Biology Reference
In-Depth Information
Evolution of River Dolphins
The Middle Miocene was a time of globally high sea levels, with three significant marine
transgressive-regressive cycles recorded worldwide (Haq et al., 1987). With the resulting
large-scale marine transgressions on to low lying regions of the continents, shallow
epicontinental seas became prominent marine ecosystems. The Indo-Gangetic plain of the
Indian subcontinent, the Amazon and Parana River basins of South America, and the Yangtze
River basin of China are vast geomorphic systems whose fluvio-deltaic regions were deeply
penetrated by marine waters during high sea-level stands. The shallow estuarine regions
created by the mixing of riverine and marine waters probably supported diverse food
resources, particularly for aquatic animals able to tolerate osmotic differences between fresh
and saltwater systems. Hamilton et al. (2001) proposed that the ancestors of the four extant
river dolphin taxa were inhabitants of Miocene epicontinental seas. Draining of epicontinental
seas and reduction of the near shore marine ecosystem occurred with a late Miocene trend of
sea level regression, which continued throughout the Pliocene, interrupted by only moderate
and relatively brief events of sea-level rise (Hallam, 1992). As sea levels fell, these archaic
odontocetes survived in river systems, while their marine relatives were superseded by the
radiation of Delphinoidea. Cassens et al. (2000) also noted the persistence of river dolphins
during the radiation of delphinoids. They suggest that extant river dolphin lineages ‗escaped
extinction' by adaptation to their current riverine habitats. By integrating phylogenetic,
palaeoceanographic and fossil data, an explicit hypothesis for the evolution and modern
distribution of river dolphins has been provided by Hamilton et al. (2001).
The Indo-Gangetic foreland basin is a broad, flat plain of sediment delivered throughout
the Cenozoic by an intricate network of migrating rivers descending from the tectonically
dynamic Himalayan Mountain (Burbank et al., 1996). The increased sea levels of the middle
Miocene would have inundated large areas of the foreland basin, creating a shallow marine
habitat. Fossils have not yet been recovered from these regions, but platanistids are known to
have inhabited Miocene epicontinental seas in North America (Morgan, 1994; Gottfried et al.,
1994).
Ganges River Dolphin
Platanista
is the only surviving descendant of an archaic odontocete that ventured into
the epicontinental seas of the Indo-Gangetic basin, and remained through its transition to an
extensive freshwater ecosystem during the Late Neogene trend of sea-level regression.
Although the paleogeography of the two river systems would suggest a history of isolation,
the genetic distance in the sample of
P. gangetica
(Ganges population) and
P. minor
(Indus
population) is surprisingly low. The Indus and Ganges populations were long regarded as
identical until Pilleri & Gihr (1971) divided them into two species based on differences in
skull structure, but Kasuya (1972) reduced the two taxa to subspecies of a single species. This
is supported by the results of Yang & Zhou (1999), who found that the difference between
cytochrome-b sequences of Ganges and Indus river dolphins was very small. Even until
historical times there was probably sporadic faunal exchange between the Indus and Ganges
drainages by way of head-stream capture on the low Indo-Gangetic plains between the Sutlej
(Indus) and Yamuna (Ganges) rivers (Rice, 1998 and refs. therein). Rice (1998), in his
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