1052
Journal of Paleontology
Figure 3. Skull of juvenile Koskinonodon perfectus (PEFO 35392) from the Chinle Formation of the Petrified Forest National Park, AZ, USA in left occipital view: (1) photograph, (2) interpretive illustration. Abbreviations: oc=occipital condyle; on=otic notch; sq=squamosal; sta=stapes; tab=tabular. Scale bar=2 cm.
the long axis oriented in the anteroposterior direction. The anteriorly projecting cultriform process is partially preserved and contributes to the medial and posterior margins of the large, oval interpterygoid vacuities. The lateral margin and the remainder of the posterior margin of the interpterygoid vacuities are formed by the pterygoid, which projects anterolaterally from its union with the parasphenoid. The left pterygoid is partially complete, with posterior portions of the anterolateral and posterolateral processes intact. Of the right pterygoid, only the main base and a portion of the posterolateral process are preserved. The occipital condyles are well exposed in ventral view and are fully attached to the pterygoid and the parasphenoid. The anteriormost extent of the skull is mostly obscured by a combination of encrusting clay matrix and bone splinters that cannot be removed without damaging the specimen because these areas are relatively thin and brittle. Several cranial features were identified as informative for
distinguishing this specimen from the nearly complete holotype of A. gregorii (UCMP 63845). In general, the profile of the skull of PEFO 35392 in dorsal view is more similar to that of large metoposaurids in that the postorbital region is more broadly expanded mediolaterally to produce a temporal lobe that is more
pronounced than in the holotype of A. gregorii. This does not appear to be the result of taphonomic damage to the postero- lateral corner in the latter considering that the subtemporal fenestra is completely enclosed on the right side (Spielmann and Lucas, 2012, fig. 10). PEFO 35392 is also notably larger; from the posteromedial margin of the postparietal to the posterior margin of the orbit, PEFO 35392 measures 15.4cm in comparison to UCMP 63845, which measures ~10.3cm over the same distance. Both are larger than a juvenile K. perfectus (TMM 31099-12B) that was photographed but not described by Hunt (1993, fig. 8A, 8B), which measures ~8.7cm over the same interval. All three skulls are significantly smaller than the largest known skulls of K. perfectus, which are documented at complete skull lengths in excess of 60cm (Long and Murry, 1995). The otic notch is deeper in PEFO 35392 than in specimens of A. gregorii, as in large metoposaurids with the exception of the intermediate depth of notches in Arganasaurus lyazidi (Dutuit, 1976). As previously noted, this incision does not appear to have been further exaggerated during preservation (Fig. 3). On the palatal surface, the cultriform process iswider in PEFO 35392 than in the holotype of A. gregorii; the ratio of the maximum skull width to the minimum process width, a metric defined by Hunt (1993), is 15.6 in PEFO 35392 and is ~19.8 in UCMP 63845 (Fig. 2.3, 2.4). The cultriform process of TMM 31099-12B is relatively wider (~9.3) than in both of these specimens. Additionally, the transverse process of the pterygoid in A. gregorii is significantly more constricted at its mid-length than appears to be the condition in the partial left transverse process in PEFO 35392. Finally, the occiputs project posteriorly in PEFO 35392 (Fig. 2.1, 2.2) as in TMM 31099-12B, whereas they are preserved but covered by the postparietals and tabulars dorsally in UCMP 63845. The lack of preserved sutures in both PEFO 35392 and in additional specimens of A. gregorii beyond the holotype renders a comparison of sutural patterns impossible at this time. The postcranial material included in PEFO 35392 consists
of nine intercentra and four partial ribs (Figs. 2.1, 2.2, 4). Three of the ribs are pressed into the dorsal surface of the skull posterior to the left orbit and cannot be removed without damaging the specimen. Two are relatively complete, while the third only preserves the proximal end. The posteriormost rib appears to be complete, with a broad proximal end that becomes more slender at the curved shaft and flattened at the distal end. The second complete rib in this area is more elongate, with a narrower proximal end that is only slightly wider than the shaft. It is slightly sinuous, curving gently toward the orbital margin in the proximal half and curving slightly away from the margin in the distal half. The proximal end of the partial rib in this region is more oval in cross-section than the other two ribs, giving it greater height. A possible fourth rib is located in the anteromedial region of the skull and is embedded in fine- grained matrix and bone splinters. It also has a more pronounced height than the two complete ribs and appears to narrow at the shaft. The preserved portion is not curved, hence our tentative identification of the element. The lack of curvature and the anteromedial position would be consistent with a basibranchial, a bone known in only one specimen of M. diagnosticus (Witzmann, 2013), but the element is too incomplete to confidently resolve its identity. Assignment of the position of
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