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TWO

Origin and Derivation of the North American Freshwater Fish Fauna

CONTENTS

Assembling a Fauna: Fish Evolution and Plate Tectonics

A Dynamic Earth

Ages of North American Fish Families

Origins of North American Fish Families

Numerically Dominant Families

Continental Positions

Ages and Origins of Major Fish Families

ASSEMBLING A FAUNA: FISH EVOLUTION AND PLATE TECTONICS

STUDIES OF FISH DISTRIBUTION and ecology are often initiated by making a series of collections in various aquatic habitats. In doing so, there is a tendency to consider fishes taken in each particular mesohabitat, such as a pond, lake shore, or stream riffle, to be part of a natural assemblage developed as a unit over evolutionary and ecological time through the interaction of local processes. However, even discounting the recent major role of humans in introducing fishes outside of their natural ranges, this assumption that species in the assemblage share a long history of coexistence may not be valid. As pointed out by Brooks and McLennan (1991), Matthews (1998), and others, in addition to local determinism in shaping assemblage composition, contemporary species assemblages may also be due to the association of the species’ ancestors in that particular geographic region, or the species may have evolved within different assemblages and one or more entered the modern assemblage through dispersal.

For instance, among the resident species in the diverse (43 species) fish assemblage of the Piney Creek watershed of Arkansas, some species have affinities with faunas to the northeast, others to the east, and yet others to the southeast (Matthews 1998). This suggests that a more realistic view is that assemblages are composed of a mixture of species that have different origins and ages, and have been interacting for widely different periods of time. Of course, this situation is now made even more extreme by the rapid and widespread introduction of nonnative species (Courtenay et al. 1986; Fuller et al. 1999) and the resulting homogenization of faunas (Rahel 2000, 2002; Olden and Poff 2003). This chapter deals with the initial largescale filters affecting the origin of the North American fish fauna (see figure in Part 1).

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 assumed to have migrated across preexisting barriers and there are many examples where this has occurred. However, when we consider the present-day distribution of related taxa over widely separated areas with no intervening populations (e.g., a disjunct distribution), then the dispersal explanation becomes more difficult.

The dispersal model was espoused by numerous early biogeographers, such as Darwin’s contemporary, Alfred Russell Wallace (1876), and later William D. Matthew (1915) and Phillip J. Darlington (1957). Among ichthyologists, Briggs (1974, 1995, and included papers) has been a strong proponent for the importance of dispersal as a primary mechanism.

VICARIANCE EXPLANATIONS

The basic tenet is that organisms are passively transported by movement of tectonic plates or by other geological means. If this occurs, then several or more taxa should share common distribution patterns, where the distribution of each taxon is referred to as a track. As such, a major starting point in vicariance biogeography is the search for common patterns of distribution (i.e., generalized tracks) among different taxa. If a common pattern of distribution exists for two or more monophyletic taxa, then this suggests that the generalized track may be due to geologic events (Croizat et al. 1974; Wiley 1981; Grande 1990). The emphasis in vicariance biogeography is on patterns generated by many, and not necessarily closely related, taxa. (In contrast, while not ignoring the generality of patterns, dispersalists have, at least historically, focused more on individual taxa.) The range of a species can be disrupted by the formation of a barrier (a vicariant event) so that a formerly contiguous population is split into separate populations (termed vicariance).

Pioneering studies by Croizat (1958; Croizat et al. 1974) helped form the basis for vicariance biogeography. For instance, Croizat’s panbiogeography (1958) ultimately worked as “a major catalyst for change during the 1960s resurgence of interest in biogeographical thought” (Keast 1991). Croizat amassed distributional patterns of species (e.g., tracks) and stressed the importance of concordant patterns (e.g., generalized tracks), even though in his 1958 book he still discounted the role of continental drift. Strong ichthyological proponents of vicariance biogeography have included Gareth Nelson and Norman Platnick (e.g., Nelson and Platnick 1981), Edward Wiley (e.g., 1981), and the late Donn Rosen (e.g., Rosen 1978).

SYNOPSIS OF PARADIGMS IN BIOGEOGRAPHY

In a dispersal model, the barrier is older than at least one of the isolated populations and the age of the barrier is older than the disjunction in range. In the vicariance model, the populations predate the age of the barrier and ages of the barrier and the disjunction in the range are the same. While there has been considerable argument among biogeographers about the relative merits of each major explanation, a synthesis of views is, undoubtedly, required to understand the distribution of fishes (Wiley 1981; Briggs 1995; Moyle and Cech 2004)

FIGURE 2.2. The configuration of the major continental landmasses at the close of the Paleozoic (250 mya) as Pangea approached its maximum extent. Based on Torsvik and Van der Voo (2002) and Torsvik and Cocks (2004).

Ages can also be determined indirectly from a well-developed phylogeny if there are fossils or geologic events available for calibration of molecular divergence times (Box 2.3; Lieberman 2003). For example, based on a calibrated molecular clock analysis, species’ divergence times within logperches (a group of darters in the genus Percina) ranged from 4.20 to 0.42 mya, with most speciation events taking place in the Pleistocene (Near and Benard 2004). The divergence times were based on the assumption that there is a constant rate of gene substitutions over time, and that by comparing the degree of genetic divergence among darter lineages, it is possible to convert the degree of divergence to a time estimate—the “molecular clock.” Because it is known that rates of gene substitution vary among taxonomic groups (Britten 1986; Avise 2004) and, consequently, that molecular clocks need to be calibrated using related taxa, Near and Benard (2004) used rates of gene substitution from the family Centrarchidae (in the same order, Perciformes, as the darters) and applied this to their analysis of logperches to generate their divergence time estimates.

