63 YEARS OF OBFUSCATION: CONCEPTUAL, FAUNAL, SPATIAL, AND TEMPORAL USES OF THE TERM “IRVINGTONIAN”

The Irvingtonian Land Mammal “Age” was proposed by Don Savage in 1951 as one of two mammal “ages” intended to encompass Pleistocene time in western North America. It was based on the fauna from the Irvington gravels in Alameda County, California, and, in Savage’s words, was “marked by the absence of Bison.” The operational definition of the Irvingtonian based on the absence of Bison resulted in considerable instability and a general lack of clarity in recognizing Irvingtonian faunas. Alternative definitions were offered in the literature for many decades, but no consensus emerged on a proper and widely applicable definition. As a result, the term still lacks clarity after 63 years of continuous usage. That lack of clarity stimulated alternative conceptual approaches to the definition of land mammal “ages,” and sparked a discussion about the geographic extent of land mammal “ages” and their applicability along latitudinal, longitudinal, and elevational gradients.

Current proposals for the definition include single-taxon definitions (using various taxa) that are conceptualized as being applicable across the entire North American continent, the continent south of 55 degrees north latitude, or only to generalized (but not rigorously specified) geographic provinces on scales smaller than the continent. Irvingtonian faunas at high elevations in western North America are distinct, both in taxonomic associations and temporal durations of taxa, from Irvingtonian faunas from lower elevation sites elsewhere in the United States. Specific proposals for diachronous and spatially distinct boundaries exist and constitute fertile areas for future discussion.

Consensus on the definition of the Irvingtonian remains an elusive goal. Historical inertia, competing egos, apathy, ignorance, and inadequate data all contributed to a long-standing uncertainty about how to recognize, conceptualize, and discuss an interesting portion of mammalian faunal history in North America. Enhanced clarity of the conceptual bounds, spatial scope, taxonomic definition and characterization of the Irvingtonian will help future generations to resolve this long-standing problem. Broad discussions of these issues are needed, and might, possibly, benefit from insights from scientists in older generations.