Boyer diss 2009 1046..

to the peroneal surface. The peroneal surface is slightly concave at the proximal end, but
becomes convex farther distally. The posteromedial surface is marked by a sharp,
posteromedially projecting crest that separates this surface into anterior and posterior
regions of roughly equal anteroposterior dimensions. The surface posterolateral to this
crest is that typically referred to as the posterior, or “flexor surface,” whereas the more
anteromedial one is that referred to as the medial or “tibialis posterior surface ” (Stern,
1988). Followed distally, the crest separates these two surfaces arcs anteriorly,
converging on another crest, the interosseous crest, which has its proximal origin from
the medial lip of the “anterior” surface.
Function.—A striking feature of the fibula is the blade-like crest projecting
posterolaterally from the proximal end. This crest extends the posterior or “flexor
surface” of the fibula proximally, far beyond its limit in humans, for instance. This crest
would greatly augment the area of origin for both the flexor digitorum fibularis muscle on
the posteromedial side of the crest and the peroneus longus muscle on the opposite side
(anterolateral). Thus, functions associated with these muscles, such as forceful flexion of
the digits and eversion of the foot, may have been enhanced. The rugose tuberosity for
the fibular notch on the medial aspect of the distal end suggests a strong tibiofibular
syndesmosis, and thus limited mobility between the tibia and fibula.
Comparison.—I was unable to locate the only other plesiadapid fibula specimen
available for comparison (a distal end of P. tricuspidens MNHN BR-11-L: Beard, 1989).
Beard only mentions the presence of a distal articular surface for the tibia being present,
and suggests a synovial joint existed, instead of a syndesmosis. Thus either these two
taxa had vastly different distal fibulae, or our interpretations differ markedly.
339