BOX 2.3 • Phylogenies and Cladistic Methodology

The study of evolutionary relationships of species or higher taxa is the field of systematic biology. The concepts and methodologies underlying how evolutionary relationships are studied, and how best to portray the resultant phylogenies, have been areas of considerable debate and advancement over the last several decades. (For an overview, see Mayden and Wiley [1992] and Mayden and Wood [1995].) Concurrent with the development of principles and procedures of systematic biology, molecular data, first as protein analyses and then broadened to include mitochondrial and genomic DNA information, have complemented morphological, ecological, and behavioral data that are used in developing phylogenies. Phylogenetic (i.e., evolutionary) relationships are shown hierarchically in branching diagrams referred to as dendrograms or cladograms, depending on the methodology used to construct them. For the most part, especially among ichthyologists, phylogenies follow methods proposed by the German biologist Willi Hennig (English translation, 1966)—the method of phylogenetic systematics. This approach has been further developed by Niles Eldredge and Joel Cracraft (1980), Gareth Nelson and Norman Platnick (1981), Edward Wiley (1981), and many others.

Basic principles of the cladistic method are that (1) relationships among groups are based primarily on branching points in evolution and not on degrees of divergence; (2) recognized groups should be derived from a single ancestral group (the principle of monophyly); and (3) taxa should be recognized on the basis of possessing shared, derived characters (synapomorphies) (Mayr and Ashlock 1991; Brooks and McLennan 1991). Sister groups are derived from the same common ancestor. A major goal of cladistic analysis is the recognition of synapomorphies, the derived (homologous) characters. Convergent evolution can result in organisms possessing structurally analogous but not evolutionarily related characters (termed homoplasies). Cladograms should be considered as hypotheses of evolutionary relationships. Support for cladistic hypotheses increases with the number of synapomorphies used in a study (i.e., studies based on few characters can be misleading), and by the number of studies, based on different characters, that come to the same or similar conclusions.

Age information from fossils or calibrated molecular phylogenies is available for 27 families of North American freshwater fishes (Figure 2.3). One family, the lampreys (Petromyzontidae), likely dates to the Paleozoic, and 5 groups (bowfins, Amiidae; pikes, Esocidae; sturgeons, Acipenseridae; paddlefishes, Polyodontidae; and gars, Lepisosteidae) have been present since the Cretaceous Period of the late Mesozoic. The remaining 21 families all date within the Cenozoic. Although 6 of the 27 families (22%) were represented prior to the Cenozoic, considering the current number of species per family, the ancestors of only 1.8% of the North American fish fauna occurred earlier than the Cenozoic. Within the Tertiary Period of the Cenozoic, 11 families are represented in the Paleogene Period (Paleocene to Oligocene epochs), and the remaining 10 families are represented in the Neogene Period (Miocene to Pliocene epochs).

Paleogene families include the ictalurids, percopsids, clupeids, salmonids, moronids, hiodontids, catostomids, centrarchids, aphredoderids, umbrids, and cyprinids. Neogene families are represented by the goodeids, poeciliids, percids, cichlids, fundulids, cyprinodontids, cottids, gasterosteids, atherinopsids, and sciaenids. Although the second most speciose family of North American freshwater fishes, the Percidae, is known from fossils only from the Pleistocene, calibrated molecular phylogenies suggest a much earlier occurrence. The separation of darters from nondarter percids dates to 19.8 mya (Carlson et al. 2009) and within the darter genus Nothonotus, the age of the most recent common ancestor dates to 18.5 mya (Near and Keck 2005). Consequently, percids likely occurred in North America at least by the early Miocene (approximately 23 mya). Seventy-eight percent of the 27 major families were present in North America by the early Miocene (23–16 mya) and were thus affected by numerous geologic and climatic events of the late Tertiary.

FIGURE 2.3. The earliest representation of major fish families in North America based on the first occurrence of fossils or from calibrated molecular phylogenies. Because the earliest fossils represent a minimal age of origin, families could be much older. Within the Cenozoic, geologic ages refer to epochs; within the Mesozoic and Paleozoic, ages refer to periods. Numbers at the top of each column are the beginning age (mya) of each geologic age or period. Numbers after families indicate sources; gaps in fossil record are not shown.

SOURCES: 1. Carlson et al. (2009), 2. Cavender (1986), 3. Cavender (1991), 4. Grande (1982), 5. Grande (1984), 6. Grande (1999), 7. Grande and Bemis (1991), 8. Grande and Bemis (1996), 9. Grande et al. (2002), 10. Mateos et al. (2002), 11. Meyer and Lydeard (1993), 12. Miller (1981), 13. Murray (2001a), 14. Murray and Wilson (1996), 15. Myers (1966), 16. Near and Keck (2005), 17. Near et al. (2005), 18. Nelson (2006), 19. Webb et al. (2004), 20. Wilson and Williams (1992).

In western North America, a freshwater fauna dominated by teleosts first appeared by the late Paleocene, followed by the expansion of an essentially modern fauna by the Oligocene and Miocene (Minckley et al. 1986). The western fauna during the Eocene (56–34 mya) and Oligocene (34–23 mya) shared forms with an eastern fauna, including paddlefishes, gars, sturgeons, bowfins, salmon and trout, mooneyes, suckers, catfishes, troutperch, and pickerel (Grande 1984; Minckley et al. 1986; Grande and Lundberg 1988; Grande 1999). The Oligocene fauna included elements from the earlier fauna, such as mooneyes, salmon and trout, and pickerel, as well as from more recent groups, such as minnows, atherinopsids, pupfishes, topminnows, sticklebacks, bass and sunfishes, surfperches, and sculpin (Minckley et al. 1986). Of the nonteleosts, sturgeon are represented by extant western forms, but gars, paddlefishes, and bowfin are now absent from the western fauna.

