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korneuburgensis
by being larger, having a thickened supraorbital process, an enlarged
supraorbital maxillary tuberosity, and a more enlarged and more elevated nuchal crest.
In species of the genus
Pomatodelphis
the lateral margin of the maxilla immediately
anterior to the antorbital notch is thickened and laterally expanded, and the supraorbital
maxillary crest is greatly enlarged and knob-like.
Pomatodelphis
is represented in the eastern
North Atlantic by the poorly known species,
Pomatodelphis stenorhynchus
(Holl, 1829),
which was based on a fragment of a rostrum from France. The holotype of this species, No.
2228, Laboratoire de Paléontologie, Muséum National d'Histoire Naturelle in Paris, is of
Middle Miocene age from Maine-et-Loire, Department de l'Orne, France, (see Kellogg
1959:19-20), and no other specimens can now be confidently referred to this species.
The type species of
Pomatodelphis
is
Pomatodelphis inaequalis
Allen, 1921, which is
known from the late Middle Miocene Agricola Fauna from central Florida, and which is
derived from near shore marine deposits of the Lower Bone Valley Formation (Morgan,
1994), and is approximately 10.5 Ma to 11.5 Ma in age (Morgan, 1994:251-252). Hulbert &
Whitmore (2006) reported this species from a Middle Miocene age river-laid deposit in
Alabama, and this is the geochronologically earliest indication of a fresh water occurrence of
a platanistoid.
Schizodelphis depressus
Allen, 1921, also named on the basis of fossils from
the marine Bone Valley Formation in Florida, has been synonymized with
P. inaequalis
(Morgan, 1994).
Another Florida pomatodelphinine species is
Pomatodelphis bobengi
(Case, 1934), which
was originally described as a species of
Schizodelphis
(see Morgan, 1994). It is also from the
marine Lower Bone Valley Formation, of late Middle Miocene age, and approximately
10.5 Ma to 11.5 Ma in age (Morgan, 1994:251-252; and see also Kellogg, 1944).
Whitmore & Kaltenbach (2007) have reported
Pomatodelphis
sp. from the marine,
Middle Miocene age, Pungo River Formation in North Carolina.
The prominent eminences over the orbits on skulls of derived species of
pomatodelphinine platanistids are formed by the frontal bones. These eminences are not solid
bony structures. A CT scan (Figure 9C) through an adult cranium (USNM 10911) that was
referred to
Zarhachis flagellator
by Kellogg (1926) shows that these structures are excavated
in their medial parts and have within their lateral parts layers of bone. We conclude that the
layers within these eminences formed during ontogeny, and that newer layers were added in
the lateral parts of the enlarging frontal bone as the skull increased in size. Much of the
medial part of each supraorbital eminence is seen to be secondarily excavated, and these
excavations cut across the different layers of interior bone. We suggest that this excavation of
the medial sides of these eminences is the result of invasion by the supraorbital lobe of the
pterygoid air sinus. The supraorbital lobe of the pterygoid air sinus, as it does in Recent
Platanista gangetica,
(see Fraser & Purves, 1960), appears to have extended dorsally from the
orbital region, via the anterior maxillary foramina, and invaded the medial sides of these
tuberosities. The invasion by the sinus tissue is interpreted to have resorbed the medial side of
the eminence, as evidenced by the way that the vacuities cut across the early ontogenetic
layers of bone. It is significant that the enlarged supraorbital eminence of
Zarhachis
flagellator
is formed by the frontal bone, and the maxillary bone is not exceptionally
thickened (Figure 9C). In contrast, in Recent
Platanista gangetica,
the frontal bone is not
exceptionally thickened, but the maxillary bone is greatly enlarged (Figure 11B), and it is the
maxilla that is invaded by the pterygoid sinus (Figure 9C). These two different manifestations
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