INVESTIGADORES
PUJOS FranÇois Roger Francis
congresos y reuniones científicas
Título:
A relative of the Ganges and Indus river dolphins from Miocene deposits of the Amazonian basin: multiple toothed whale invasions of freshwater environments?
Autor/es:
OLIVIER, LAMBERT; G., BIANUCCI; RODOLFO, SALAS-GISMONDI; JULIA, TEJADA; FRANÇOIS, PUJOS; MARIO, URBINA; PIERRE-OLIVIER, ANTOINE
Lugar:
Bruxelle
Reunión:
Congreso; 4th Internacional Geological Belgica Meeting 2012 ? Moving Plates and Melting Icecaps. Processes and Forcing Factors in Geology; 2012
Resumen:
During
their transition to the marine environment, in addition to numerous
morphological changes, early cetaceans had to evolve physiologically
to face the excess of salt due to the ingestion of seawater. But
evolution is complex, far from being a process with one way. After
this critical Eocene step of their evolutionary history, followed by
several phases of diversification of fully marine cetaceans (leading
to the two extant suborders, Mysticeti, the baleen whales, and
Odontoceti, the toothed whales), members of several odontocete clades
independently recolonized freshwater environments. Nowadays, four
species in the genera Inia,
Lipotes (possibly
recently extinct), and Platanista
(two species), exclusively occupy river and lake habitats,
respectively in the Amazon-Orinoco, Yangtse, and Ganges-Indus river
basins. Based on morphological and molecular data, as well as on
paleontological information on their fossil relatives, it has been
demonstrated that Inia,
Lipotes, and
Platanista are
relict members of ancient lineages with distantly related marine
ancestors.
Among
them, the endangered Platanista
is interesting for several reasons. In addition to its
unusual cranial morphology, this nearly blind dolphin is the sole
survivor of the superfamily Platanistoidea, much diversified from the
late Oligocene up to the middle Miocene. Among platanistoids, the
fossil record of Platanistidae is dominated by members of the extinct
subfamily Pomatodelphininae, most of them discovered in marine to
estuarine Miocene layers. Only one fragmentary mandible, from marine
deposits of the early Miocene of Oregon, has been tentatively
referred to the subfamily Platanistinae, the clade typified by the
extant Platanista.
We
report on the discovery of an isolated platanistid periotic (ear
bone, one of the most diagnostic elements in crown cetaceans) found
in late middle Miocene sediments of the Fitzcarrald Arch area,
Peruvian Amazonia. This periotic is identified as belonging to a
platanistine, based on several derived characters shared with
Platanista. Even
if fragmentary, this is likely the most significant fossil record of
a platanistine, partly filling the temporal gap between the emergence
of the subfamily and the Recent Platanista.
Interestingly, this specimen was found in a geographic area now
occupied by Inia.
Sedimentology and associated fauna indicate that this middle Miocene
depositional area corresponds to an inland tidally-influenced
freshwater/oligohaline basin. Although a marine origin cannot be
discarded for this new fossil platanistine record, it is tempting to
hypothesize that it illustrates an early step of the colonization of
a freshwater habitat, in a region very far from the area of
Platanista. If
confirmed, this interpretation would further support the scenario of
multiple Miocene freshwater invasions by members of several
odontocete clades.
Reasons
for such drastic ecological changes are not yet clear, and several
factors, both biological (competition with diversifying pelagic
delphinoids, flight from oceanic predators, migration of prey in
freshwater regions) and physical (sea level changes and temperature
fluctuation), might have played a role. Species of three strictly
freshwater odontocete genera survived until the Holocene, and one of
them probably went extinct at the dawn of a twenty-first century that
will be decisive for the fate of the remaining others.