THE EVOLUTIONARY HISTORY AND PHYLOGENETIC RELATIONSHIPS OF THE SUPERFAMILY PLATANISTOIDEA - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 457

the dorsal narial opening, and it is separated by a considerable distance from the premaxillary
bone. This is the primitive condition for the Odontoceti. Members of the family Platanistidae,
in contrast, have a derived character state, in which the posterior maxillary foramen is located
very close to the posterolateral corner of the posterior end of the premaxilla, and in some
individuals and in some species, this foramen is under the edge of the premaxilla.
The most primitive member of the Platanistidae is Araeodelphis natator Kellogg, 1957,
which occurs stratigraphically low within the Miocene Calvert Formation in Maryland
(Godfrey et al., 2006; Bohaska et al., 2007), and also in the Middle Miocene age Pungo River
Formation in North Carolina (Whitmore & Kaltenbach, 2007). The exact relationships of this
species have yet to be determined, so it cannot now be assigned to either of the following two
platanistid subfamilies. When Kellogg (1957) described Araeodelphis natator, he did not
recognize the platanistid affinities of the holotype skull because it included only the rostrum
and part of the mandible. A more complete cranium (USNM 526604) is now known from the
lower part of the Calvert Formation (Early Miocene in age, approximately 17 MA), and it has
a rostrum that is similar in morphology but slightly smaller than that of the holotype of
Araeodelphis natator . This skull, USNM 526604 (Figure 7), we now identify as
Araeodelphis, cf. A. natator , and it allows Araeodelphis to be identified as a member of the
family Platanistidae. As in all Platanistidae, the posterior maxillary foramen is located at the
posterior-most end of the premaxilla, behind the nasals, but unlike other Platanistidae, the
bone over the large, dorsally convex supraorbital processes is not particularly thickened
(Figure 7B), and there is no development of pneumaticity above these processes. The large
size of its orbits suggests that sight was still an important sense in this fully marine
platanistid, in contrast to the living Platanista gangetica.
Figure 7. Cranium of a very primitive member of the platanistoid family Platanistidae, Araeodelphis, cf.
A. natator Kellogg, 1957, USNM 526604, early Miocene in age, approximately 17 Ma, from the
Calvert Formation, western North Atlantic Ocean; A , dorsal view; B , left lateral view; scale bar equals
10 cm.