The anatomy of the ribs, pectoral girdle, and forelimb of Chasmosaurus irvinensis is closely comparable to that described in other ceratopsids (e.g., Hatcher et al. 1907; Brown 1917; Lull 1933, Dodson et al. 2004).

Almost all of the ribs and presacral vertebral column of CMN 41357 are preserved. Although the vertebral column is badly distorted and some of the ribs are crushed, there is a good representative sample of relatively undistorted ribs from all regions of the rib cage. They confirm that the posterior cervical and anterior-most thoracic ribs turn sharply ventrally at their necks, and are otherwise almost completely straight, resulting in a distinctly narrow chest between the pectoral girdles (Paul and Christiansen 2000). A gentle curvature develops by the fourth or fifth thoracic rib, and by about the ninth thoracic rib, the rib cage forms a broad barrel.

Both scapulocoracoids are preserved (Figure 1). Although the coracoids have been folded under slightly, the morphology of the elements is clear. The medial surface of the scapular blade is only slightly concave. The strongly concave coracoid contribution to the glenoid "closes" the articular surface proximally, resulting in a glenoid that faces at right angles to the long axis of the scapulocoracoid.

The humerus, ulna, and radius (Figure 2, Figure 3) are all slightly crushed, but are well preserved and can be articulated without difficulty. As in other neoceratopsids (e.g., Johnson and Ostrom 1995), the proximal humeral condyle is located on the dorsal (external) surface of the proximal humeral expansion and is offset to a position posterior to the axis of the humeral shaft. The distal expansion bears two distal condyles separated by a groove (the trochlea) to receive a ridge on the proximal articular surface of the ulna. The anterior (preaxial) condyle bears a convex capitular facet on its ventral (extensor) surface to receive the radius. The proximal articular surface of the ulna is divided into two concave surfaces by a ridge that fits into the trochlear notch of the humerus. The posterior of these two surfaces articulates with the anterior half of the posterior distal humeral condyle, and the anterior surface with the posterior half of the anterior humeral condyle. A prominent olecranon projects proximally from the rim of the articular surface. The radius bears an oval, concave terminal facet that articulates with the ventral (capitular) surface of the anterior humeral condyle. When the epipodium is in articulation with the humerus, the proximal head of the radius lies in a bowl-shaped depression on the flexor surface of the ulna. The long axes of the ulna and radius diverge distally. The distal end of the epipodium, formed by the expanded distal ends of the two bones, forms a broad arc.

As in other well-preserved neoceratopsid forelimbs (e.g., Centrosaurus AMNH 5351, Brown 1917), all but the third and fourth distal carpals are absent, and probably never ossified. The entire metacarpus and manus (Figure 4) are well preserved in articulation. Five digits are present. As in Centrosaurus apertus (Brown 1917), the phalangeal formula is 2-3-4-3-2 (Figure 5). Terminal phalanges of digits 1-3 show clear evidence of keratinous hooves. Terminal phalanges 1 and 2 are distinctly larger, suggesting that the preaxial (medial) side of the manus bore most of the weight or sustained more stress during locomotion. The distal-most phalanx on each of digits 4 and 5 bears what appears to be a terminal articular facet. However, as in the similarly articulated and well-preserved mani of Centrosaurus apertus (Brown 1917), there is no trace of a hoof-bearing terminal phalanx associated with either digit, suggesting that they were not present in life, or possibly never ossified. The distal articular facets of the metacarpals have extensive dorsal exposure, indicating considerable potential for dorsiflexion of the manus, permitting a digitigrade stance. The combined proximal articular surface of the articulated metacarpus forms a broad arch (Figure 4.1). As a result, the digits are distinctly splayed (Figure 4.1).