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20 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 304 Another possibility is that the Mayrellinae might have dispersed along the coasts of the Tethy Seaway during the early to middle Eocene. The Tethy Seaway was an important avenue of dispersal for the boreotropical flora across Eurasia, connecting eastern North America through western Europe to southeastern Asia (Tiffney, 1985). Paleontological evidence reveals an Indomalaysian affinity of early Eocene to Oligocene floras of eastern Europe, Egypt, and England (London Clay) (Tiffney, 1985; Taylor, 1990, and references therein). However, the existence of basal lineages of the Mayrellinae and Paramblynotus in the eastern Palearctic but not in the southeastern Asia indicates that the Tethy Seaway was not the route of dispersal in this case. That the Bering land bridge is more likely to be the dispersal route for Mayrellinae between North America and eastern Palearctic/and Oriental is also supported by biogeographical reconstructions based on paleoclimatology and paleogeology. The Holarctic has been split and reconnected by various barriers at different locations since the early breakup of Pangaea in the Late Jurassic (e.g., Hallam, 1981; McKenna, 1983; Briggs, 1987). These events have profoundly affected the evolution of organisms of the Northern Hemisphere (McKenna, 1983) as well as the present distribution patterns of these organisms. These barriers are (1) the Turgai Sea, (2) the North American mid-continental seaway, (3) the northern Atlantic Ocean, and (4) the Bering area (e.g., Noonan, 1986; Briggs, 1987; Enghoff, 1995). The Turgai Sea was formed in the Middle Jurassic (160 Ma) and separated Asia from Europe (Hallam, 1981; Briggs, 1987). Species of Kiefferiella and Paramblynotus are only known to be associated with angiosperm trees. Although the angiosperms could have diverged before the Cretaceous (145 Ma) (Doyle and Donoghue; 1986, Crane et al., 1995), and the earliest angiosperm fossils are from the Valanginian (140–130 Ma) (Crane et al., 1995), angiosperms were generally lacking in the early Cretaceous (Wing and Sue, 1992). Therefore, early connections between North American and Asian elements of the Mayrellinae through Europe should have occurred after the formation of the Turgai Sea. By the Oligocene (30 Ma), when the Turgai Sea retreated (Hallam, 1981, Briggs, 1987), no land connection existed between Europe and North America (Hallam, 1981; Scrivastava and Tapscott, 1986). Therefore, the early connections between eastern Asian and North American elements of the Mayrellinae are more likely to have been by way of the Bering land bridge than by way of Europe. In the northern Pacific, the Bering land bridge connected Asia and North America continuously since at least the Late Cretaceous (Briggs, 1987), possibly earlier (Hallam, 1981; Hickey, 1981). However, the Bering area as a route of biotic exchanges was severely constrained by climate and daylight length because the area was then situated closer to the North Pole than it is today, and it has since slowly and gradually moved southward to its current position (McKenna, 1983). Nonetheless, the global climates were much warmer and equitable during the late Cretaceous and early to middle Tertiary than they are today (Wolfe, 1978, 1980, 1987; Tiffney, 1985; Wing and Sues, 1992). Climates became particularly favorable during the early Tertiary warm intervals, when the Bering land bridge was primarily covered with broadleaved, deciduous forest, perhaps with a thin southern fringe of evergreen, megathermal communities (Tiffney, 1985; Wolfe, 1985). This definitely made it possible for the early Mayrellinae to disperse between the two sides of the land bridge. Within the historical framework laid out above, plausible reconstructions of the early biogeography of the Mayrellinae based on the three competing alternative hypotheses can be presented as outlined below. The first two reconstructions (figs. 16, 17) suggest that the ancestral species of Mayrellinae was distributed both in the Nearctic and in East Asia, and it was divided into disjunct populations when a geographical barrier separated these two regions, giving rise to the genera Paramblynotus and Kiefferiella. The stem species of Paramblynotus expanded its distribution by dispersal into the Nearctic and became split by a subsequent vicariance event, leading to the origination of the P. virginianus clade and the clade
2007 LIU ET AL.: REVISION OF PARAMBLYNOTUS (HYMENOPTERA) 21 Fig. 16. Summary of the DIVA reconstructions of historical biogeography based on the majority-rule consensus tree of type III islands that postulate a widespread ancestor of the Mayrellinae distributed in both Gondwana and Laurasia. With clades collapsed when the species included are endemic to a single biogeographical area as defined in the present study, the majority-consensus tree over all shortest trees is identical to that of type III islands. Results based on type I and type II islands are similar to the one presented here with minor differences. Dispersal events are indicated on the branches. Arrows with dashed tails indicate alternative positions of dispersal events. These explanations also apply to figures 10–12, except as stated.
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