Gee and Parker—Juvenile Metoposaurid from Arizona
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disarticulated ribs is difficult in metoposaurids given the lack of articulated reference specimens with the ribs in place and the subsequent difficulty in differentiating morphological changes associated with position from those associated with ontogeny (Sulej, 2007). Based on comparisons to the described ribs of M. diagnosticus krasiejowensis (Sulej, 2007), the posteriormost rib can be tentatively assigned as an anterocaudal rib based on its strong curvature and relatively short length. The second rib, located posterior to the orbit, does not conclusively match the description of any described rib, which suggests that it may have been taphonomically deformed to produce the sinuosity. The anteriormost rib, located posteromedial to the orbit, is similar in morphology to the rib assigned as an anterocaudal based on its curvature and length. The potential fourth rib is too fragmented to assign a position. Because of the disarticulated nature of North American metoposaurid material, ribs that can confi- dently be assigned to A. gregorii have never been described, preventing any comparison with the taxon. A total of nine intercentra were found associated with the
skull, including one that is pressed into the dorsal surface anterior to the pineal foramen (Figs. 2.1, 2.2, 4; Table 1). The intercentra are of various sizes and proportions, but eight of the nine are modestly elongate (Fig. 4.1–4.5 4.7–4.8; Table 1). The remaining intercentrum is of elongate proportions that are characteristic of A. gregorii (Fig. 4.6, Table 1). Based on comparisons of the diameter-to-length ratio in these intercentra to those of other North American metoposaurids of various sizes and provenance, several intercentra of this specimen are within the upper limits of the range of values for A. gregorii, but a few of the larger intercentra fall within the lower limits of the range of larger metoposaurids (unpublished data, B.M. Gee, 2017). Accordingly, we consider them to be of a more intermediate degree of elongation. As with the ribs, the precise positions of the vertebrae are somewhat difficult to extrapolate, especially since all comparisons of postcrania are generally made with M. diagnosticus krasiejowensis, which might not be representa- tive of the North American metoposaurids. Intercentra can be generally divided into precaudal and caudal based on the presence of prezygapophyses and parapophyses in the former, deep ventral furrows extending from the anterior to the posterior surface in the latter, and the nature of the anterior articular surface (flat versus convex and overall depth of depression when convex) (Sulej, 2007; Spielmann and Lucas, 2012).All assignments are tentative given the uncertainty regarding potentialmorphological variation in the axial column among metoposaurids. The intercentrum impressed onto the skull (Fig. 2.1, 2.2) is caudal in position based on the absence of any lateral projections and the presence of the broken base of the chevron. It was probably relatively close to the sacrum given the reduced elongation in comparison to the posteriormost caudal elements. Of the separately collected intercentra, the largest four (Fig. 4.1–4.4) are similar to the caudal intercentrum in both size and proportions, allowing for a confident associationwith this element and the
cranium.Based on
Figure 4. Intercentra of juvenile Koskinonodon perfectus (PEFO 35392) from the Chinle Formation of the Petrified Forest National Park, AZ, USA; intercentra are ordered by overall size from 1 to 8, with the anterior profile in the left column and the lateral profile in the right column; number designations and order correspond to those in Table 1. Scale bar=2 cm.
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