Although ecologists, until recently, have tended to focus more on current faunas and less on historical aspects, knowledge of the varying ages of occupation of fish groups in North America is of paramount importance to our understanding of fish assemblages and the extent and duration of coevolutionary processes. In addition, recent studies have stressed the importance of incorporating information on evolutionary relationships of component species (i.e., phylogeny) into studies of community ecology (Webb et al. 2002).

As is evident from Figure 2.3 and the previous paragraphs, fish groups vary widely in their ages of occupation in North America. Consequently, the forces shaping the evolution of morphology, physiology, and behavior of species making up present-day assemblages are likely not to be found by only looking within the contemporary assemblage. Instead, selective pressures leading to various traits may date to earlier time periods and may not even include the present-day assemblage. As an example, assemblages that have included pike have changed over time since the Paleocene (65–56 mya), when pike would have been part of fish assemblages including osteoglossomorphs, percopsiforms, amiids, gonorynchids, lepisosteids, asineopids (now extinct), osmerids, clupeids, cyprinoids (possibly catostomids), and ictalurids (Wilson and Williams 1992). Although feeding and morphological specializations show little change in pike, the community relationships have since changed a great deal, consequently, and “major adaptations of pike evolved before modern predator-prey systems existed” (Wilson and Williams 1992).

Origins of North American Fish Families

As with differences in ages, North American fish families exhibit a variety of origins, including archaic groups and some more recent groups whose origins can be traced to Pangean and Laurasian faunas (Figure 2.4). Of the 50 North American fish families listed by Burr and Mayden (1992), half show a marine origin. In some groups the radiation from the marine environment into fresh water occurred early, as with the bowfin subfamily Amiinae (family Amiidae) that has occupied freshwater habitats in the Northern Hemisphere since the late Cretaceous, some 90 mya (Grande and Bemis 1999). The second-largest group has a North American origin (including those originating in old landmasses of Pangea or Laurasia), followed by groups originating in Central and South America and Eurasia (Figure 2.4).

FIGURE 2.4. General origins of North American fish families. Phylogenetic and/or fossil information generally does not allow enough resolution to determine modern continental origins listed as arising in Laurasia/Pangea.

SOURCES: 1. Berra (2001), 2. Briggs (1986), 3. Burr and Mayden (1992), 4. Cavender (1986), 5. Cavender (1991), 6. Collette and Banarescu (1977), 7. Echelle and Echelle (1992), 8. S. A. Foster et al. (2003), 9. Gilbert (1976), 10. Grande (1984), 11. Grande (1999), 12. Grande and Bemis (1991), 13. Grande and Bemis (1996), 14. Grande and Bemis (1998), 15. Grande and Bemis (1999), 16. Grande et al. (2002), 17. Hrbek and Larson (1999), 18. Miller and Smith (1986), 19. Moyle and Cech (2004), 20. Parenti (1981), 21. Patterson (1981), 22. G. R. Smith and Stearley (1989), 23. Wiley (1976), 24. Wilson and Williams (1992).

The 15 fish families containing 90% of the modern species have their origins in Eurasia (minnows and suckers); Central America (livebearers and topminnows); North America, including Pangean/Laurasian elements (catfishes, trouts and salmons, goodeids, sunfishes, and perches); the marine environment (New World silversides, pupfishes, sculpins, lampreys, herrings); and South America (cichlids). The histories of these groups are treated in more detail in the following section.

Numerically Dominant Families

Plate Tectonics, Ages, and Origins

This section examines in more detail how potential fish assemblages have changed over time by the addition of new taxa and relates the arrival of these taxa to the positions of major landmasses at various times in the past. It is primarily limited to families composing 90% of the North American fish fauna, although sturgeons and pickerels are included because of conservation interest and past or present economic importance. Families are listed by decreasing age of occurrence (largely as determined from fossils) in North America. For the goal of relating fish distribution to continental positions and connections over geological time, I have followed Cracraft (1974) and Matthews (1998) in portraying landmasses as a series of blocks (Figure 2.5). Although temperatures in Antarctica were generally too cold for the survival of freshwater fishes (Matthews 1998), I have included it in the figure to help with orientation. Simple diagrams might seem to suggest otherwise, but movements of landmasses were not necessarily unidirectional, so that connections between different elements may have been made and broken numerous times. A case in point is the union of eastern North America and Europe during the Jurassic and early Cretaceous when these landmasses were joined and then separated several times (A. G. Smith et al. 1994).

A. Early Triassic (245 mya)

B. Late Jurassic (161 mya)

C. Late Cretaceous (70 mya)

D. Cenozoic-Paleocene (60 mya)

E. Cenozoic-Middle Eocene (45 mya)

F. Cenozoic-Middle Miocene (10-15 mya)

G. Present day

FIGURE 2.5. Schematic diagrams of the relative positions of oceans and landmasses from the early Mesozoic to the present. Arrows indicate possible connections between landmasses. Sources are given in the accompanying text. Landmasses and oceans are not drawn to scale.

Continental Positions

MESOZOIC, EARLY TRIASSIC (245 MYA) The Pangean supercontinent reached its maximum extent in the early Triassic, then began to break apart by the late Triassic (Figure 2.5A). At its maximum, the supercontinents of Laurasia and Gondwana were joined, providing dispersal routes for numerous mobile terrestrial and freshwater organisms from pole to pole and east to west (A. G. Smith et al. 1994; Scotese 2002). During this time, tetrapod vertebrates were essentially cosmopolitan with no evidence of latitudinal variation (Briggs 1995).

MESOZOIC, LATE JURASSIC (161 MYA) The northern and southern supercontinents of Laurasia and Gondwana separated from each other as the young central Atlantic Ocean began to increase in size (Figure 2.5B). The southern landmasses were still grouped within Gondwana, but the components of Laurasia had begun to drift apart. Western and eastern North America were joined along the southern margin, although A. G. Smith et al. (1994) portrayed the northern elements as separated by a large, north-to-south-oriented inland sea (not shown by Scotese 2002). Greenland was part of eastern North America, whereas Europe had “recently” (e.g., approximately 20 million years earlier) separated from eastern North America and was also separated from northern Asia by the Obik Sea to the north, and by the Turgai Straits to the south (A. G. Smith et al. 1994; Zwick 2000; Scotese 2002).

