Boyer diss 2009 1046..

groove measures about 0.29 mm in diameter. This is the groove that MacPhee et al.
(1983: fig. 1) labeled as “s2.” In their figure caption they attribute this morphology to
AMNH 17388, and describe USNM 309902 as possessing the morphology of the former
specimen. MacPhee et al. (1983: 509) stated that this groove has a blind end, posteriorly
against a “bridge of bone uniting the promontory to the posterior wall of the bulla.” This
description, however, did not reveal that the specimen is broken in some critical areas.
How the bulla and promontory connected can actually only be inferred, because the
whole bulla is shifted anteriorly and the posterior part of the bulla is broken out where the
posterior carotid foramen would have been located (Figs. 2.9-13) on both ears. This is
especially observable with the CT scan. The g2 and g5 grooves are not preserved and
may or may not have been present. The g4 groove may be present on the right ear where
the medial side of the promontorium is best exposed. A groove running mediolateraly on
the left promontorium appears to be the g3 groove (Fig. 2.11: g3), because HRxCT data
reveal that it leads towards the s2.
Both ears preserve the bone-enclosed facial nerve canals (Fig. 2.9: 41).
Semicircular canals are also preserved (Figs. 2.9, 11: 42). Their diameters are provided in
Silcox et al. (in press). The right ear preserves a large epitympanic recess (Fig. 2.9: 43), a
bone-enclosed chamber dorsolateral to the fenestra vestibuli and posterior to the
epitympanic crest.
The left bulla, although shifted relative to the promontorium, is fairly undistorted
(Figs. 2.10-13). As indicated above, the posterior wall of the bulla is missing, however,
the limestone that infills it creates a natural cast, which allows visualization of its intact
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