We selected and discussed what we view to be the most influential components of Charles A. Repenning's work on arvicoline rodents, particularly because those components will likely impact research in the field for years to come. Casual perusal of his published works and personal correspondence will reveal that we have only scratched the surface. Other aspects of his work elucidate additional complexities in his thinking about arvicolines. For example, Rep used taxonomy to express his concepts of biogeographic history. That is best exemplified by his persistent use of the name Mimomys as the proper generic assignment for arvicolines otherwise classified as Cosomys, Ogmodontomys, and Ophiomys (the latter he accepted at generic level in 2003). That was a taxonomic convention that emphasized his focus on a connection between the Eurasian and North American arvicolines. Specialists on arvicolines have a strong and general tendency to quibble over details of taxonomy, and many aspects of Rep's taxonomic practices remain controversial. Most notable are the status of Mimomys in North American Pliocene faunas, and the merits and drawbacks of inclusion within Microtus of the taxa he variously placed within Allophaiomys, Lasiopodomys, Pedomys, Pitymys, Proedromys, and Terricola.
An appreciation of his evolutionary approach to biostratigraphy and its influence on his taxonomic practices is essential for placing his work in its basic cognitive context. His emphasis on polyphyletic origins of arvicolines and his explicit claims of paraphyly of many lineages he recognized are not compatible with modern phylogenetic methods, nor are they supported by recent phylogenetic analyses of rodents. That much of his work withstood the ascendancy of cladistic methodologies is testament to the flexibility of his framework and to the fact that (despite his intransigence on philosophical matters) the details of his biochronological schemes are, to a substantial degree, immune to the philosophical context under which they were born.
Rep's extensive explorations of intercontinental correlations in the northern hemisphere forced new ways of thinking about the North American arvicoline fauna. The hypothesis of trans-Beringian dispersal is now amenable to new forms of testing that are independent of the distribution of fossils in the rock record. His claims that much of the North American arvicoline rodent fauna is the result of repeated pulses of immigration followed by endemic radiation is an obvious and fertile area for molecular phylogenetic assessments. Initial efforts to test those claims (e.g., for Microtus) found them to be wanting and recovered a monophyletic lineage of extant North American endemic species (Conroy and Cook 2000). A limitation of such methods is, of course, that they include only the extant representatives of a diverse clade that left a rich fossil record. That record was Rep's special purview and bailiwick, and he was skeptical that molecular data alone could yield an informative view of the evolutionary history of the arvicolines. The challenges he faced in establishing immigrant status for many North American arvicolines remain unresolved in many cases, but the promise of detailed Holarctic biostratigraphic correlations remains, and the clear integration of European, Asian, and North American chronologies remains a desirable goal.
Continuing investigations of Pliocene and Pleistocene fossil deposits across North America regularly yield new data on the evolutionary history and temporal and spatial distribution of arvicoline rodents. Pioneering efforts in the Great Plains by Hibbard and his colleagues and students provided a foundation upon which Rep built his more expansive biochronology. In addition to modifications based on his own extensive research, Rep constantly strove to update his chronology and his thinking in light of renewed efforts to establish reliable and detailed correlations of faunas in the Great Plains (e.g.,
Martin et al. 2000,
2003b) and new insights from west of the Great Plains (e.g.,
Barnosky and Rasmussen 1988;
Gillette et al. 1999;
Bell and Barnosky 2000;
Barnosky and Bell 2003;
Bell et al. 2004a,
Bell and Jass 2004). These new data revealed that some characteristics of proposed arvicoline provinces would be overturned by new data (e.g., the purported restriction of Allophaiomys and Microtus paroperarius to faunas east of the Rocky Mountains), that high-elevation sites might require independent biochronologies, and that formerly unexpected and anomalous taxonomic associations were becoming common in the west.
Although persistent health problems plagued him in his last years, Rep continued to struggle with new data and the challenges they raised to his arvicoline rodent biochronology. Even following his 'retirement' he was a formidable antagonist, a cantankerous collaborator, and an inspiring and thoughtful connoisseur of the interplay among geology, paleontology, climate change, and biotic history. None of the problems with which he struggled were resolved to his complete satisfaction, and his work stands in large part as a roadmap for future investigations. His published works leave a legacy of insights and proposals to lace the struggles of future practitioners who seek to follow his interests. Those who do follow will do so in the context of their times and under the scientific paradigms that govern their fields of inquiry. Some may at times be confronted with a temptation simply to dismiss Rep's ideas and proposals as anachronistic, born and developed as they were in a pre-cladistic, though firmly evolutionary, context. Any such dismissal would be unwarranted. Certainly, Rep's proposals are best understood in the context in which he framed them, but they can and should be modified and reshaped in modern contexts. We are confident that his ideas will continue to provide stimulating challenges for years to come. We regret that his arguments now must be carried largely by his formally published works, and not by the colorful, humorous, and sometimes biting wit that was a hallmark of his personal interactions and correspondence.