FIGURE 2.1. Paleozoic and Mesozoic landmarks in fish evolution. Based on Moy-Thomas and Miles (1971), Nelson (2006), and Helfman et al. (2009).

      Fish evolution began in the early Paleozoic Period in the late Cambrian or early Ordovician, approximately 500–470 million years ago (mya) (dating of geologic time periods follows Walker and Geissman 2009). During the Silurian and Devonian (444 to 359 mya), there was widespread radiation of both jawless and jawed fish lineages (Figure 2.1). Because of this, the Devonian is often referred to as the Age of Fishes. By the close of the Paleozoic, approximately 250 mya, body forms had evolved that differed very little from those living today (Moy-Thomas and Miles 1971). Modern bony fishes (fishes in the division Teleostei) appeared in the lower Mesozoic (middle or late Triassic, 245–202 mya), and representative forms of most major groups (i.e., orders or divisions) of modern fishes were present at least by the middle Mesozoic (Jurassic, 202–145 mya [Figure 2.1]) (Nelson 2006; Helfman et al. 2009). In fact, almost half of the currently recognized orders of teleostean fishes have fossil records that reach to the Cretaceous Period of the Mesozoic, some 145 to 65 mya (Figure 2.1) (Helfman et al. 2009). Consequently, given the great age of many of the major fish lineages, any understanding of fish biogeography requires looking at a broad slice of the earth’s dynamic geologic history.

      A Dynamic Earth

      During the Carboniferous Period of the late Paleozoic, a time of active radiation in fishes, precursors to the present-day continents collided to form the single large, but highly dynamic and ephemeral, landmass of Pangea (Figure 2.2; Box 2.1). Although subsequently reworked by geological processes such as uplift and erosion, Pangean geography included familiar elements, including an ancestral Mississippi River in central Pangea that flowed through a gap bounded by the Southern Appalachian Mountains and the Ouachita Mountains (Redfern 2001). The process of mountain building (orogeny) that formed the Ouachita and Alleghenian-Appalachian ranges was driven by the collision of the supercontinents Laurasia and Gondwana along the region now recognized as the Mississippi Embayment, as one of the final stages in the assembly of Pangea (Redfern 2001). During the maximum extent of Pangea, what is now the eastern continental margin of North America was adjacent to northwestern Africa and the northeastern region of North America abutted Western Europe (Figure 2.2). Pangea gradually began to break up during the middle Jurassic (176–161 mya) with the intrusion of the central Atlantic Ocean between the northern (Laurasia) and southern (Gondwana) landmasses (Torsvik and Van der Voo 2002). Continued rifting, especially through the late Jurassic to middle Tertiary, resulted in the present-day arrangement of continents (Cracraft 1974; Briggs 1987; Hocutt 1987). Because forms ancestral to most modern fish lineages (but generally not modern orders or families) were present prior to the breakup of Pangea, the subsequent movements of tectonic plates and their associated faunas were primary factors in the formation of fish assemblage composition (Figure 2.1), although in some cases, phylogenies are too poorly understood or fossil material is lacking to clearly establish an area of origin.

      BOX 2.1 • Plate Tectonics

      The idea that continents change position over time, or drift, is understood today as a scientific fact and is as important to geology and biogeography as evolution is to an understanding of biology and biogeography. However, it was not that long ago that continental drift was quite a controversial issue, in spite of the fact that a quick glance at a world map suggested that continental shapes could be fitted together like a crude jigsaw puzzle—a point recognized as early as 1620 by Francis Bacon (Cattermole 2000). The idea that continents move was proposed early in the twentieth century (1910 and 1912) by Frederick B. Taylor, H. D. Baker, and A. L. Wegener, with the German meteorologist Wegener generally recognized as the father of the modern theory of continental drift (Briggs 1995; Cattermole 2000). However, the reigning view at that time was that the earth’s crust was solid and that crustal movement was therefore not possible, and that former land connections (so-called land bridges) were largely responsible for the movement of many organisms between continents. In fact, my first course in biogeography used the classic text by Darlington (1957) in which the issue of continental drift, although mentioned briefly, was largely discounted. By the late 1950s and 1960s the issue of continental drift was gaining increasing attention, and by 1970 the theory was generally accepted as fact (Cattermole 2000).

      The reasons for the fairly rapid turnaround had to do with advancement in the tools of physical geography, paleontology, and biogeography. The understanding that molten rocks can capture the orientation of the magnetic field at the time that they cool provided important insight into past continental orientations. If continents were fixed in their positions, then past magnetic orientations should be coincident with present-day orientations. However, this is obviously not the case. Indeed, magnetic anomalies (the nonalignment of the magnetic field in rocks with the present-day magnetic field) are an important data source in the reconstruction of the earth’s surface (Torsvik et al. 2001). Today the earth’s surface is understood to comprise a series of semirigid plates that are moving relative to one another, driven by the powerful convection currents of the underlying mantle. Because the continents are riding on the plates, the term continental drift has been replaced by plate tectonics (Cattermole 2000; Cox and Moore 2005). Additional evidence for plate tectonics comes from modern studies of the alignment of continents and the paleodistribution of organisms as shown by fossils.

      Ages of North American Fish Families

      Understanding how long particular fish groups have inhabited North America is, at best, a difficult endeavor. The direct evidence is based on the fossil record, which is usually incomplete and requires access to rock layers of various ages (i.e., the rock formations of particular ages must be exposed by weathering or excavation such as during road construction). In addition, even if there is a rock outcrop of a particular age, it might only represent a particular kind of habitat from one particular region and thus show great ecological bias (Patterson 1981). This would be akin to trying to understand the entire modern North American fish fauna by sampling only a few low-gradient rivers where they enter into the sea or several isolated lakes. In addition, the ages represented by fossils are likely minimum ages because, from the standpoint of vicariance biogeography, groups are older than their oldest fossil (Box 2.2; Parenti 1981). However, in spite of its shortcomings, the fossil record is often the best evidence that is available.

      BOX 2.2 • Biogeographic Theory

      From the standpoint of interpreting broadscale distribution patterns of organisms, there are two major paradigms of biogeography: dispersal explanations and vicariance explanations.

      DISPERSAL EXPLANATIONS

      There is a natural tendency of organisms to disperse within areas of suitable habitat, and certainly many organisms, including fishes, have welldefined long-distance dispersal patterns, achieved either through adult movement or through dispersal of larval stages. In the dispersal model, organisms are