The last 30 years of Rep's career were spent in large part in pursuit of a deeper understanding of the evolutionary history, classification, biostratigraphic distribution, and biochronologic significance of arvicoline rodents. The evolutionary history of arvicolines extends back to at least the early Pliocene, and is revealed through a rich and geographically widespread Holarctic fossil record and an impressive array of extant taxa, including at least 151 species distributed within 28 genera (Musser and Carleton 2005). The relatively rapid rate of evolutionary change within arvicolines, combined with their reproductive and dispersal capabilities, makes them one of the more important faunal groups for biostratigraphic correlation of Pliocene and Pleistocene deposits in the Holarctic (Repenning 1987;
Repenning et al. 1990;
Bell et al. 2004b).
Rep recognized the possible significance of arvicolines for biostratigraphic correlation in the early 1960s, but his development of a fully mature research program did not occur until the 1970s. His first research study of arvicolines was conducted in pursuit of paleoenvironmental and habitat reconstructions of a late Pleistocene deposit in Alaska, but in that context arvicolines were simply listed as part of a larger faunal assemblage (Repenning et al. 1964). Within two years, he established the foundations of late Cenozoic intercontinental correlations between North America and Eurasia (Repenning 1966), a subject that would become one of the cornerstones of his research program in the later decades of his life.
His first paper that was centered entirely on arvicolines was an anatomical study of mandibular myology and osteology (Repenning 1968) that was prepared initially as a term paper for a comparative anatomy course at the University of California at Berkeley. That, his first immersion into arvicoline anatomy, was simultaneously his first major assessment of the evolutionary lineages within the group. Arvicoline rodents soon assumed a prominent role in Rep's attempts to establish intercontinental correlations between North America, Asia, and Europe. Initially, he used arvicolines in conjunction with other mammalian taxa (Repenning 1967), but they subsequently emerged in his thinking as prominent, if not dominant, biostratigraphic tools in that endeavor (Repenning and Fejfar 1977). In the decade between 1967 and 1977, Rep developed his ideas about trans-Beringian correlations, and struggled with the potential implications of his research for the established evolutionary and biogeographic framework then accepted in North America.
Early research on fossils of arvicolines in North America began in earnest in the 1940s, and was conducted mostly by Claude Hibbard and his students working in the central Great Plains. Hibbard recognized the value of arvicoline rodents for correlation and environmental reconstruction, but always centered his attention on local and regional correlations (e.g.,
Hibbard 1970a). His initial reticence to consider intercontinental correlations was a result of his conviction that correlations within North America were insufficiently understood to permit reliable correlations across continents, and he considered such attempts to be "unwise" (Hibbard 1941, p. 93), "futile" (Hibbard 1949, p. 1426), and contributory to additional confusion (Hibbard 1953, p. 410). By the mid 1960s Hibbard did recognize paleontological evidence for mammalian immigration into North America from Asia across the Bering Land Bridge (Hibbard et al. 1965), but he maintained that there was no evidence of immigration among arvicoline rodents (Hibbard 1970b). Instead, he conceived of the North American arvicoline fauna as being essentially an endemic radiation (Hibbard and Zakrzewski 1967). Rep took an opposite approach, seeking to explore the possibility of high-resolution intercontinental correlation through arvicoline rodents. However, he postponed publication of his ideas until after Hibbard's death in 1973, at least in part out of respect for Hibbard's accomplishments and Rep's concern that his new ideas were insufficiently well developed to contradict Hibbard's established reputation and authority (Repenning, email letter to Christopher Bell, 16 October, 1998, p. 1).
Repenning's first significant publication on arvicolines that post-dates Hibbard's death was prepared in collaboration with Oldřich Fejfar. They described the framework for an intercontinental correlation scheme based exclusively on arvicolines (Repenning and Fejfar 1977), and emphasized the hypothesis that the North American fauna achieved diversification not through endemic evolution, but through repeated pulses of immigration across the Bering Land Bridge. They openly acknowledged that their proposals would be controversial, and seemed to welcome the imminent challenge when they wrote "The thesis of this paper, as just outlined, is heresy according to the dogma of North American microtid paleontology which, for the past 35 years, has maintained that there were no successive invasions from the Old World" (Repenning and Fejfar 1977, p. 235). The omissions, awkward syntax, and typographic errors in that paper suggest it was a preliminary draft of a manuscript that saw print prematurely, but the paper was important because it preliminarily established the arvicoline rodent divisions of the Blancan North American land mammal age that would later figure prominently in Rep's correlation schemes.
In an extended abstract published in conjunction with the 1978 meeting of the American Quaternary Association, Rep expanded discussion of his divisions of the Blancan, and adopted the convention of using sequential roman numerals (e.g., Blancan I, Blancan II) to denote successive faunal changes that he recognized (Repenning 1978). Divisions of the younger Irvingtonian land mammal age were proposed, but not numbered, in that abstract, and Rep called specific attention to the importance of cave faunas for establishing a chronologic framework for the Pleistocene in North America. A fully developed and internally consistent presentation of an arvicoline biochronology appeared two years later in an expanded paper based on the 1978 abstract (Repenning 1980). In that paper, he established numbered divisions of the Blancan, Irvingtonian, and Rancholabrean land mammal ages, with a five-fold division of the Blancan, and two divisions each for the Irvingtonian and Rancholabrean. That scheme relied heavily on purported immigrant taxa, and the dispersal events responsible for introducing new taxa into North America were outlined in detail by 1984 (Repenning 1984). A full explication of this scheme, including the faunas and specific arvicoline occurrences upon which it was based, was published in 1987 (Repenning 1987) as a chapter in the influential Cenozoic Mammals of North America volume (Woodburne 1987a). At the time that book was conceived, the emergence of intercontinental arvicoline rodent biochronology was relatively new, justifying inclusion of a separate chapter dedicated entirely to the group (Woodburne 1987b). In that chapter, documented and/or purported dispersal events were sequentially numbered for the first time, and tied to the temporal divisions of each mammal age. That chapter became a standard reference for North American arvicoline rodent biochronology, and remained so even following the 1990 publication of substantial revisions in details of the timing of dispersal events, and the numbering sequence for the Irvingtonian and Rancholabrean divisions (Repenning et al. 1990). The later paper was missed, or ignored, by non-specialists for many years after its publication.
The establishment of a functional, biostratigraphically based biochronology for arvicoline rodents stands as one of Rep's most significant contributions to Neogene paleontology. In his many papers on arvicolines written subsequent to the core biochronology paper (Repenning 1987), Rep concentrated on refining, expanding, and updating his biochronology in light of new discoveries and interpretations emerging from various disciplines (e.g.,
Repenning and Grady 1988;
Fejfar and Repenning 1992,
Rogers et al. 1992,
Repenning and Brouwers 1992;
Repenning et al. 1995;
Bell and Repenning 1999).
Taken as a complete body of work, Rep's publications concerning arvicolines reveal significant contributions that extend beyond the establishment of the biochronology itself. Many of those contributions were born from controversy surrounding Repenning's ideas, or because they stimulated additional work by a new generation of arvicoline enthusiasts. Some of those areas of controversy reveal not only the methodological and cognitive approaches Rep used in his work, but also his thoughts on the modern and continuing development of the evolutionary history, classification, temporal duration, and biogeography of arvicolines. We recognize and explore here three significant contributions Rep made to the study of arvicolines. We selected areas that encompass his philosophical approach to the study of fossils, taxonomy, and biostratigraphy, as well as his emphasis on intercontinental correlations, and the importance of faunal provinciality in establishing biostratigraphic correlations and their biochronologic implications.