Boyer diss 2009 1046..

of the promontorium of the Pellouin skull (Fig. 2.28), strongly suggest a suture in this
vicinity. HRxCT scans of UALVP 49105 do not, however, strengthen the support for
this interpretation because they do not reveal any separation between these bony layers,
and furthermore, do not show any “planes of canals” along a possibly previously unfused
boundary.
A further test of the significance of the apparent similarity in the medial tympanic
process morphology between plesiadapids and mammals known to have non-petrosal
bullae is to examine the cross-sectional morphology of more ventral and lateral
components of the bullar wall. If plesiadapids do in fact have a bullar construction
similar to that of Sciurus, for instance, then these more ventral regions of the bullar wall
should be thin and comprised of a single lamina of bone as they are in Sciurus. This test
is not possible in the UALVP specimens. However, it is possible in the Pellouin skull of
P. tricuspidens.
The cross-sectional morphology of the more ventral and lateral parts of the bullar
wall in the Pellouin skull exhibit two distinct layers. This is contrary to the prediction of
the hypothesis that the two layers of bone on the medial tympanic process represent two
different bones (Fig. 2.30). Furthermore, the broken open bulla in the otherwise intact
skull of the euprimate Adapis looks extremely similar to the condition in P. tricuspidens
in having two layers comprising the bulla (Fig. 2.30). Finally, the morphology of a
petrosal specimen of the euprimate Indri indri contradicts the most straightforward
interpretation of the significance of multiple bony layers on the medial process of the
promontorium. Indri also exhibits the double layer morphology at the lateral margin of
the medial process extending from the promontorium (Fig. 2.37).
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