MESOZOIC, LATE CRETACEOUS (70 MYA) High sea levels, resulting in shallow epicontinental seas, separated western and eastern North America as well as northern and southern sections of eastern North America (A. G. Smith et al. 1994) (Figure 2.5C). A large landmass comprising northern Asia, Beringia, and western North America dominated the Northern Hemisphere. Greenland was apparently close to, but perhaps separated from, eastern North America. There is disagreement about whether eastern North America, Greenland, and Europe were connected. Briggs (1986, 1995) and Rage and Rocek (2003) supported a connection, whereas other recent authors (e.g., A. G. Smith et al. 1994; Scotese 2002) show separations between these landmasses (as I have done in Figure 2.5C). Europe was separated from Asia by the Obik Sea to the north and the Turgai Straits to the south (the combined water body is the Uralian Sea [Rage and Rocek 2003]). There is also disagreement about the connection of North and South America during this period. Briggs (1995) and Rage and Rocek (2003) indicate a connection via Central America, whereas A. G. Smith et al. (1994) and Scotese (2002) do not.

CENOZOIC, EARLY TERTIARY

PALEOCENE (60 MYA) The Northern Hemisphere was largely ringed by a single landmass comprising Asia, Beringia, North America, Greenland, and Europe (Figure 2.5D). Asia was linked to western North America via Siberia and Beringia, the large inland sea separating eastern and western North America had partially receded, and eastern North America was connected to Europe via Greenland. Europe continued to be totally (Rage and Rocek 2003) or partially (A. G. Smith et al. 1994) separated from northern Asia, and North and South America were not connected.

CENOZOIC, EARLY TERTIARY

MIDDLE EOCENE (45 MYA) The Northern Hemisphere was dominated by a landmass comprising northern Asia, Beringia, and North America (Figure 2.5E). There were possible northern connections between Greenland and North America, and Greenland and northern Europe (A. G. Smith et al. 1994; Briggs 1995), although more recent data seem to cast doubt on this since Torsvik et al. (2001) showed Greenland separated from North America by the Labrador Sea in the early Tertiary (54 mya) and also separated from northern Europe; this is reflected in Figure 2.5E. The Turgai Straits had reopened from the south and, along with the Obik Sea to the north, separated Europe from northern Asia. South America was separate from North America, with Central America existing as island archipelagos (A. G. Smith et al. 1994; Briggs 1995; Scotese 2002). Although less critical for understanding the North American fauna, the position of India during the Eocene is uncertain—Briggs (1989) argued, based on faunal evidence, that India must have already contacted the Asian continent by the early Eocene, whereas other studies based on geophysical evidence indicate that it did not make contact until the Miocene (e.g., A. G. Smith et al. 1994; Scotese 2002).

CENOZOIC, LATE TERTIARY, MIDDLE MIOCENE (10–15 MYA) The large, Northern Hemisphere landmass of northern Asia, Beringia, and North America not only continued to persist but had expanded with the closure of the Uralian Sea (Turgai Straits and Obik Sea) and the union of Europe with northern Asia (Figure 2.5F). South America was still separate, but Central America was joined with North America. By the early Miocene, Africa had contacted Asia along the Arabian Peninsula, allowing potential interchange of freshwater fishes and resulting in the formation of the Mediterranean Sea (A. G. Smith et al. 1994; Briggs 1995; Scotese 2002; Rage and Rocek 2003). The present configuration of landmasses differs from the middle Miocene by the submergence of Beringia, the expansion of the Atlantic and Pacific oceans, and the union of South and Central America (Figure 2.5G).

Ages and Origins of Major Fish Families

PETROMYZONTIDAE (21 SPECIES) Lampreys represent one of the two surviving groups of jawless fishes, the other being the strictly marine hagfishes (Myxini). All lampreys have a prolonged larval stage (termed ammocoetes) during which they burrow into soft sediments of streams and feed on small organisms at the sediment-water interface. Some lampreys have a parasitic adult stage where they feed on body fluids of other fishes, whereas others do not feed after their metamorphosis to adults (Hardisty and Potter 1971). Petromyzontid lampreys likely represent the oldest living group of North American freshwater fishes, although there are no North American fossils that can be conclusively placed within the family (Cavender 1986). Lamprey fossils, described as Mayomyzon pieckoensis, were from Pennsylvanian marine shale deposits in Illinois, dating to approximately 310 mya (Bardack and Zangerl 1968), and another species, Hardistiella montanensis, was described from lower Carboniferous (ca. 350 mya) marine formations from what is now Montana (Janvier and Lund 1983). Although both were initially placed within the Petromyzontidae, Mayomyzon is now recognized as being the sole described species in the extinct family Mayomyzontidae, whereas the family relationship of Hardistiella is uncertain (Nelson 2006). A third fossil species, Pipiscius, also dates from Paleozoic formations in North America (Janvier 1997a). Of these extinct lampreys, Mayomyzon is most similar in body form to modern petromyzontids and certainly demonstrates that the lamprey body plan, and most likely mode of life, has been around since the Paleozoic. Recent work on lamprey phylogeny supports the notion that the fossil family Mayomyzontidae is as old or older than any extant families and that the North American Petromyzontidae are monophyletic (Gill et al. 2003). Freshwater lampreys are likely derived from marine ancestors (Gilbert 1976) and the Petromyzontidae probably originated in Pangean North America (Figure 2.2) in the Paleozoic (Cavender 1986).

