Locality information

PEFO 35392 was collected in 2008 from locality PFV 215 (Zuni Well Mound) in the Petrified Forest National Park, Arizona, USA. Exact locality information is protected from disclosure by the Paleontological Resources Protection Act of 2009, but is available to qualified researchers from the Resource Management Division at the PEFO, see Figure 1.

Stratigraphic information

PEFO 35392 was collected from the middle portion of the Petrified Forest Member (middle Norian) of the Chinle Formation (sensu Woody, Reference Woody2006). The quarry is situated at the top of the Painted Desert Sandstone #4 (Parker and Martz, Reference Parker and Martz2011), ~30m below the Black Forest Bed of the Petrified Forest Member. The Black Forest Bed is a sandstone unit dominated by reworked tuffaceous material (Ash, Reference Ash1992) and has provided a U/Pb date (sample BFB) of 209.926±0.072Ma from detrital zircons (Ramezani et al., Reference Ramezani, Hoke, Fastovsky, Bowring, Therrien, Dworkin, Atchley and Nordt2011). This provides a minimum age for the quarry. The maximum age at the northern end of the PEFO is set by sample KWI, which is from the Brown sandstone (Billingsley, Reference Billingsley1985) and which has a date of 213.870±0.078Ma (Ramezani et al., Reference Ramezani, Hoke, Fastovsky, Bowring, Therrien, Dworkin, Atchley and Nordt2011). The Brown sandstone is ~120m below the quarry (Parker and Martz, Reference Parker and Martz2011). Thus the estimated age of the quarry is closer to 211 Ma (Nordt et al., Reference Nordt, Atchley and Dworkin2015).

Other taxa found at this locality include phytosaurs (PEFO 36745), large-bodied metoposaurids (PEFO 31173), drepanosaurids (PEFO 36743), the archosauriform Vancleavea campi Long and Murry, Reference Long and Murry1995 (PEFO 34035; Parker and Barton, Reference Parker and Barton2008), a rauisuchid paracrocodylomorph (PEFO 36740), the aetosaurian Typothorax coccinarum Cope, Reference Cope1875 (PEFO 16668), the pseudosuchian Revueltosaurus callenderi Hunt, Reference Hunt1989 (PEFO 16671), and a theropod dinosaur (PEFO 36741). Numerous small, elongate metoposaurid intercentra from this locality have been previously assigned to Apachesaurus gregorii (e.g., PEFO 31209; Parker and Martz, Reference Parker and Martz2011), but these identifications are less certain given the conclusions of the present study. Typothorax coccinarum and R. callenderi are index taxa of the Revueltian biozone (Parker and Martz, Reference Parker and Martz2011).

Holotype

Complete skull (UMMP 7475) from the Dockum Group, Crosby County, Texas, USA (Case, Reference Case1922, pls. 1, 2A, 2B, figs. 1–3).

Referred material

Partial skull with associated ribs and intercentra (PEFO 35392) from the Chinle Formation, Petrified Forest National Park, Arizona, USA (Figs. 2–5).

Remarks

Previously, the inclusion of the lacrimal into the orbital margin was the sole diagnostic character of Koskinonodon, which separated it from Metoposaurus, but the subsequent observation of a comparable condition to the former in the latter by more recent workers (Sulej, Reference Sulej2002, Reference Sulej2007; Brusatte et al., Reference Brusatte, Butler, Mateus and Steyer2015; but see Lucas et al., Reference Lucas, Spielmann and Hunt2007) has rendered it invalid. Therefore, the diagnosis of Koskinonodon is in need of revision, particularly with regard to its differentiation from other large-bodied metoposaurids. However, we do not attempt to amend the diagnosis in this paper in the absence of a broader revision of the Metoposauridae, which similarly requires extensive revision to evaluate existing autapomophies of the clade and to assess the potential for morphological differences to be attributable to intraspecific variation, ontogeny, or factors other than speciation. Additionally, although this specimen provides additional information on K. perfectus, the paucity of juveniles of other taxa inhibits an appropriate comparison of autapomorphies at this ontogenetic stage. Our taxonomic assignment of this material is based on the marked differences from the diagnosis of the small metoposaurid, Apachesaurus gregorii, and the current biogeographic distribution of metoposaurids in which K. perfectus is the only large-bodied metoposaurid west of Texas.

Description

PEFO 35392 consists of a partial skull associated with several ribs and intercentra. The skull is modestly well preserved, with landmark features such as the pineal foramen, the left orbit, the left stapes, the left otic notch, and both occipital condyles visible in dorsal view, and features such as the parasphenoid and pterygoid visible in ventral view. The skull measures ~15.4 cm from the posteromedial margin of the postparietals to the posterior margin of the orbit.

