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varying degrees, by the palatine bone and the lateral lamina of the pterygoid bone (see Figure
1C, where it is labeled on the holotype skull of
Allodelphis pratti
Wilson, 1935), which
extend posteriorly and connect with the alisphenoid bone at a location anteromedial to the
glenoid fossa. Muizon (1994) discussed this structure, noting that it exists in all platanistoids
except for species of
Prosqualodon.
He described the lamina as being comprised of the
palatine bone, covered in its middle by maxilla and pterygoids, and divided into a small
anteroventromedial part, and a larger posterodorsolateral area. This laminar plate of bone was
called the
―reduplicated pterygoid‖
by Miller (1923
).
This structure is in part a modification
of the smaller pterygoid of generalized mammals. Among the Odontoceti the pterygoid
hamulus is invaded by the pterygoid sinus (see Fraser and Purves, 1960). The participation of
the lateral lamina of the pterygoid in this prominent bony wall within the orbit of the skull is
noteworthy, because in most other Odontoceti except members of the Platanistoidea, it is an
atrophied and thin sheet of bone, or it is replaced by a membranous sheath.
Several platanistoids have been shown to lack a coracoid process on the anterior surface
of the scapula (Fordyce, 1994; 2006), and because this process is present in more primitive
Cetacea, and in most other more derived cetacean groups, its loss from the scapula in the
platanistoids is interpreted as being a synapomorphic character. This character, the lack of a
coracoid process, exists in the primitive platanistoids, the family Squalodontidae, and in the
Recent
Platanista,
but because scapulae are not yet described for many other platanistoids,
we did not include it in our phylogenetic analysis.
Muizon (1994) also suggested that the superfamily Platanistoidea is characterized by
having the acromion process located on the anterior edge of scapula rather than the lateral
surface, the latter location being where it is in generalized mammals, and in primitive
Cetacea. The apomorphic condition, however, might characterize some but not all
Platanistoidea, because on the scapula of an as yet un-named late Early Miocene allodelphinid
platanistoid from Japan (Kimura & Ozawa, 2001) the acromion is not located on the anterior
edge of the scapula, but is on its lateral side, as is typical of primitive Cetacea. If it should be
demonstrated that this primitive position of the acromion, on the lateral side of the scapula, is
characteristic of species of the family Allodelphinidae, then this might re-enforce other
indications from the phylogenetic analysis presented herein that the Allodelphinidae are basal
members of the superfamily Platanistoidea.
Platanistoids retain relatively large dorsal vertebrae, a primitive character state for
Cetacea. Commensurate with their large dorsal vertebrae, the ribs of platanistoids are also
relatively large for their body size. Furthermore, through time, members of the family
Allodelpinidae evolved progressively larger cervical vertebrae. This phenomenon is the
opposite trend that is exemplified by most other groups of Cetecea, in which the cervical
vertebrae become progressively very much shortened anteroposteriorly, and in some taxa
becoming fused to one another to varying degrees.
A primitive condition that is exemplified by squalodontids is the persistence of very large
and anteriorly procumbent incisors and canines. In life these anterior teeth certainly must
have protruded from the front of the mouth, probably appearing in a fan-like array of small
tusks. In members of the later and more highly derived platanistoid families Waipatiidae and
Squalodelphinidae, these anterior teeth became more reduced in size, but they remained still
relatively prominent. In members of the families Allodelphinidae and Platanistidae, the
anterior teeth are no longer tusk-like, but they did remain larger in comparison with the
posterior teeth.
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