Geography Reference
In-Depth Information
Figure 7.11
Cladogram showing the posterior divergence ages of placental taxa obtained
by Poux et al. (2006; reproduced with permission of
Systematic Biology
). The vertical zones
span the periods between the origin and radiation of Caviomorpha and Platyrrhini, with an
overlapping period during which they could have reached South America synchronously.
EOC, Eocene; MIO, Miocene; OLI, Oligocene; P, Pliocene; PAL, Paleocene.
With regard to the possible dispersal histories, the results are consistent with
an African origin for caviomorphs and platyrrhines, from phiomorph and anthrop-
oid stocks, respectively, followed by transatlantic migration. Despite the distance
between Africa and South America in the Middle Eocene-Early Oligocene, colon-
ization could have occurred, aided by marine currents, paleowinds, or stepping
stone islands along with rafts. This transatlantic route is the preferred hypothes-
is for platyrrhine dispersal for two reasons: Fossils of early platyrrhines and ca-
tarrhines so far have been found only in Africa, and migration through Antarctica is
unlikely for this group because at the time of the platyrrhine-catarrhine divergence
(37 mya) Australia, Antarctica, and South America were no longer connected, and
Antarctica was covered by ice sheets. With respect to rodents, South American
caviomorphs and African phiomorphs may share an Asian ancestor, but it is not
clear whether they diverged already in Asia or after dispersal of their ancestor into
Africa, leaving the dispersal route to South America open to speculation. The cavi-
omorph fossil record indicates that by 31 mya these rodents had probably started
to diversify in South America and that their arrival predated the Early Oligocene,
implyingthat extant lineagesderivefromearlydiversification events. Therearestill
paleontological uncertainties about the time of radiation of living South American
platyrrhines. However, fossils from the Late Miocene of La Venta (Colombia) are
highly similar to modern taxa, consistent with the dating of the platyrrhine diversi-
fication in the Early Miocene. Poux et al. (2006) concluded that the most plausible
scenario for primates was a transatlantic dispersal at the end of the Miocene, fol-
lowed by the extinction of all but one of the earlier diverging lineages and the radi-
ation of extant taxa in the Early Miocene. The arrival of rodents and primates might
have been contemporaneous; however, in contrast to platyrrhines, representatives
of the early diversification of caviomorphs, which occurred before the Oligocene
glaciations, survived until the present.
References
Arnason, U., A. Gullberg, A. S. Burguete, and A. Janke. 2000. Molecular estimates of
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Philippe, H. 1993. MUST: A computer package of management utilities for sequences
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