ESOCIDAE (4 SPECIES) The pike and pickerel are one of the five North American families dating from the Mesozoic, a group that includes two subclasses: Chondrostei (sturgeon, paddlefish) and Neopterygii (bowfins, gars, and pickerels) (Nelson 2006). Fossil and living pikes and pickerels are found only in the Northern Hemisphere, including North America, Europe, and Asia (Grande 1999; Berra 2001). The family includes major recreational species such as Muskellunge (Esox masquinongy) and Northern Pike (E. lucius). The earliest known North American fossils are from late Cretaceous deposits of the Green River Formation (Grande 1999), when western North America was separated from eastern North America by the Late Cretaceous Seaway but linked with eastern Asia via Beringia (Figure 2.5C). The oldest species, Esox tiemani, was described from Paleocene lake and river deposits in Alberta and Saskatchewan (Wilson and Williams 1992). The discovery of this fossil species demonstrated the highly conserved body plan of esocids. The family perhaps originated in Laurasian North America, although data remain inconclusive about possible origins in northern Europe or Asia (Patterson 1981; Grande 1999; Wilson and Williams 1992).

ACIPENSERIDAE (8 SPECIES) Sturgeons are also an ancient group with fossils from Asia, Europe, and North America. The family is, and has been, essentially limited to temperate regions (Bemis and Kynard 1997). The order Acipenseriformes, containing sturgeons and paddlefishes, originated during the Triassic in Western Europe at a time when Laurasian elements were beginning to separate (Figure 2.5A, B) and sturgeons likely had their earliest diversification in Central Asia (Bemis and Kynard 1997). North America fossils date from the Paleogene and Upper Cretaceous deposits in areas that are now Montana and Alberta (Cavender 1986; Bemis and Kynard 1997). There is also a Miocene fossil from Virginia (Bemis and Kynard 1997). Sturgeon have a Laurasian/Pangean origin (Grande and Bemis 1996), although whether ancestral forms were present in Laurasian North America, or whether Sturgeon arrived via Beringia or from Western Europe is uncertain. Sturgeon body form is highly conserved, having changed little since the Mesozoic (G. R. Smith 1981; Sulak and Randall 2002). The family includes both freshwater and anadromous species, has a Holarctic distribution, and includes some of the largest and longest-lived species of freshwater fishes (Berra 2001). In fact, the Beluga Sturgeon (Huso huso) of the Black and Caspian seas reaches 9 m and 1300 kg and may live for nearly 100 years (Berra 2001). North American sturgeons in the genus Acipenser also reach large sizes (> 2m TL) and may live 100 years or more (Sulak and Randall 2002). Anadromous North American forms (all in the genus Acipenser) are derived from freshwater forms and were one of the first groups of fishes to solve the physiological challenges of moving from fresh to salt water (Bemis and Kynard 1997; Sulak and Randall 2002).

CLUPEIDAE (10 SPECIES) Although this family is primarily marine, there are a number of freshwater species worldwide, all considered to be derived from marine forms. In North America, clupeids first appeared in the fossil record in the middle Paleocene (ca. 60 mya) in what is now Montana and apparently occupied western North America until the middle Eocene (Grande 1982). After the middle Eocene, freshwater clupeid fossils did not show up again in the fossil record for approximately 40 million years until the Pliocene/Pleistocene (Cavender 1986). The Plio/Pleistocene fossil was identifiable as a modern species—the Threadfin Shad (Dorosoma petenense) (Miller 1982).

ICTALURIDAE (46 SPECIES) Ictalurid catfishes date from the Paleogene and are restricted to North America. The earliest North American fossils referable to the Ictaluridae date from the late Paleocene (60 mya) (Cavender 1986; Lundberg 1992). The most complete specimens are of the extinct genus Astephus and are from Eocene lake deposits in the Green River Formation, a large system of lakes located in intermontane basins, in what is now Utah, Colorado, and Wyoming, that were formed by the uplift of the Rocky Mountains in the Tertiary (Grande 1984; Grande and Lundberg 1988). From the Paleocene through the late Eocene, the Green River system comprised one of the world’s largest and longest-lived Great Lakes systems (Grande 2001). Ictalurids likely originated in North America (Gilbert 1976; Burr and Mayden 1992; Lundberg 1992) and, although there were connections with Asia and Europe in the Cretaceous and Paleocene (Figs. 2.5C, D), this group has never been found outside of North America. Of the modern genera, Ictalurus, Ameiurus, and Trogloglanis occurred in the early Oligocene, and Pylodictis fossils are known from the middle Miocene (Lundberg 1992). The genus Noturus (madtom catfishes) is younger, with fossil material dating only from the early Pleistocene (1–2 mya) (Cavender 1986).

SALMONIDAE (38 SPECIES) The oldest fossil salmonids are of the extinct genus Eosalmo, found in Eocene deposits located in what is now British Columbia and northern Washington (Wilson and Williams 1992). Eosalmo apparently occupied Pacific drainages and, based on phylogenetic analysis, was basal to all other members of the Salmonidae (i.e., considered a stem group) (Wilson 1992). Although Gilbert (1976), among many others, considered the Salmonidae to likely have a marine origin, later work on salmonid phylogeny and ecology points to a freshwater origin for the group. All primitive salmonids are restricted to freshwater habitats, whereas derived groups all contain anadromous species (i.e., those spawning in fresh water and then moving to the sea to feed) (Stearley 1992). The fossil species Eosalmo provides additional support for the freshwater origin hypothesis because of the discovery of a large size series of specimens, ranging from young to adult fish, in the same lake deposit (Stearley 1992; Wilson and Williams 1992). The anadromous life cycle shown by some modern members of the family, such as Steelhead (Oncorhynchus mykiss), Pacific salmon species, and Atlantic Salmon (Salmo salar), is thus considered a secondary adaptation. Anadromy may have been triggered by increased seasonality caused by the cooling of the climate in the middle Cenozoic, such that the marine habitat offered greater productivity and more constant temperatures that would have favored increased growth in the marine compared to the freshwater habitats (Gross et al. 1988; Stearley 1992).

