TAPHONOMIC TRENDS IN THE CENOZOIC MARINE FOSSIL RECORD OF WESTERN NORTH AMERICA

While the Cenozoic fossil record of western North America has been extensively studied over the last century, until recently this rich knowledge has not been accessible for broad-scale paleobiological research. The NSF-funded Eastern Pacific Invertebrate Communities of the Cenozoic digitization initiative not only resulted in the mobilization of extensive museum collections from this region and time interval, but has also provided an opportunity to compile supporting taxonomic, stratigraphic and geographic range, and trait data. Such digitized collections complement, but do not substitute, efforts to document the same fossil record through the Paleobiology Database.

These data (collectively some 500,000 specimen records and 8.5 million specimens) allow for both synoptic- and occurrence-based analyses and interpretation of biodiversity trends. Here the taphonomic characteristics and potential of the approximately 4,000 Cenozoic fossil and extant molluscan species found in western North America are explored.

A first order pattern on the preserved biodiversity of this fauna is the poor preservation of particular parts of the environmental gradient (e.g., assemblages of bathyal depth) and specific habitats (e.g., rocky intertidal), which are controlled by basin-scale tectonic regimes. An additional bias is imposed by lithification and/or dissolution in older enclosing sediments. These biases are geographically and stratigraphy patchy though, such that many individual assemblages may act as surrogates for the diversity of stages that are generally depauperate or fragmentary in completeness. Our knowledge of biodiversity is not surprisingly greatest in the Quaternary where nearly 1000 species are document. Of those known from the recent fauna (>1600 named species) many occur in depths greater than 50 m (23%), are numerically rare among living faunas (22%), are known only from their type locality (22%), and are less than 5 mm in maximum diameter (21%). Rocky intertidal faunas are particularly well preserved in the Quaternary (~300 spp.) as coastal environments are influenced by eustatic sea level fluctuations. Fewer than 50 extant shelled molluscan species reported from the rocky intertidal of western North America lack a fossil record, and again these are explained by factors such as ecological rarity, geographic restriction, and small body size.