The Present Day Drainage Patterns of the Congo River System and their Neogene Evolution - Geology and Resource Potential of the Congo Basin - page 322

This also means there will be a lag time from when a river
becomes a barrier and its effects are seen in the genetic
signal.

The recently discovered species of guenon monkey in the
genus Cercopithecus ( C. lomamiensis ), based on morpho-
logical and molecular evidence, provides proxy evidence of
the emplacement of the Lomami River (Hart et al. 2012 ). C.
lomamiensis is separated from its nearest congener,
C. Hamlyni , by both the Congo and the Lomami rivers
(Hart et al. 2012 ) (Figs. 15.5 and 15.8b ; Table 15.3 ). C.
lomamiensis is endemic to the region confined by the upper
Tshuapa River in the west through to the Lomami River in
the east, whilst the eastern-most distribution of C. hamlyni is
bounded by the CR (Hart et al. 2012 ). Hart et al. ( 2012 )
suggest the Lomami and upper Congo rivers are the biogeo-
graphic barriers responsible for this speciation event. Molec-
ular divergence dates estimate the time to MRCA of the C.
lomamiensis - C. hamlyni clade based on two different
genetic makers, provide age estimates of ca . 1.7 Ma
(3.2-0.5 Ma at 95 % confidence level) and ca . 2.8 Ma
(4.3-0.6 Ma at 95 % confidence level) respectively (Hart
et al. 2012 ).
The congruence of the divergence data for the bonobos-
chimpanzees, gorillas and guenon species, supports the
emplacement of the modern, crescent shaped CR river by
1.5 Ma. The CR became an effective barrier to gene flow
(i.e. larger than 100 m—assumed effective barrier size for
chimpanzees see Piel et al. 2013 ) between 2.5 and 1.5 Ma
based on the most recent common ancestor of bonbos and
chimpanzees. This places an age constraint on the drainage
pattern of the Cuvette Central (D-Sd2 in Fig. 15.3 ), with its
relatively young age accounting for the mixture of dendritic
and sub-dendritic features. The separation of Cercopithecus
around 1.7 Ma by the northerly flowing Lomami River
strengthens the emplacement age of the modern CR.
The north-western boundary of the CB is formed by the
watershed between the Congo and Ogou´ rivers is marked by
the Bat ´ k ´ Plateau. The eastern portion of the plateau slopes
gently toward the CR and is incised by numerous, actively
eroding headwaters of tributaries of the Ogou´ River
(Seranne et al. 2008 ). Telfer et al. 2003 report a divergence
time of the mandrill ( Mandrillus sphinx ) populations north
and south of the Ogou ´ River at ca . 800 Ka, constraining the
emplacement of the modern Ogou´ River to at least 800 Ka.
With a sediment load of 19.7

15.4.2.1 Phylogenetic Age Estimates
of Geomorphic Events
In central Africa, a synthesis of phylogenetic studies of
primates provide an estimation of river emplacement
(Fig. 15.8a, b ; Table 15.3 ) in the Late Pliocene and Early
Pleistocene. Evidence from the divergence of bonobos-
chimpanzees suggests that the Equatorial arc of the Congo
River was an effective barrier to large primate dispersal,
with Pr¨fer et al. ( 2012 ) suggesting a 2.5-1.5 Ma, although
a different genetic marker provides a 1 Ma age estimate
(Kawamoto et al. 2013 ). The divergence estimate of
1.75 Ma for the western ( Gorilla gorilla ) and eastern ( G.
beringei ) gorilla populations (Scally et al. 2012 ) suggests
that the CR (and possibly the Oubangui) formed a barrier to
gorilla dispersal at a similar time of the bonobo-chimpanzee
divergence (Fig. 15.8a ; Table 15.3 ). Genetic data for the
lowland gorillas indicate a subsequent, moderate genetic
exchange between the two groups, since the initial split
suggesting movement of individuals between the two
populations until the late Pleistocene; with the eastern popu-
lation experiencing a genetic bottleneck that is indicative of
low population numbers (Ackermann and Bishop 2010 ;
Scally et al. 2012 ). Nevertheless both molecular and mor-
phologically studies support the classification of the two
species, eastern and western gorillas with further subdivision
down to subspecies possible. This is further evidence for
genetic structuring within the western and eastern gorilla
populations, suggesting a barrier role of rivers smaller than
the Congo River (Anthony et al. 2007 ; Ackermann and
Bishop 2010 ) Yet more extensive sampling of both gorilla
populations is needed to allow for better resolution of diver-
gence times (Scally et al. 2012 ).
While the CR is an effective barrier to bonobos dispersal
northward, almost all the dendritic to sub-dendritic drainage
of the Cuvette Central (D-Sd2 in Fig. 15.3 ) have had little
affect on gene flow between populations (cohorts), apart
from the lower Lomami River (Fig. 15.5 ; Kawamoto et al.
2013 ). The barrier effect of the Lomami River is seen in the
greater genetic distance of the bonobo cohort east of the river
(the area between the Congo and Lomami rivers) compared
to the central and western cohorts (Kawamoto et al. 2013 ).
Additionally the genetic distances among the central and
western bonobos cohorts suggest that in the central region
of the basin that rivers only provide a weak barrier
(Kawamoto et al. 2013 ).

10 6 ta 1 the Ogou ´ River
has a comparable sediment to the CR (22.7

10 6 ta 1 )
although the Ogou ´ water volume is a magnitude less than
the Congo

s River, suggesting the Ogou´ watershed is
undergoing active erosion (Seranne et al. 2008 ). Thus the
headwater regions of the Ogou´ are actively incising back-
ward, and have likely captured smaller north-western
tributaries of the CR within the last 800 ka.

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