Based on the worldwide distribution of the family throughout northern Asia and also Europe (Berra 2001), and the presence of the stem-group fossils in western North America, the family likely evolved in the region of Laurasia that included western North America and perhaps northern Asia (Figure 2.5E). The modern species of Pacific trout (genus Oncorhynchus) likely originated in the Miocene and have had at least six million years of history (Stearley and Smith 1993). The Atlantic basin salmonids (genus Salmo) are primarily a European lineage, and the separation of the eastern Salmo and the western Oncorhynchus lineages likely occurred via a vicariant event across the northern coast of Asia in the Miocene (Stearley 1992).

CATOSTOMIDAE (71 SPECIES) Suckers represent the median in terms of age in North America (Figure 2.3). The oldest North American fossils, of the extinct genus Amyzon, date from the middle Eocene (ca. 49 mya). Amyzon is represented in various western fossil deposits, including the Green River Formation of Wyoming (Grande et al. 1982; Grande 1984; Cavender 1991). The habitat was likely swamp-like and included crocodiles and alligators (Grande et al. 1982). Like the modern subfamily Ictiobinae with which it is closely related, Amyzon was a fairly large-bodied fish. More derived species of suckers have tended toward smaller body sizes (G. R. Smith 1992). Catostomids occupied much of western North America and eastern Asia by the late Eocene, a time when the Asian and North America landmasses were connected via Beringia (Figure 2.5E) (Cavender 1986, 1991). Of the modern genera of suckers, Ictiobus occurred by the middle Miocene and Chasmistes by the late Miocene (Cavender 1986). Suckers are thought to have originated in Eurasia and then reached North America via the Pacific connection of Beringia (Figs. 2.4 and 2.5E) (Gilbert 1976; Briggs 1986; Burr and Mayden 1992; Berra 2001).

CENTRARCHIDAE (31 SPECIES) The centrarchids are endemic to North America and likely evolved there (Gilbert 1976; Burr and Mayden 1992). The earliest fossils of this primarily eastern North American family date from the Eocene epoch of northwestern Montana (ca. 45 mya) in drainages that flowed eastward from the continental divide (Cavender 1986). During this time, North America had separated from Europe but was still connected to Asia via Beringia (Figure 2.5E). Based on a fossilcalibrated molecular phylogeny, Near et al. (2005) estimated the age of the most recent common ancestor to the Centrarchidae to be 33.6 million years, providing another line of evidence supporting the Eocene age estimate for the group. Because the earliest fossils have not been linked to species, they could not be used in calibrating the molecular phylogeny. By the Miocene, modern genera including Lepomis, Micropterus, and Pomoxis were well represented and, especially by the early Pleistocene, centrarchids had become a dominant element in the North American freshwater fish fauna (G. R. Smith 1981). Ages of modern species, based on molecular phylogenies, are 8–11 million years for Micropterus, at least 11 million years for Pomoxis, and 14 million years for Lepomis (Near et al. 2003, 2005). Centrarchids also were widespread by the middle Miocene, based on fossils found west of the continental divide and including fossils of the extant western genus Archoplites (Cavender 1986).

CYPRINIDAE (297 SPECIES) The largest family of North American fishes has a Eurasian origin and likely reached North America via Beringia. The earliest fossil evidence in North America is from several Oligocene deposits in the northwestern United States in a region that in the middle Tertiary would have been near the western continental margin (Cavender 1986, 1991). By the late Miocene and Pliocene, cyprinids had taken their place as a major component of the North American fish fauna (Cavender 1986, 1991). In the New World, cyprinids are restricted to North America with no records, past or present, from South America. They are well represented both in lineage and species diversity throughout Europe, Asia, and sub-Saharan Africa (Howes 1991; Berra 2001). Although there are a variety of hypotheses of relationships within the Cyprinidae, there appear to be two main lineages (treated as subfamilies), the Leuciscinae and the Cyprininae (Cavender and Coburn 1992). North American minnows are all within the subfamily Leuciscinae, which is also well represented in Eurasia. Two phyletic groups are recognized within the subfamily Leuciscinae, the Phoxinini and the Leuciscini. The majority of North American minnows are phoxinins, with only the monotypic genus Notemigonus placed in the Leuciscini (Cavender 1991).

Cyprinids likely reached North America via Beringia during periods of lowered sea level that occurred coincident with a period of climatic cooling during the late Eocene to early Oligocene—a cooling event perhaps caused by changes in ocean currents related to the separation of Australia from Antarctica and the opening of a seaway between Greenland and Norway, allowing an exchange between North Atlantic and Arctic waters (Figs. 2.5D, E) (Cavender 1991). By the time that cyprinids reached North America, the Atlantic Ocean had filled the gap between the North American and European plates, precluding movement from eastern North America and western Europe (Figure 2.5E). As a group, the Cyprinidae likely originated in the Oriental region where all major cyprinid groups are represented (Cavender 1991).

From an ecological standpoint, the speciose cyprinids are relatively recent arrivals to North America and were thus a new element incorporated into fish assemblages already composed of older groups such as salmonids, esocids, ictalurids, and others (cf. Figure 2.3). The rapid radiation of minnows was perhaps related to the rise of many insect families such as the dipterans (Cavender 1991).

GOODEIDAE (45 SPECIES) The distribution of the goodeids includes the western Great Basin of the United States (subfamily Empetrichthyinae) and the Mexican plateau (subfamily Goodeinae) (Berra 2001; Webb et al. 2004). The oldest fossils are Miocene—the subfamily Goodeinae is represented by the extinct genus Tapatia from deposits in the state of Jalisco, Mexico (Cavender 1986), and the subfamily Empetrichthyinae by material of the extant genus Empetrichthys from deposits in Southern California (Webb et al. 2004). A molecular-based phylogeny indicates that the family originated 23 mya, which corresponds well to the Miocene fossils. The two subfamilies diverged between 11.5 and 16.8 mya (Doadrio and Dominguez 2004; Webb et al. 2004).

