the cynipoid genus paramblynotus - American Museum of Natural ...

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22 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 304 Fig. 17. Summary of the DIVA reconstructions of historical biogeography that postulate an ancestor of the Mayrellinae with a distribution limited to the Nearctic, eastern Palearctic, and Oriental region. Based on the majority-rule consensus tree of type III islands. comprising the rest of Paramblynotus. The ancestral species of the Mayrellinae might have appeared first in either East Asia or the Nearctic and have expanded its distribution across the Bering land bridge during the warm intervals from the late Cretaceous to the middle Tertiary. Rather than being the result of a vicariance event separating East Asia and Nearctic as indicated by the DIVA analysis, the divergence between Parambly-
2007 LIU ET AL.: REVISION OF PARAMBLYNOTUS (HYMENOPTERA) 23 notus and Kiefferiella may have been caused by the formation of the Rocky Mountains in western North America. The Rocky Mountains were mainly formed during the latest Paleocene to early Eocene (ca. 56–40 Ma) (Leopold and MacGinitie, 1972), and they created direct topographic barriers to plant movement between southwestern North America and the rest of the then continuous land mass Asiamerica (Briggs, 1987). The endemic distribution of Kiefferiella in western and southwestern North America (Idaho, California, and Texas) is apparently also due to the formation of the Rocky Mountains, which caused the interior of North America to become increasingly drier. The remnants of the earlier, moistureadapted boreotropical flora were largely confined to the western slopes or retreated southward (Leopold and MacGinitie, 1972; Tiffney, 1985). Kiefferiella might have been associated with the earlier boreotropical flora and represents a relic taxon that has retreated to the south. The fossil species K. connexiva from Upper Eocene beds in Colorado (34.1 Ma) (Ronquist, 1995a) indicates that the genus was once more widespread in interior North America than it is today. The disjunction within Paramblynotus between the Nearctic and East Asia, that is, between P. virginianus and the rest of the genus, probably resulted from the Terminal Eocene Event (34–33 Ma), when the global temperature dropped drastically (Wolfe, 1978, 1980, 1987; Potts and Behrensmeyer, 1992) and the climatic seasonality increased (Wolfe 1978, 1980, Tiffney, 1985). This disrupted the warm-temperate to subtropical broadleaved evergreen forests that had stretched from Asia to North America and replaced them in the Bering area with a cooltemperate deciduous forest (Wolfe, 1980; Tiffney, 1985). Following the Terminal Eocene Event, the climate continued to deteriorate with short, warmer intervals in the Oligocene and Miocene, and this trend of climate deterioration gradually led to the extreme cold of the Pleistocene (Wolfe, 1978; Tiffney, 1985). Thus, suitable habitats for hardwood-associated organisms like Paramblynotus were generally lacking in the Bering area after the Terminal Eocene Event (Nordlander et al., 1996, and references therein), making dispersal of such organisms across the land bridge difficult. According to the third alternative hypothesis (fig. 18), the stem species of the Mayrellinae was limited to the Nearctic. After a vicariance event within the Nearctic, the ancestral species of Paramblynotus expanded its distribution by dispersal into the eastern Palearctic, and a subsequent vicariance event gave rise to P. virginianus and the clade comprising the rest of Paramblynotus. In this scenario, the formation of the Mid-continental Seaway in North America in the mid- Albian (ca. 100 Ma) (Hallam, 1981; Crabtree, 1987) might correlate with the splitting between Paramblynotus and Kiefferiella. The retreat of the seaway and reconnection of the two parts of the continent shortly before the end of the Cretaceous (70 Ma) (Hallam, 1981) allowed dispersal of Paramblynotus from eastern North America to the eastern Palearctic by way of Beringia. Alternatively, as suggested in the previous scenario, the event was caused by the formation of the Rocky Mountains during latest Paleocene to early Eocene, and the newly evolved Paramblynotus then dispersed to eastern Palearctic by way of Beringia. In either case, the dispersal, as suggested earlier, most likely took place before the Terminal Eocene Event. The genus Paramblynotus was probably widely distributed in East Asia, and its species may have differentiated in response to the differentiation of the forest vegetations of the region into temperate elements of the north and subtropical to tropical elements in the south. Currently, the terrestrial ecosystems in continental East Asia stretch from above the polar circle well into the tropics. This large expanse of continuous inland has existed since the latest Triassic (Metcalfe, 1988). Within this region, there has been no major physical barrier impeding the migration of plants (Latham and Ricklefs, 1993) or associated organisms, such as Paramblynotus. The cooling trend since the Oligocene led to the selection of cool-adapted taxa from the boreotropical flora and their addition to the mixed mesophytic forest of the middle latitudes (Wolfe, 1978; Tiffney, 1985). The continued cooling of climates of later times, which gradually led to the extreme cold of the
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