. Bulletin. Natural history; Natuurlijke historie. FIG. 27. Deinonychus antirrhopus, YPM 5210. Atlas neural arch (A) in left lateral (1) and medial (2) views, and atlas intercentrum (B) in left lateral (1) view. C) outline diagram of neural arch and intercentrum in articulation in lateral (1) and medial (2) views. (Neural arch drawn reversed from a right half.) Abbreviations: ic—articular facet for atlas intercentrum; na—facet for atlas neural arch; oc—facet for occipital condyle; po—postzygapophyseal facet. concave for contact with the occipital condyle and faces sharply upward and forward (G
. Bulletin. Natural history; Natuurlijke historie. FIG. 27. Deinonychus antirrhopus, YPM 5210. Atlas neural arch (A) in left lateral (1) and medial (2) views, and atlas intercentrum (B) in left lateral (1) view. C) outline diagram of neural arch and intercentrum in articulation in lateral (1) and medial (2) views. (Neural arch drawn reversed from a right half.) Abbreviations: ic—articular facet for atlas intercentrum; na—facet for atlas neural arch; oc—facet for occipital condyle; po—postzygapophyseal facet. concave for contact with the occipital condyle and faces sharply upward and forward (Gilmore [1920: p. 31] noted this same condition in Allosaurus). The condylar articular facet is well-defined, but the limits of the atlanto-occipital articular capsule are not clearly preserved. The posterior surface is slightly con- vex both transversely and vertically. The right margin of this facet is sharply defined by a lip-like margin that must represent the limits of the atlanto-axial capsule and the attachment of the capsular ligament. The ventral surface is marked by paired, laterally placed tubercles, which are most prominent posteriorly. These are separated by a slight, median de- pression immediately anterior to the margin of the articular facet for the axis. Because these tubercles are situated well outside the margins of both articular capsules (atlanto-occipital and atlanto-axial), they cannot represent attachment sites of either the ventral or the anterior oblique atlanto-occipital ligaments. Also, there are no similar features preserved on the ventral surfaces of either the axis intercentrum or centrum. Accordingly these are considered the probable origin sites of M. rectus capitis anterior, the principal flexor of the atlanto- occipital joint. They might be parapophyses for articulation of atlantal ribs but this seems improbable in view of the much more dorsal position of the axial parapophyses. Triangular, rugose, sutural surfaces for the pedicels of t
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