The family is thought to have originated in North America (Gilbert 1976; Burr and Mayden 1992). Increasing aridity during the Tertiary may have fragmented the once continuous range of the family, resulting in the divergence of the two subfamilies (Parenti 1981; Webb et al. 2004).

POECILIIDAE (69 SPECIES) The livebearers are primarily a Neotropical group with most of the diversity centered in Mexico, Central America, South America, and the West Indies, and with relatively few species in temperate North America (Parenti 1981; Rauchenberger 1988). There are no known fossils of this group in North America and no pre-Quaternary fossils known at all (Hedges 1996). This led Matthews (1998) to suggest a recent (< 1 mya) North American age for the family. However, recent molecular phylogenetic studies of Poeciliopsis, a large genus within the family that occurs on the central Mexican Plateau, indicate that divergence within this genus occurred 6–18 mya (Mateos et al. 2002). As a consequence, the age of the family in North America must be at least Miocene. In further support of this, both morphological (Parenti 1981) and molecular phylogenies (Meyer and Lydeard 1993) show that the clade containing the Poeciliidae and the clade containing the Goodeidae are sister groups (i.e., derived from a common ancestor). Given that a molecular phylogeny (Webb et al. 2004) places the origin of the Goodeidae in the Miocene (ca. 23 mya), a similar age should apply to the Poeciliidae. North American poeciliids are most likely derived from Central American ancestors (Gilbert 1976; Burr and Mayden 1992; Lydeard et al. 1995).

CICHLIDAE (16 SPECIES) This large family has a broad Neotropical distribution occurring in Mexico, Central and South America, and the West Indies, with one species, the Rio Grande Cichlid (Cichlasoma cyanoguttatum), even reaching into the United States. Cichlids are poorly represented in the North American fossil record. In the Paleotropics, cichlids occur in Africa, Madagascar, and parts of southern Asia (India, Sri Lanka, Syria, and Iran) (Murray 2001a). There is a Miocene fossil of the modern genus Cichlasoma that was found in Haiti (Cavender 1986; Hedges 1996; Murray 2001a), and the oldest known cichlid fossil was found in an African deposit and dates from the middle Eocene (Murray 2001b). One view is that the separation between African and South American cichlids may postdate the formation of the Atlantic Ocean and that South American cichlids were derived from marine dispersal of cichlids from Africa, with molecular phylogeny suggesting a divergence time of 58–41 mya (Vences et al. 2001). However, a more recent review supports a vicariance hypothesis and thus requires an older age for divergence of New and Old World cichlids (Chakrabarty 2004).

The family as a whole is thought to have an early Tertiary origin, and movement from South America to Central America perhaps occurred in the late Tertiary (ca. < 20 mya) (Myers 1966; Murray 2001a). Consequently, the origin of cichlids in North America (primarily Mexico) could have occurred as early as the middle Miocene (ca. 12–15 mya).

PERCIDAE (186 SPECIES) Percids are the second most speciose family of North American freshwater fishes. The family occurs in Europe, northern Asia, and eastern North America (Collette and Banarescu 1977; Berra 2001) and seems to represent a Laurasian clade (Wiley 1992; Carney and Dick 2001). The family likely originated in the early Tertiary (Paleocene; ca. 65 mya) when land connections existed between eastern North America and Europe (Figure 2.5D) (Wiley 1992; Carney and Dick 2001). However, fossil remains of percids are only dated to the Miocene (26 mya) (Carney and Dick 2001), and the earliest North American percid fossils are from Pleistocene deposits (ca. 2 mya) in areas now located in Texas and Oklahoma (genus Perca) and South Dakota (Percina and Etheostoma) (Cavender 1986). Given the currently available information, it is not possible to distinguish between a dispersal hypothesis, with percids evolving in Europe and then dispersing to North America via the North Atlantic connection, or a vicariance hypothesis, with percids evolving in Laurasia and then being separated by the formation of the North Atlantic Ocean (Carney and Dick 2001). However, given that fossils likely underestimate ages of percids, I have shown the Laurasian origin in Figure 2.4.

The first occurrence of darters in North America is likely far earlier than indicated by the Pleistocene fossils. A molecular phylogeny of the darter family (Percidae), with the rate of genetic change (i.e., the molecular clock) based on a fossil-calibrated phylogeny of centrarchids, shows the separation of darters from nondarter percids occurring 19.8 mya (Carlson et al. 2009). The 18.5 mya age of the most recent common ancestor to the darter genus Nothonotus provides further evidence of at least a Miocene origin of darters (Near and Keck 2005). Finally, a recent molecular phylogeny of logperches (genus Percina) showed that divergence began in the Pliocene (ca. 3–5 mya), although most speciation events in this group did occur within the Pleistocene (Near and Benard 2004).

FUNDULIDAE (34 SPECIES) Members of this group occur in North and Central America as well as Cuba; in North America all but two species occur east of the continental divide (Berra 2001). The oldest fossil evidence in North America dates from the middle Miocene (ca. 16 mya) and perhaps is of the modern genus Plancterus (Cavender 1986). North American fundulids apparently are derived from Central American ancestors (Briggs 1986, 1987) and, in support, phylogenies based on morphological and molecular data are consistent in placing the Central American family Profundulidae as basal to the Fundulidae (Parenti 1981; Wiley 1986; Bernardi 1997).

