FIGURE 3.1. The approximate early Jurassic continental margin of North American. Based on Stokes (1986), Aberhan (1999), and Dickinson (2004)

      In addition to the major geomorphic changes of mountain building and volcanism, the composition and distribution of Western fish assemblages have been shaped by a general climatic trend toward increasing aridity, resulting in the drying of large lakes and shrinking or loss of streams present during the Miocene and Pliocene, and the concomitant extinction of many populations (G. R. Smith 1978). The restriction or extirpation of some of the early components of the western fish fauna, such as mooneyes, smelts, pikes, sunfishes, and catfishes, was likely associated with habitat alterations brought on by uplift and climatic shifts that changed the low-gradient, meandering rivers of the Oligocene to higher-gradient streams flowing over diverse landforms, and with altered drainages that characterized the Miocene and later streams (Minckley et al. 1986

      The general trend of increasing aridity, coupled with aperiodic severe droughts lasting decades or even hundreds of years, prompted Matthews (1998) to suggest that western fishes must have suffered through periodic extirpations followed by long periods of recolonization. In fact, in reference to the development of the western North American fish fauna, Minckley et al. (1986) commented that “taxa that persist have dealt with far more spectacular geologic and climatic events than their counterparts in other parts of the Continent.” In fact, fishes of western North America have suffered higher extinction rates compared to eastern fishes. As a result, many western species are relics of groups that were once more speciose but which have lost species through extinction over the past 1–3 million years (G. R. Smith et al. 2010).

      Colorado Plateau

      Distribution patterns of modern western fish faunas tend to correspond to the continental subplates formed from the accreted terranes, and drainages that extend into adjacent subplates tend to have faunas that have been derived from several sources (Minckley et al. 1986). For example, the Colorado Plateau (Figure 3.2) is both a tectonic and physiographic province that has remained internally stable. At the close of the Cretaceous, the region of the Plateau was near sea level; it then experienced approximately 2 km of uplift during the Cenozoic, especially during the Pliocene. The uplift occurred in two phases, with the second phase taking place only within the last 5 million years (Morgan and Swanberg 1985). The Colorado Plateau is drained by the Colorado River; hence, elevation changes of the Plateau have had major impacts on the directions of flow and drainage connectivity. The uplift resulted in the isolation of the Colorado Plateau as north-flowing streams from central Arizona were interrupted. Also, the uplift of the Wasatch Front, starting in the early Eocene, and the subsequent drop of the Great Basin in late Oligocene isolated the upper Colorado River fauna from that of the Great Basin along its northwestern margin (Figure 3.2).

      Origins of the upper Colorado River fish fauna (including watersheds of the Green and Colorado rivers) are ancient, likely having begun in the Oligocene and Miocene in streams draining the uplifted Rocky Mountains and flowing across the Colorado Plateau to interior basins in Colorado and New Mexico (the Miocene Bidahochi Basin) or northwestern Arizona (the Miocene Hualapai Basin) (Figure 3.2) (Oakey et al. 2004). A middle section of the Colorado River, currently comprising the Little Colorado, Virgin, and White rivers, drained to the southwest, while a third section, including the Gila River, was incorporated into the drainage after the retreat of the Bouse Miocene/Early Pliocene Embayment (Minckley et al. 1986)—an embayment that, while first thought to be a marine or estuarine extension of the Gulf of California, now appears to have been a series of lakes, with perhaps the lower being saline (Dillon and Ehlig 1993; Spencer and Patchett 1997; Roskowski et al. 2007). The upper and lower Colorado River systems were joined perhaps 10.6 to 3.3 mya, following headward erosion of streams of the middle and lower Colorado watersheds and through reoccupation and reversal of flow in older channels (Figure 3.2). Prior to this time, the upper Colorado River drainages flowed into a closed basin. It was not until the Pliocene that the Colorado River reached the Gulf of California (Minckley et al. 1986; Powell 2005).

      The origins of the dominant components of the mainstem Colorado fish assemblage (Colorado Pikeminnow, Ptychocheilus lucius; Humpback Chub, Gila cypha; Roundtail Chub, G. robusta; Bonytail Chub, G. elegans; Speckled Dace, Rhinichthys osculus; Razorback Sucker, Xyrauchen texanus; Bluehead Sucker, Catostomus discobolus; and Flannelmouth Sucker, C. latipinnis) can be traced to these various geological events (Minckley et al. 1986; G. R. Smith et al. 2002). Most of the species have their closest relationships with populations in the north and west, but some also show relationships to the south. Colorado Pikeminnow, and Roundtail, Humpback, and Bonytail chubs, show relationships to the north and west, including the Sacramento-San Joaquin Basin in what is now California, but also to the Miocene Bidahochi Lake deposits to the southeast (Figure 3.2).