SHIFTING BOUNDARIES: BIOCHRONOLOGIC PROVINCIALITY

Rep recognized geographic distinction in biochronologic patterns of arvicoline rodents (i.e., provincialism) as early as 1980, when he noted that Alaskan arvicoline faunas retained a distinct biochronology relative to other portions of North America (Repenning 1980). By 1987, his thinking on the topic was clearly developing, and he indicated the necessity of using different lineages as time markers within distinct geographic regions (Repenning 1987). He thus merged faunal provinciality and biochronology of arvicolines into what was for him an inseparable conceptual whole. A preliminary attempt at a synthesis of provincialism in arvicoline rodent biochronology was published in a paper co-authored with Oldřich Fejfar, in which they outlined provincial distinctions in both Eurasia and North America (Fejfar and Repenning 1992).

Faunal regions were proposed by Fejfar and Repenning (1992) on the basis of similarity in known arvicoline faunas. Eurasia was broken into four broad faunal provinces identified as the Oriental, Beringian, Paratethyian, and Central and Northern European faunal regions. North America was split into the Beringian, Canadian, Western United States, Eastern United States, and Mexican faunal regions. Topography, climate, and latitude were seen as primary factors that controlled regional dispersal, and those factors were important in discussions concerning the evolutionary history of individual taxa (e.g., Mimomys; Repenning 1990, 2003; Repenning et al. 1995).

Specific aspects of dispersal patterns within North America were explored in detail by Rep and others (e.g., Fejfar and Repenning 1992; Repenning et al. 1995), and within these works he explored the diachronous nature of arvicoline biochronology within North America. As a specific example, he recognized differences in the faunal characterization of the Irvingtonian in the eastern and western conterminous United States (Repenning 1987; Repenning et al. 1990), and eventually came to open acknowledgment of diachroneity for the Blancan-Irvingtonian boundary (Repenning et al. 1995). Immigration of Allophaiomys into the United States was known only east of the Rocky Mountains and was interpreted to have occurred at 1.9 Ma. In the west, the Irvingtonian taxa Microtus and Phenacomys did not appear until 300,000 years later, and Allophaiomys was thought to be absent. Although new data now indicate the presence of Allophaiomys in the Rocky Mountains (Porcupine Cave, Bell et al. 2004a) and areas farther to the west (Cathedral Cave, Jass 2007; Jass and Bell 2011), those occurrences are younger in age than the eastern occurrences, and the principle discussion points of Rep's work are still salient.

Rep recognized mammal ages as faunal units in geographic space and not as indicators of specific time units (Repenning et al. 1995). He proclaimed that the co-occurrence of distinct mammal ages in geographically separated regions was a reality. That remains one of his more controversial and thought-provoking suggestions. It was within such a conceptual framework that he noted the 300,000 year time disparity in the Blancan-Irvingtonian boundary in the conterminous United States. He nonetheless concluded that application of a single faunal age name would still be acceptable in this particular situation (Repenning et al. 1995). The circumstances under which the usage of a single name would no longer be appropriate were not clearly outlined, but longer time frames (two million years) were alluded to (Repenning et al. 1995), implying that single names for faunal ages across broad geographic regions might obscure complex patterns of change in different regions (Repenning et al. 1995).

Differences in timing of immigration events in Beringia appear to have met the criteria for subdivision because Rep was not opposed to establishing separate mammal age names for Beringian faunas (Repenning et al. 1995). Philosophically, Rep appeared to approve of other attempts to create regional mammal age names (Sher 1986), even though some weaknesses were thought to exist in how those divisions were supported (Repenning and Brouwers 1992; Repenning et al. 1995). Other regions simply lacked sufficient data for mammal age terms to be applied (e.g., Mexican faunal region; Repenning et al. 1995).

One of the interesting aspects of the initial discussion of provinciality by Fejfar and Repenning (1992) was the stated potential for further sub-division of provinces, an issue re-emphasized in later publications (Repenning et al. 1995). We interpret this as an indication that they recognized the fluid nature of the boundaries that they proposed, and that future work likely would serve to test and refine their model. Rep had long recognized the potential influence of latitudinal climatic differences on faunal character, as well as longitudinal differences governed by physiographic barriers (e.g., the Rocky Mountains). In later years he openly embraced the idea that elevational differences could contribute to distinct and recognizable faunal histories as a logical extension of that thinking (e.g., Repenning 1998). Rep emphasized that the complexities of biotic history played out in the context of changing climatic conditions, geologic perturbations, and resultant modification to landscapes, all of which he considered to be tractable, understandable problems. The complications associated with teasing apart the complexities of such a mosaic were not lost on Rep, but he clearly viewed them as a challenge worth undertaking. They remain a significant challenge for future generations.