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it is missing from the other side of MNHN CR 125 and as well as all other specimens.
This suggests to me that it is not a major conduit for branches of CNs III-VI, as in other
taxa with a superior orbital fissure. In MNHN CR 965, the location of the
alisphenoid/orbitosphenoid suture is located more dorsally such that, given the breakage,
it is possible that the orbitosphenoid did not reach the sphenorbital fissure in a significant
way. These variations introduce some uncertainty to the interpretation. Even so, in light
of the above observations, it seems that “suboptic foramen” is a better-supported
designation for Russell’s “t.d.a.” and that Russell’s “t.r.” should be considered the
sphenorbital fissure. This conclusion is illustrated in Figure 2.41.
Composition of bulla: sutural evidence on medial tympanic process
The total morphological pattern for the paromomyid ear suggests that its medial
tympanic process is petrosally derived. Even if one accepts that there are two “types” of
bone present in the paromomyid petrosal, as argued for Acidomomys hebeticus (Fig. 2.39)
with bone relating to the medial tympanic process having apparently migrated down onto
the promontorium to create a sutural effect, the fact that bone comprising this process
also extends dorsal to much of the promontorium (onto the bone’s endocranial surface)
suggests that it is also a petrosally-derived bone: entotympanics and ectotympanics are
not known to have endocranial expression in other taxa (MacPhee, 1981). Given the
documented mode of development of entotympanic elements in treeshrews and various
insectivorans (MacPhee, 1981), it is difficult to imagine how they ever could intervene
between the petrosal and the endocranium. If two types of petrosally-derived bone are
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