DETERMINING THE GEOLOGIC AGE OF THE DUFF BROWN TANK CONTINENTAL MOLLUSK LOCAL FAUNA (COCONINO COUNTY, ARIZONA): AN IMPROVING SCIENCE (OR, WHY DOES IT HAVE TO BE SO HARD?)
The Duff Brown Tank lacustrine assemblage is dominated by relatively well-preserved viviparid, pluerocerid, hydrobiid, physid, and planorbid taxa. These are generalized morphologies that C.A. White (>1870s) would likely have identified as previously named taxa (as a lumper) and F.B. Meek (<1870s) would likely have named as new (as a nonlumper). In either case, a problem of age assessment would exist. Currently, continental mollusks species identification accepts too much morphological variability, which promotes species of long duration, living over wide areas.
When paleontologists began to document the adaptive radiation of mammals and construct "time" intervals with diagnostic taxa in the early 1900s, molluscan paleontologists thought of formations much like assemblage zones (e.g., Fort Union time). Not until the mid 1940s did T.-C. Yen correlate mammalian age data with associated mollusks. However, a persistent issue was the use of White's species concepts. Thus A. La Rocque, J. H. Hanley, and others assigned Paleocene names to Eocene specimens that represented an unreasonable spatial and temporal extension of certain taxa. Mammalian studies have continued to progress and refine the NALMA system, and generally provide a quality basis for placing mollusk fossils into a biochronology framework. Studies to date indicate that the Duff Brown Tank local fauna is likely early Eocene (Wasatchian), although a late Paleocene (Tiffanian-Clarkforkian) age cannot be ruled out.