margins during the Mesozoic and Cenozoic.
WEB SOURCES
Scotese, C. R. Paleomap Project. http://www.scotese.com/Earth.htm.
Paleogeography and Geologic Evolution of North America. http://jan.ucc.nau.edu/∼rcb7/nam.html.
THREE
Reshaping North American Fish Faunas
THE ROLE OF LATE CENOZOIC CLIMATIC AND TECTONIC EVENTS
CONTENTS
Tertiary and Quaternary Events
Examples from Western North America
Examples from Northern and Eastern North America
Changes in Drainage Patterns and Stream Connections in Eastern North America
Changes in Drainage Patterns and Stream Connections in Northern and Northwestern North America
FISH ASSEMBLAGES AND POPULATIONS are continually challenged by changes in their local and regional environments. These changes could be relatively minor, such as local shifts in stream habitats caused by alterations in pools or riffle structure, or changes in access to habitats caused by shifts in the distribution of large piscivores. More extreme changes might include annual shifts in water level and/or flow rates caused by variation in precipitation. On an even larger scale, changes could reflect long-term climatic shifts, such as the onset of the Pleistocene Ice Ages, or major tectonic events, such as the uplift of the Colorado Plateau in the late Cretaceous that resulted in major alterations of stream connections and drainage patterns (G. R. Smith et al. 2010). Examples of some of the large-scale events that characterized the middle to late Cenozoic and their impacts on fishes are the topic of this chapter.
TERTIARY AND QUATERNARY EVENTS
From the previous chapter it is apparent how North American fish assemblages have been shaped by large-scale geologic and climatic events, with some fish lineages, such as lampreys, having experienced events as far back as the late Paleozoic. Such climatic and geologic events have, over time, shaped the pattern of fish diversity that characterizes North America (Figure 1.5). In addition, fish assemblages, like other biotic assemblages, have undergone continual breakup and rearrangement (Jablonski and Sepkoski 1996). In this section the emphasis is on events occurring during the Cenozoic, particularly the late Tertiary and early Quaternary Periods, while still recognizing that many of the long-term climatic and geological impacts are part of continual processes shaping our planet.
Examples from Western North America
In the early Paleozoic (Cambrian), western regions of North America subsided and were covered by seas that lasted, to varying extents, into the Jurassic (Stokes 1986). The exposed continental margin of North America cut through portions of Alberta, Montana, and Utah, essentially following the Wasatch Line in Utah (Figure 3.1) (Stokes 1986; Aberhan 1999; Dickinson 2004). Furthermore, as the eastern margin of the Pacific Plate collided with the North American Plate, primarily by subduction (sliding beneath), terranes of largely oceanic origin were added to the western margin of North America. (A terrane is a discrete, fault-bounded crustal element that is added to a craton through plate movement—a craton is a continental nucleus.) Consequently, much of the extreme western margin of present-day North America is a collage of crustal fragments that have been tacked on to the North American Craton in a complex series of events extending from the Paleozoic through the Miocene (Schermer et al. 1984; Dickinson 2004)—a process termed the accretion of allochthonous terranes.
Middle to late Tertiary changes in landform and climate were extensive throughout North America, but were particularly so in the West. The subduction of the Pacific Plate resulted in orogeny (mountain building), including the formation of the Cascade and, much later during the middle Miocene, the Sierra Nevada ranges, as well as periods of intense volcanism (Schermer et al. 1984; Dickinson 2004). Because these processes have extended well into the Cenozoic, they are certainly recent enough to have impacted the flora and fauna of western North America (Minckley et al. 1986); if not modern species, then certainly their evolutionary lineages.
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
