Boyer diss 2009 1046..

position from the tympanic plexus groove photographed by Gingerich (1976) for the
Pellouin skull and represents a different structure (although both could certainly have
held tympanic plexus fibers). The “s2” groove is not identified. The authors noted that it
has a similar location to a canal previously identified as the internal carotid canal and
illustrated by Russell (1964: fig. 15) in MNHN CR 125, and photographed and labeled by
Gingerich (1976: Pl. 9c). They stated, however, that the groove does not connect to a
foramen leading to extratympanic space, implying that it could not actually have held a
functional internal carotid artery or nerve. They argued that this fact, combined with a
lack of the “s2” groove on the other specimen, indicates that the internal carotid system is
diminished in importance to the point that it was only sporadically retained in the adult
plesiadapid specimens. MacPhee et al. (1983) also argued bullar composition cannot be
determined in fossil taxa lacking a suture between the bulla-forming bone and
promontorium of the petrosal (Russell, 1959, 1964; Gingerich, 1976) because only
ontogenetic evidence can reveal whether the bulla started as an ossification separate from
the petrosal bone.
MacPhee and Cartmill (1986) characterized plesiadapid basicranial anatomy
according to previous publications. They provided an expanded argument against
petrosal contribution of the auditory bulla based on further observations of the Pellouin
skull (Gingerich, 1975; 1976). Specifically they pointed out that, although there is no
visible suture between the petrosal bone and bulla medial to the promontorium, there is
no suture between the ectoympanic and lateral aspect of the bulla either. This means that
it is not clear whether the bulla is petrosal, entotympanic or ectotympanic in composition.
It is important to note, however, that all of the specimens available to them lacked the
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