CYPRINODONTIDAE (35 SPECIES) Cyprinodontid fishes include both marine/estuarine and freshwater forms that are found primarily along coastal regions (with some notable exceptions) in North, Central, and South America, and the Mediterranean region including North Africa (Parenti 1981; Berra 2001). One view is that the distribution of the cyprinodontiform fishes suggests, in part, a reduced Pangean pattern (Figure 2.2), with members of the group absent from Australia and Antarctica (Parenti 1981). Correspondingly, the order Cyprinodontiformes likely existed at least from the late Triassic before the breakup of Pangea (Figure 2.5A), and New World cyprinodontids perhaps date from the early Tertiary (Parenti 1981). An alternative view places the origin of the group at a later time in the early Cretaceous when Africa and South America were only divided by a narrow saltwater passage (Figure 2.5B, C) (Briggs 1986).

The relatively great age of New World cyprinodontids is also supported by molecular studies of the amount of divergence between New and Old World species (Echelle and Echelle 1993). The North American cyprinodontids are likely derived from a marine ancestor (Gilbert 1976; Parker and Kornfield 1995). However, the hypothesis that lineages of inland species of Cyprinodon in North America have been derived independently from the widely distributed coastal species, although initially supported by a reduced data set of western species in the Cyprinodon variegatus complex (Echelle and Echelle 1992), has not been substantiated by a more complete study of the family (Echelle et al. 2005).

In spite of the suggested age of the order and of New World Cyprinodontidae, the fossil record for the family in North America is meager. The only known North American fossil, of the extinct species Cyprinodon breviradius, is from late Miocene/early Pliocene deposits near Death Valley, California (7–9 mya) (Miller 1981; Cavender 1986). However, there are recent phylogenies, based on water-soluble proteins (allozymes) and mitochondrial DNA, that provide times of divergence for North American genera and species (Echelle and Echelle 1992; Echelle et al. 2005). Molecular data suggest that modern New World genera of the Cyprinodontidae began diverging in the Miocene (7–9 mya)—dates that are earlier than those proposed by Miller (1981) based largely on geological inferences. Some species within the family are of much more recent origin, dating to less than one million years (Echelle et al. 2005).

COTTIDAE (30 SPECIES) In terms of the diversity of genera, the sculpins are primarily a marine family. However, the genus Cottus is well represented in North American fresh waters with 28 species, and there are at least two freshwater species of Myoxocephalus. The oldest fossils of the genus Cottus are from late Miocene (ca. 11 mya) deposits of North America in what is now Oregon (Linder 1970; Cavender 1986). Freshwater forms in both genera are derived from marine species (Gilbert 1976; Burr and Mayden 1992). In contrast to the Miocene age of Cottus, freshwater species of Myoxocephalus are more recent, likely invading freshwater habitats in the early to middle Pleistocene around the beginning of the major continental glaciations (ca. 0.9 mya) (Kontula and Väinölä 2003).

ATHERINOPSIDAE (39 SPECIES) The New World silversides are primarily a marine family, although there are five genera that occur widely in freshwater habitats. Fossil silversides are known from Pliocene formations (ca. 4–5 mya)—one in what is now Arizona and the other from the Mesa Central of the Mexican Plateau (Cavender 1986). The Arizona fossils were most likely from a marine or brackish water habitat and have been assigned to the modern species Colpichthys regis (Todd 1976). The Mexican material also represented modern species of the genus Menidia (formerly placed in Chirostoma) (Barbour 1973; M. L. Smith 1981). Freshwater silversides are derived from marine ancestors, with perhaps several independent invasions of fresh water by species groups occupying the Mexican Plateau and those found in more northern regions of North America (Barbour 1973; Gilbert 1976; Burr and Mayden 1992).

SUMMARY

Fish evolution began in the early Paleozoic, perhaps 500–470 million years ago (mya). Modern bony fishes, the teleosts, appeared by the lower Mesozoic (230–206 mya), and by the middle Mesozoic (195 mya), representatives of most major groups of fishes were present. The distribution of marine and freshwater fishes worldwide, and the occurrence and distribution of North American freshwater fishes, have been strongly shaped by the movements of landmasses—plate tectonics.

The diverse fish fauna of North America came to occupy North America over a span of hundreds of millions of years, from as early as the late Paleozoic through the Pleistocene, and continuing into the present as populations respond to changing environmental conditions. Over two-thirds of the fauna, in terms of family origins, has occupied North America since the Paleogene (ca. 24 mya) or earlier, whereas ages of particular species can be much more recent. The minnows and darters, the two most speciose North American families, are also among the more recent (Miocene and Oligocene) arrivals.

Lineages of fishes in North America have various origins as well. Half of North American freshwater fishes have a marine origin, followed by those originating in North America (or in ancient landmasses of Pangea and Laurasia), Central and South America, and Eurasia. Because of the long and varied histories of fish lineages, contemporary assemblages of fishes should be viewed as being composed of suites of species with potentially widely differing histories, with adaptations that have likely been shaped to a greater or lesser extent by interactions in fish assemblages that were greatly different from those they are currently occupying.

SUPPLEMENTAL READING

Cavender, T. M. 1986. Review of the fossil history of North American freshwater fishes, 699–724. In The zoogeography of North American freshwater fishes. C. H. Hocutt and E. O. Wiley (eds.). John Wiley and Sons, New York, New York. An important reference to the fossil history of North American fishes.

Grande, L. 2001. An updated review of the fish faunas from the Green River Formation, the world’s most productive freshwater Lagerstätten, 1–38. In Eocene biodiversity: Unusual occurrences and rarely sampled habitats. G. F. Gunnell (ed.). Kluwer Academic/Plenum Publishers, New York, New York. A comprehensive review of one of the most complete series of fossil fish faunas.

Smith, A. G., D. G. Smith, and B. M. Funnell. 1994. Atlas of Mesozoic and Cenozoic coastlines. Cambridge University Press, Cambridge, United Kingdom. An important reference for understanding continental 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.