations (often the kind of locality sampled as a result of surface prospecting). The disadvantages of this method include: 1) fossils are not abundant in these floodplain mudstones and large quantities of matrix must be collected and processed; 2) the resultant concentrate consists of very small bone that is time consuming to sort; 3) this method tends to produce a poor record of larger organisms (e.g., turtles and dinosaurs); and 4) the taxa recovered (teleost fish, lizards, salamanders, frogs) are relatively poorly studied in North America. Despite the limitations of the method, it is clear that integrative study of microvertebrate faunas recovered by "blind washing" will lead to improved terrestrial biostratigraphic schemes and more detailed paleoecologic interpretations of faunas.