Boyer diss 2009 1046..

the bulla. In Sciurus carolinensis and Tupaia glis the promontoria have modest medial
and rostral tympanic processes. In Sciurus, the ectotympanic forms the bulla and septa,
which contact the medial and rostral tympanic processes of the petrosal in an externally
on-lapping, squamous suture. The overlap is extensive and can easily be visualized with
HRxCT data (Fig. 2.32). In Tupaia, the condition is similar except that the bulla-forming
bone is the entotympanic and the overlap is not so extensive, because the rostral and
medial processes of its promonotorium are relatively smaller (MacPhee, 1981) (Fig.
2.33). In some rodents with this basic pattern (e.g., Marmota: Fig. 2.34), foramina for
neurovasculature relating to the tympanic and internal carotid plexi can be observed to
enter the tympanic cavity at the sutural boundary between the ectotympanic and petrosal
on the medial aspect of the promontorium. Other rodents, specifically Lagostomus (Figs.
2.35, 36) and Dipodomys, fuse the suture between the ectotympanic and petrosal (and
thus superficially appear to have a petrosal bulla), but still preserve evidence of this
suture via the canals for tympanic plexus nerves that reach the middle ear cavity through
this sutural conduit (Fig. 2.36). Thus, if plesiadapids have a non-petrosal bulla, one
might expect the suture to be expressed as overlapping laminae of bones on the tympanic
processes, as in Sciurus and Tupaia, or by the planar organization of canals for
neurovasculature along a previously unfused suture.
Almost all the plesiadapid specimens that preserve part of the medial tympanic
process show it to be comprised of two layers of bone. In fact, Pronothodectes gaoi
(UALVP 49105) looks nearly identical to a Sciurus carolinensis ear that has been
prepared in a way so as to mimic the inferred breakage on the fossil (Fig. 2.32). This
fact, combined with the observation of what appears to be a suture on the medial aspect
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