Boyer diss 2009 1046..

Trapezium
Description.—The left trapezium is preserved (Figs. 4.11A, 4.12). This bone is
not known for any other plesiadapid, or plesiadapiform for that matter. It is identifiable
as the trapezium mainly by its ability to articulate with the proximal articular surface of
the first metacarpal (MC I). The facet of MC I is “kidney bean”-shaped in distal view
(Fig. 4.11A; 4.12A: 2). It measures 3.73 mm by 1.91 mm. Its surface area is 5.6 mm 2 . In
ulnar and radial view, the the outline of the trapezium is trapezoidal. This is due to
dimensions that differ from the dorsal side of the bone (1.97 mm) to the ventral side (3.00
mm). The ulnar surface has a proximal facet for the trapezium (Fig. 4.12A: 3) that slopes
distally toward the dorsal side. The proximodistal axis of the trapezium deviates from the
plane of the trapezoid facet by ~45° toward radial.
Function and comparison.—When the trapezium is articulated with the scaphoid
and trapezoid (Fig. 4.12), the MC I facet of the trapezium faces radially and ventrally
compared to the MC II facet of the trapezoid. This suggests the presence of a divergent
pollex. Furthermore, the MC I facet is broadly separated from the MC II facet when the
two bones are articulated, indicating that MC I and MC II would not have contacted one
another in the articulated hand (Figs. 4.10-12). Beard (1989) recognized that N.
intermedius had a widely divergent pollex. However, he considered MC I and II of N.
intermedius to have had a relatively immobile contact with one another, based on study
of the metacarpals alone. This interpretation now seems unlikely given overall similarity
between P. cookei and N. intermedius elements. The convex, slightly incongruent
articulation between trapezoid and trapezium suggests the presence of substantial
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