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Amazon ones. This means that, at least, two migrations from the Amazon penetrated
into the Orinoco basin, but, at least, one migration from the Orinoco migrated to the
Amazon basin. This means that the affirmation of Grabert (1984a, b, c) is false and
the black-waters are not a barrier for the dispersion of the pink river dolphins. In
agreement with my affirmation, Meade & Koehnken (1991) showed that the Negro
River in the Brazilian Amazon or the Atabapo River in Venezuela also contained
considerable
Inia
populations. However, Pilleri & Pilleri (1982) affirmed that the
density of
Inia
was 50 times less in the black-water systems such as in the upper
Orinoco and the Casiquiari channel compared to a white-water river like the Apure
River in Venezuela. In fact, some black water tributaries, for example, of the Napo
River in Ecuador do not contain
Inia
populations (Tagliavini & Pilleri, 1984).
All these results reveal that is possible that the ancestors of
Inia
could have originated in
the Atlantic Ocean (but
Inia
appeared ―in situ‖ in the Amazon) in discordance with Grabert
(1984 a, b, c), but in agreement with the opinions of Brooks et al. (1981) and Gaskin (1982).
It is known, and previously mentioned, that in the late Miocene, the Iquitos-Purús arch
divided the Amazon basin in two. At that time, the Paraná River basin (Cozzuol, 1996), the
eastern Amazon basin (Hoorn, 1994), most of the South American Atlantic coast and a big
fraction of the Orinoco basin were covered by epicontinental sea waters (Entrerriense
transgression; Cozzuol, 1992; Hoorn et al., 1995; Lovejoy et al., 1998). Therefore, the
Iquitos-Purús arch interconnected these three river basins (Orinoco, Amazon and Paraná-La
Plata systems). The Rebeca Lagoon, the Grande Tremedal River, or the Paranense Sea, for
instance, interconnected the Amazon and the Paraná basins (Klammer, 1984), at different
moments, permitting the inter-change of fauna. This internal epicontinental sea is supported
by sedimentological data and the fauna distribution of mollusk and foraminifera (Räsänen et
al., 1995; Nuttall, 1990; Boltovsky, 1991). Recall,
Pontoporia blainvillei
is an estuarine
dolphin living in the Atlantic coasts of Argentina, Uruguay and Brazil (mouth of La Plata
River) and is the sister clade of
Inia
(Cassens et al., 2000; Hamilton et al., 2001). Henceforth,
it is easy to imagine that
Inia
and
Pontoporia
had a common ancestor which could penetrate
in that interconnected Orinoco-Amazon-Paraná system via the Atlantic Ocean. Later, when
the orogeny of the Andes in the late Miocene and Pliocene, the influence of the Iquitos-Purús
arch was lost and the Guayaquil Gate was closed (Pacific Amazon drainage) and the complete
Amazon river drainage was towards the Atlantic ocean. In that moment, the interconnection
between the three basins diminished and disappeared between the Amazon and the Paraná
basins. With these changes, it is possible that
Inia
(or better an
Inia
ancestor) was isolated in
the Amazon Basin, meanwhile
Pontoporia
(or a
Pontoporia
ancestor) returned, or simply
stayed, at the estuarine or coastal habitats of the South America Atlantic Ocean. Some
palaeontological data could be in agreement with this hypothesis. It seems that the unique and
clear Iniidae fossil found in Central Florida, outside South-America, is
Goniodelphis
from the
Early Pliocene (Morgan, 1994), although Muizon (1988) was not absolutely convinced that it
was an Iniidae. Three clear Iniidae, such as
Ischyorhynchus
,
Saurodelphis
and
Saurocetes
,
were found for the Late Miocene of the Paraná basin in Argentina. It could contribute
additional proof that a marine ancestor of
Inia
could penetrate into the continental South-
America via the Paraná basin and then be isolated in the Amazon Basin when the sea level
declined and the Amazon drainage was only towards the Atlantic Ocean. It is easy to imagine,
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