In neoceratopsian dinosaurs, the centra, neural arches, and neural spines of several anteriormost cervical vertebrae are fused together to form a structure called the cervical bar (Langston, 1975) or syncervical (Ostrom and Wellnhofer, 1986). This structure characterizes all neoceratopsians (Dodson and Currie, 1990; Dodson et al., 2004; You and Dodson, 2004) for which the anterior cervicals are known, but it remains controversial how many vertebrae are fused together to produce this structure, namely, whether it consists of the first three (atlas through the third cervical) or four (atlas through the fourth cervical) elements. Analyses of ceratopsian interrelationships have employed from two (Sereno, 1984; Chinnery, 2004) to five (Xu et al., 2002) characters related to the cervical bar; accordingly, sorting out the exact identities of the elements contributing to this structure may have implications for our understanding of support, branch lengths, and rates in ceratopsian evolution.

Hatcher in Hatcher et al.(1907) described the first three cervicals as being fused together to produce the cervical bar in YPM 1822, Triceratops prorsus Marsh, 1890 (Fig. 1.1). Hatcher inferred that the atlantal contribution consists of a paired neural arch and a centrum that has a concave anterior surface for articulation with the occipital condyle. Each half of this neural arch was considered as arising from the dorsolateral edge of the centrum and uniting with each other dorsally to produce a neural spine. Hatcher inferred that the neural arch and spine of the atlas were completely fused with those of the axis, leaving no trace of a zygapophyseal articulation. Here we call this the three-element hypothesis. Lull in Hatcher et al.(1907), however, argued that the cervical bar in T. prorsus consists of four cervicals, with the atlas being reduced to just a ringlike bone on the anterior margin of the bar, demarcated from the following bones by a “distinct suture” in YPM 1822 (Fig. 1.2). In this hypothesis, Lull considered the part of Hatcher's “atlas” that is posterior to this suture as the “axis.” In other words, Hatcher's “atlas” comprises Lull's “atlas” and “axis.” We here call Lull's idea the four-element hypothesis. Brown (1917) observed that there are only three segments present in the cervical bars of Triceratops Marsh, 1889 and Monoclonius Cope, 1876, and argued that there are only three co-ossified cervicals in the cervical bar of all neoceratopsians. Lull (1933) later followed this argument and agreed that the third cervical is the most posterior vertebra incorporated in the cervical bar. However, Lull (1933) still maintained that the “distinct suture” is present near the anterior margin of the cervical bar of YPM 1822, and inferred that the part in front of this “suture” may represent a proatlas, an idea discredited on developmental grounds by Brown and Schlaikjer (1940).