Taphonomy of mammalian fossils across the Blancan-Irvingtonian boundary: Palm Spring Formation, Anza-Borrego desert of southern California

A taphonomic analysis of terrestrial mammal fossils from the Fish Creek-Vallecito Creek area of the Salton Trough in southern California is presented. The analysis is the first of its kind for these biochronologically important fossils and is the first step toward future paleoecological studies of these fossils. Paleoecological study of the Fish Creek-Vallecito Creek fossils is important because the study may reveal changes in paleocommunity parameters related to faunal change across the Blancan-Irvingtonian boundary. Fossils were collected from the Palm Spring and uppermost Imperial formations, consisting of 4300 m of continuously deposited tidal flat, lacustrine and fluvial sediments. Based on superposed faunas, deposition occurred during the Blancan and Irvingtonian Land Mammal “Ages”. Seventy-five genera of terrestrial mammals are recorded. The analysis is based mainly on 109 productive sites (> 10 identifiable specimens). Small mammals (< 5 kg) are considered separately from large mammals. s most often are found in low-energy, fine-grained fluvial sediments (floodplain, channel-fill). Floodplains acted as the source area for skeletal elements found in channel-lag, channel-fill, lacustrine and tidal flat sediments so that the assemblages from these depositional environments are comparable to each other throughout the 3.3 m.y. history of the Fish Creek-Vallecito Creek area. Fluvial transport and sorting of skeletal elements of from the death site is minimal as shown by the lack of hydraulic sorting, lack of abrasion and presence of elements characteristic of Voorhies groups I–III in any one fossil assemblage. Physical condition and proportions of preserved skeletal elements suggest that physical, chemical and biological processes occurring prior to fluvial transport were more important in biasing assemblages than hydraulic sorting by currents. The consistency of the results suggests that the assemblages departed from the biocenoesis to the same degree. Most fossils accumulated in sediments deposited by streams originating in the local drainage basin. The probability of mixing species from different paleocommunities is negligible and only one paleocommunity existed at any one time. The significant aspect of the taphonomic history of the fossil assemblages is concerned with the degree to which paleoecologic information (e.g., structure, diversity, relative abundance) is preserved in the paleocommunity. Diversity and relative abundance appear to be accurately preserved for large mammals and underestimated to some degree for small mammals. The analysis concludes that robust paleoecologic data can be retrieved from the suite of assemblages.