THE EVOLUTIONARY HISTORY AND PHYLOGENETIC RELATIONSHIPS OF THE SUPERFAMILY PLATANISTOIDEA - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 460

of bone invasion by the dorsal expansion of the pterygoid sinus may be diagnostic differences
between the two named subfamilies of Platanistidae: invasion of the enlarged supraorbital
process of the frontal bone in the subfamily Pomatodelphininae, and invasion of the enlarged
supraorbital process of the maxillary bone in the subfamily Platanistinae. If this is true, then
Araeodelphis natator is indeed a basal platanistid, in which neither the maxilla nor the frontal
are extremely thickened dorsal to the orbit, and neither bone is invaded by the pterygoid sinus
dorsally.
Figure 9. Cranium of a fossil western North Atlantic marine-adapted member of the platanistoid family
Platanistidae, subfamily Pomatodelphininae, Zarhachis flagellator Cope, 1868, of Middle Miocene age
from the Calvert Formation in Maryland, U.S.A.; A, dorsal view; and B, left lateral view; C, high
resolution CT scan made transversely through a cranium, USNM 10911, referred to Zarhachis
flagellator by Kellogg (1926) transecting the orbital area, showing vacuities within the supraorbital
tuberosities created as the supraorbital lobe of the pterygoid sinuses invaded them; figures not to scale;
the images in A and B are composites of two referred specimens, USNM 10485 and USNM 10911, and
are modified from Kellogg (1924:pls. 1, 2; 1926:pl. 1, 4); anatomical abbreviations in C are: Fr-frontal
bone, Mx-maxillary bone, Na-nasal bone, Pmx-premaxillary bone, Pt(ll)-lateral lamina of the pterygoid
bone, zp-zygomatic process of the squamosal bone.