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.
