Biology Reference
In-Depth Information
The holotype (Ameghino, 1891) is a fragment of a rostrum with worn teeth. Additional
material, including cranial and mandibular specimens, has been found and referred to this
species (see Burmeister, 1891, 1892; Pilleri & Gihr, 1979; Cozzuol, 1985).
Descriptions of the skull published by Burmeister (1891, 1892) of
Ischyrhorhynchus
vanbenedeni
were actually composites of fossil fragments from
I. vanvenedeni
and
Saurocetes
argentinus
, then sketched in completion with the known structure of
Pontoporia.
Ischyrhorhynchus
vanbenedeni
is characterized by a very long, laterally compressed
beak, a pneumaticized maxillary crest, and very small orbits. These characters, all of which
are convergent with the living Ganges river dolphin
Platanista
gangetica
, have been
interpreted as adaptations to a highly turbid fluvial environment (Cozzuol, 1985, 1988).
Pilleri & Gihr (1979) reconstructed the sonar field of
Ischyrhorhynchus
and suggested that, it
had a pattern of sound production and behavior similar to the living
Platanista,
that is a side-
swimming cetacean with rapid scanning movements of the head. For the purposes of
understanding river dolphin phylogeny and evolution, the importance of
Ischyrhorhynchus
is
that it has been widely accepted as a valid member of the family Iniidae, and that this Late
Miocene form was already highly specialized for existence in the freshwater riverine
environment.
Fossils of
I. vanbenedeni
have been recovered originally from the fluvial Ituzaingó
Formation, along the southeastern cliffs of the Paraná River in Entre Rios Province,
Argentina, but it was also recorded in the Late Miocene Solimões Formation in the State of
Acre, northern Brazil (Boquentin et al., 1990; Cozzuol, 2006, personal observation).
The fossil assemblage, specially vertebrates, in both localities are very similar, with
several species in common. They are dated as Late Miocene and include freshwater fishes,
aquatic and terrestrial turtles, Aligatoriidae, Gavialiidae and Netosuchidae crocodiles, birds
and land mammals (see Cione et al., 2001; Cozzuol, 1993; Rancy et al., 1989, Cozzuol,
2006). The marine influence is stronger in the Entre Ríos (Argentina) deposits, which is clear
from the underlying marine Paraná Formation and from the presence of marine fishes in the
continental sediments (Cione et al., 2001). Some marine influence was detected in the Acre
region too, but its importance is controversial (Cozzuol, 2006).
Remains
referred
to
Ischyrorhynchus
were
reported
originating
from
the
contemporaneous deposit from Venezuela (Urumaco Formation; Linares, 2004).
Genus
Saurocetes
S. argentinus
Burmeister, 1871
The classification history of this species and its synonyms, has also been quite confusing,
as detailed by Pilleri & Gihr (1979). The holotype of the first described species,
Saurocetes
argentinus
Burmeister, 1871 is a mandibular fragment, still containing teeth. Its stratigraphic
origin was described only as Tertiary outcrops of the eastern bank of the Paraná River.
Subsequently, much fragmentary cranial, rostral, and mandibular fossil material has been
collected from the Ituzaingó Formation and attributed to
Saurocetes argentinus
(Pilleri &
Gihr 1979; Cozzuol, 1985, 1988).
S. argentinus
is principally distinguished from
Ischyrhorhynchus
by a much lesser degree of lateral compression in the mandible and rostrum
and by distinct tooth morphology consisting of a very wrinkled enamel, anterior and posterior
carinae, and sharp and posteriorly recurved apex.
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