Hydrodynamic sorting and ecology of a Pleistocene mammalian assemblage from California (U.S.A.)

Abstract

Bulk sediment samples and surface collections were made from deltaic distributary channel deposits for a paleoecological investigation in the Late Pleistocene Montezuma Formation at Rodeo, California (U.S.A.). Over 1300 kg of bulk samples were screenwashed, and all particles greater than 1.3 mm were saved for analysis. About 3400 identifiable vertebrate specimens of 36 genera (40 species) were recovered. Variations from the expected per cent preservation for selected skeletal elements of small mammal general ( N = 1889), for elements of nine rodent genera compared ( N = 1136), for cheek teeth of six rodent genera ( N = 447), and sediment particle size distribution of a representative locality (464 kg sample) provide qualitative and quantitative information about the effects of hydrodynamic sorting on the preservation of small mammal fossils. Elements which are small, long, and slender, or have large surface to volume ratios are extremely rare. Frequency of preservation and size of cheek teeth decrease in the order: M1, M2, P4-M3, P3. The greatest mass of sediment is in the 1.3–1.4 mm interval where 25% of the identifiable mammal bones and teeth appear. About 80% of the fossils are in the 1.3–3.0 mm range. Hydrodynamic sorting is responsible for the low per cent preservation and relative abundance of each of three species from the Rodeo site ( Reithrodontomys raviventris, Scapanus latimanus, and Sorex sp.). Size and shape of skeletal elements are the critical factors, not distance of transport. To ascertain if a species belonged to the community near the site of deposition and to determine its original relative abundance there requires study of many factors in addition to sorting.