Molecular Panbiogeography of the Tropics

4. Biogeography of New World Monkeys

145 Detailed information on the molecular phylogeny of New World monkeys (Opazo et al., 2006; Osterholz et al., 2009) and on their distribution and ecology (IUCN, 2009) is now available, and so the group is an excellent subject for biogeographic and evolutionary study. Researchers now agree on many descriptive aspects of the phylogeny and distributions, although the interpretation of the data remains controversial . As yet, there is little agreement on how, when, or where the different primate clades evolved in America or even how primates came to be there in the first place. Most previous work has suggested that primates colonized America from somewhere else, for example, by rafting . Instead, it is suggested here that New World primates “arrived” in America by evolving there, in situ, from earlier generalized ancestors that were widespread in South and Central America before the breakup of Gondwana. In Chapter 3 it was suggested that the American primates, the platyrrhines , were isolated by the opening of the Atlantic. In a similar way, the lemurs may have been isolated in Madagascar with the opening of the Mozambique Channel. Both these well-dated tectonic events were used to calibrate the time course of primate evolution, rather than relying on the scanty fossil record of early primates. The geology of the South American plate is outlined in Figure 4-1. The plate is delimited by a divergent plate margin to the east (the 4 Biogeography of New World Monkeys The more we study the distribution of organized life on the globe, the more we tend to abandon the hypothesis of migration. —Humboldt, 1814–1825 (1995: 139). Heads_6480007_Ch04.indd 145 Heads_6480007_Ch04.indd 145 10/20/11 2:35 PM 10/20/11 2:35 PM 146 | Biogeography of New World Monkeys mid-ocean ridge in the Atlantic), a convergent margin to the west (the Chile–Peru trench), and transform faults to the north and south. As the South American plate moved west, beginning in the Cretaceous, it collided with the plate in the Pacific and the continent underwent great deformation. The most obvious result was the uplift of the Andes along the convergent plate margin. South America is composed of older cratons that are exposed as the Guiana and Brazilian shields, the Andean orogenic belt, and an intermediate zone (Fig. 4-2). The latter comprises a fold and thrust belt (the Subandean hills) and a foreland basin (in front of the craton) and has special significance for biogeography. Apart from the marginal deformation—the Andes—the Cretaceous events led to significant intraplate rifting and magmatism throughout the continent. Figure 4-3 (from Costa et al., 1996) shows the effects in the Amazon region: normal (extensional) faults in the northeast, reverse (thrust) faults indicating convergence in the southwest, and strike-slip FIGURE 4-1. The South American plate (gray) and its current boundaries: the divergent margin at the Mid-Atlantic ridge, strikeslip transform faults (black arrows) at the northern and southern boundaries, and the convergent margin (subduction zone) to the west, along the Peru–Chile trench. The rise of the Andes is linked to the opening of the Atlantic. South American plate Nazca plate Caribbean plate Scotia plate Antarctic plate Cocos plate Heads_6480007_Ch04.indd 146 Heads_6480007_Ch04.indd 146 10/20/11 2:35 PM 10/20/11 2:35 PM Biogeography of New World Monkeys | 147 faults striking close to 90° found throughout. The course of the rivers is determined by the faults, which also give rise to lines of hills with an elevation of about 200 m. The other figures in Costa et al. (1996) should be consulted for many cases of fault control at local scales. It is well known that many platyrrhine distributions reach limits at rivers, but biogeographers often neglect the factors that have caused the rivers to flow where they do. The geometry of the stress regime responsible for the Amazon faults was initiated with the opening of the Atlantic in the Cretaceous and has remained more or less the same until now. For example, in 1983 there was an earthquake of magnitude 5.5 at Codajás, 200 km upstream of Manaus (Costa et al., 1996). Because the landscape in the interior of the cratons is so flat, the slightest inclines and the smallest faults can cause significant changes in vegetation and FIGURE 4-2. Geology of South America. R ⫽ Romeral fault zone, the boundary between accreted terranes in the west and...