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radiation of delphiniids and that they represent relict species whose adaptation to fluvial
habitats might have insured their survival against environmental changes in the marine
ecosystem or the emergence and dominance of delphinids (Cassens et al., 2000). In the case
of franciscana, because it is mostly marine and diverged from
Inia
before the radiation of
delphinids, it is unclear whether the common ancestor of the two South American species was
marine or riverine. If it was riverine, it can be considered a recent ecological reversal with the
reinvasion of the marine coastal habitat. If otherwise, then the franciscana's ancestors might
have escaped extinction because it was ecologically specialized with adaptations to live in
coastal habitat and coupe with competitive pressures after the radiation of small delphinids.
The Amazon and Paraná river basins in South America were deeply invaded by marine waters
during Miocene high sea levels stands. The shallow estuarine regions created by the mixing
of oceanic and riverine waters probably supplied a diversity and abundance of food resources,
particularly for the species able to tolerate osmotic stress. The draining of epicontinental seas
occurred with sea level regression which took place during the Late Miocene and Pliocene.
During the highest global sea level, the Amazon and Paraná basins may have been connected,
originating an interior sea known as Paranaese Sea (sensu Von Ihering, 1927), dividing the
continent. It has been hypothesized that dolphins entered the Paranaese Sea from the north,
diversified within its complex fluvial-estuarine-marine system and colonized as far as the
western South Atlantic Ocean (Hamilton et al., 2001). Evidences include: isolated periotics
bones of
Pontoporia
from the late Miocene-Pliocene found in the eastern ravines of the
Paraná River (Cozzuol, 1985); pleistocene specimens recovered in ―Piso Querandino‖, near
La Plata, Argentina (Ameghino 1918) and; incomplete, Late Pleistocene, skulls of
P.
blainvillei
found on Rio Grande do Sul State coast, Brazil (Buchmann and Rincón, 1997;
Ribeiro et al., 1998). Furthermore, closely related early Pliocene Miocene (
Pliopontos
littoralis,
de Muizon, 1983) and middle Miocene (
Brachydelphis mazeasi,
de Muizon, 1988)
pontoporiids fossils were collected as north as the Pisco Formation in Peru and Pontistes
rectifrons (Bravard, 1885) was recovered from the late Miocene in the Paraná Formation,
Argentina (Cozzuol, 1985). The lowering of the global sea level the inland sea was drained
separating the northern and southern river basins and isolating river dolphins. While Inia
remained isolated to fluvial system in the Amazon and Orinoco,
Pontoporia
followed the
marine waters, the recede of the Paraná basin, to colonize the coastal zone north and south of
the La Plata estuary (Hamilton et al., 2001).
D
ISTRIBUTION AND
H
ABITAT
The franciscana is endemic to the western South Atlantic Ocean, ranging from Itaúnas
(18
o
25´S), Espírito Santo State, Brazil (Siciliano, 1994) to Golfo San Matias (~42
o
10S'), Rio
Negro Province, Argentina (Crespo et al., 1998) (Figure 2). Although it has been considered
by many to be a member of the so-called river dolphins (superfamily Platanistoidea -
currently thought to be a polyphyletic taxon -
e.g.
Cassens et al., 2000), franciscanas are
found mainly in coastal marine waters with occasional occurrences in estuaries (
e.g.
Santos et
al., 2007; 2009).
It is, however, relatively common in the Uruguayan part of the La Plata
River (Praderi, 1986) and Babitonga Bay estuaries (Cremer & Simões-Lopes, 2005). There is
evidence that the species is not continuously distributed throughout its range. Siciliano et al.,
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