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Indus River dolphin (
P. g. minor
) were probably two subspecies of a single species. They also
suggested that among the four river dolphin families, Platanistidae was the earliest divergent
clade, the Lipotidae was the next, and then the Iniidae and Pontoporiidae. Further they
suggested that the river dolphins were paraphyletic, and it was reasonable to place
Platanista
at a superfamily level as no affinity was revealed between the Platanistidae and other river
dolphin families. The long-suspected polyphyly of river dolphins is supported by the
mitochondrial sequence data. In both trees,
Platanista gangetica
(and
Platanista mino
r,
representing Platanistidae), is sister to the remaining odontocetes, although bootstrap support
for this node is low (Hamilton et al., 2001).
Extinct taxa assigned to the Platanistidae are well documented, particularly
Zarhachis
and
Pomatodelphis
, long beaked Middle to Late Miocene cetaceans recovered primarily from
shallow epicontinental sea deposits of the Atlantic coast of North America (Kellog, 1995;
Gottfried et al., 1994; Morgan 1994). Possible platanistid relatives are Squalodelphinidae and
at least some members of Squalodontidae (Muizon, 1994; Fordyce, 1994), two well known,
extinct families of archaic, medium sized heterodonts. Other fossil relatives of the
Platanistidae include members of the Delpiaziniidae (Muizon, 1994) and Waipatiidae
(Fordyce, 1994). If these lineages are monophyletic, then Platanista is the sole extant member
of a once-abundant and diverse clade of archaic odontocetes. The side-swimming, blind and
highly endangered Indian River dolphin has long been recognized as ‗the genus presenting
the greatest total of modifications known in any cetaceans' (Miller, 1923). However, both
fossil and extant platanistids warrant further investigation for potential insights into cetacean
evolution. Verma et al. (2004) established the evolutionary relationship of the Ganges River
dolphin with extinct and extant cetaceans based on comprehensive analyses of the
mitochondrial cytochrome b and nuclear interphotoreceptor retinoid-binding protein gene
sequences, obtained from 15 specimens of Ganges dolphin from India and Bangladesh. The
study suggested that
P. g. gangetica,
a toothed cetacean, is significantly closer to Mysteceti
(Toothless whales) than to any other group of toothed whales. However, Yan et al. (2005)
observed that the
Platanista
lineage is always within the odontocete clade instead of having a
closer affinity to Mysticeti. Nevertheless, they opined that the position of the
Platanista
is
more basal, suggesting separate divergence of this lineage well before the other one. And they
agree that they could not resolve with high significance the exact phylogenetic position of
Platanista. The more basal position of Platanistidae is also supported by the records of
platanistoid fossils in the late Oligocene (Fordyce & Barnes, 1994). Muizon (1991), Heyning
(1989) as well as Messenger & McGuire (1998) proposed that Platanistidae branched after the
divergence of sperm and beaked whales. However, others placed Platanistidae and beaked
whales in a clade between the sperm whale and more crown-ward odontocetes (Cassens et al.,
2000) or placed Platanistidae between sperm whale and beaked whales (Hamilton et al., 2001;
Nikaido et al., 2001). The position of Platanistidae is not very clear. This may be, at least in
part, because the susu, sperm, beaked, and baleen whales lineages seem to have been
produced through a very rapid succession of splitting events in the Eocene (Nikaido et al.,
2001). Meanwhile, additional evidence is needed to resolve this issue. There is a consensus
that these four river dolphins belong to two different groups of dolphins: Platanistoidea,
which is an early divergent superfamily of odontocetes, and nonplatanistoid river dolphins, a
monophyletic clade closely related to superfamily Delphinoidea.
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