Geography Reference
In-Depth Information
South America was an isolated continent after it separated from Africa in the
Cretaceous (90-100 mya), until it reconnected with North America in the Pliocene
(3-2.5 mya). According to the fossil record, in the Eocene-Early Oligocene (50-30
mya), South America was colonized by two groups of placental mammals, rodents
and primates, of which the extant caviomorphs (guinea pigs, chinchillas, agoutis,
New World porcupines, and arboreal spiny rats) and platyrrhines (sakis, spider
monkeys, capuchins, and marmosets) are the descendants, but the exact times
and ways of colonization are debated (Arnason et al. 2000; Hoffstetter 1972;
Springer et al. 2003). Poux et al. (2006) undertook a molecular clock analysis in
order to demarcate the periods of possible colonization by estimating the time win-
dows between their divergence from their respective Old World sister groups (up-
per or oldest bound) and the subsequent diversifications in South America (lower
or most recent bound).
Poux et al. (2006) analyzed separate and combined sequences of three nucle-
ar genes, coding for the alpha 2B adrenergic receptor, the von Willebrand factor,
and the interphotoreceptor retinoid binding protein. For each gene, the authors se-
lected sixty species using three criteria: Species should represent all placental or-
ders and two marsupial mammals as outgroups; the sampling should reflect the
diversity of primate and rodent taxa, with a broad representation of platyrrhines
andcaviomorphs; andspeciesshouldenablethe useofpaleontological calibration
constraints from various lineages, thus minimizing the dependence of the results
on a single fossil reference. Sequences were aligned by hand with the ED editor of
MUST version 2000 (Philippe 1993). The phylogenetic analyses were performed
on the complete DNA data set by maximum likelihood with PAUP version 4 (Swof-
ford 1999), and the Bayesian analyses were performed with MrBayes version 3.0
(Huelsenbeck and Ronquist 2001). Molecular dating analyses were performed ac-
cording to the Bayesian relaxed molecular clock approach (Thorne et al. 1998),
using the MULTIDIVTIME package (Thorne and Kishino 2002). For fossil calibra-
tions, Poux et al. (2006) selected eight time constraints previously used in oth-
er molecular dating studies: outside the Primates and Rodentia, the diversifica-
tionagesofPaenungulata(54-65mya),Perissodactyla(54-58mya),Cetartiodac-
tyla (55-65 mya), and Lagomorpha (minimum age of 37 mya); within rodents, the
splits
Glis-Dryomys
(minimum age of 28.5 mya) and
Aplodontia-Marmota
(minim-
um age of 37 mya); and within Primates, the basal primate radiation (63-90 mya)
and the Cercopithecoidea-Hominoidea divergence (25-35 mya).
The maximum likelihood and Bayesian analyses yielded identical topologies
theria, and Euarchontoglires) were strongly supported. Within primates and ro-
dents, the monophyly of platyrrhines and caviomorphs had maximal support. All
cladesrelevanttotheunderstandingoftheproblemanalyzedwerewellsupported,
providing a reliable framework for the assessment of divergence times. According