The dorsal surface of the skull (Fig. 2.1, 2.2) preserves a good amount of detail and landmark features, although it has been fractured in several locations, and the original sutures are not visible. Where the ornamentation is preserved, much of the skull is characterized by the typical honeycomb pattern found in temnospondyls, mostly consisting of semi-circular pitting of variable size with a small area of elongate grooves in the posterolateral lobe of the temporal region. The pits contain small, distinct foramina that provide a method to distinguish fragmentary temnospondyl material from ornamented osteoderms of amniotes such as aetosaurs, which lack these foramina (Witzmann et al., Reference Witzmann, Scholz, Mueller and Kardjilov2010). The posterior region of the skull is the best preserved. A large anteromedial portion of the skull, probably the snout, is present but obscured by encrusting matrix and bone splinters, which could not be removed without damaging the skull because this area is very thin. The left orbit is mostly preserved with the entire lateral and posterior margins intact (Fig. 2.1, 2.2). Portions of the anterior and anteromedial margins of the orbit are missing, and the posteromedial portion is dislocated into the orbit.

Several postcranial elements are pressed into the dorsal surface of the skull, including two nearly complete ribs and one partial rib posterior to the left orbit and a caudal intercentrum anterior to the pineal foramen; these are discussed further below. The pineal foramen is well defined, circular, and connected to a slightly raised ridge that extends both anteriorly and posteriorly along the midline (Fig. 2.1, 2.2). The best-preserved and most-diagnostic portion of the skull is the posterior region, containing the otic region and the occiput. The otic notch (Fig. 2) is the most common feature used to identify specimens of North American metoposaurids, and in this specimen, it is relatively deep in comparison to described specimens of A. gregorii (Spielmann and Lucas, Reference Spielmann and Lucas2012). The surface is smooth and uninterrupted except for a small fracture that caused slight dislocation of the squamosal directly upward, suggesting that the incision is not the product of taphonomic damage. Further supporting the conclusion of a well-preserved otic region is the presence of an articulated stapes, the cylindrical distal end of which can be seen projecting posterolaterally in dorsal and occipital views (Figs. 2.1, 2.2, 3). The exposed region of the stapes is identical in form to that seen in other metoposaurids (Case, Reference Case1931; Dutuit, Reference Dutuit1976; Sulej, Reference Sulej2007; Spielmann and Lucas, Reference Spielmann and Lucas2012; Brusatte et al., Reference Brusatte, Butler, Mateus and Steyer2015). Medially, the otic notch forms a continuous surface with the tabular and the postparietal. Although no posterolaterally projecting tabular horn is present, this margin of the tabular appears to be weathered; thus we cannot conclusively determine whether the tabular horn was absent in this individual. The occiput can be seen projecting posteriorly beyond the posterior extent of the dorsal skull table. The right otic notch, in addition to most of the right side of the cranium, is not preserved in this specimen.

The palatal surface of the skull (Fig. 2.3, 2.4) is similar to the dorsal surface in that the posterior portion is the best preserved and that the anterior portions are heavily damaged. The basal plate of the parasphenoid is fully preserved, although its sutures to the exoccipitals and the pterygoids are not identifiable. The basal plate is roughly diamond-shaped, with 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 posterolateral corner in the latter considering that the subtemporal fenestra is completely enclosed on the right side (Spielmann and Lucas, Reference Spielmann and Lucas2012, 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 15.4 cm in comparison to UCMP 63845, which measures ~10.3 cm over the same distance. Both are larger than a juvenile K. perfectus (TMM 31099-12B) that was photographed but not described by Hunt (Reference Hunt1993, fig. 8A, 8B), which measures ~8.7 cm 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 60 cm (Long and Murry, Reference Long and Murry1995). 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, Reference Dutuit1976). As previously noted, this incision does not appear to have been further exaggerated during preservation (Fig. 3). On the palatal surface, the cultriform process is wider 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 (Reference Hunt1993), 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, Reference Witzmann2013), but the element is too incomplete to confidently resolve its identity. Assignment of the position of 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, Reference Sulej2007). Based on comparisons to the described ribs of M. diagnosticus krasiejowensis (Sulej, Reference Sulej2007), 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 confidently 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, Reference Gee, Parker and Marsh2017). 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 representative 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, Reference Sulej2007; Spielmann and Lucas, Reference Spielmann and Lucas2012). All assignments are tentative given the uncertainty regarding potential morphological 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 association with this element and the cranium. Based on the posterior position of the parapophyses and the relatively flat anterior surface, all four are assigned to the range from mid-dorsal to perisacral. Two of these intercentra were selected for the histological analysis that is presented in part below. The other four intercentra (Fig. 4.5–4.8) can also be identified as pre-caudal based on the presence of the parapophyses and the smooth anterior surface. It is unclear whether these are associated with the cranium and the larger intercentra given the stark difference in size. Only one of the nine intercentra described here has a deep convexity on the anterior surface similar to previously described intercentra of A. gregorii (Hunt, Reference Hunt1993; Spielmann and Lucas, Reference Spielmann and Lucas2012). Although all nine are opisthocoelous, the notochordal pit is relatively shallow in the remaining eight, more so on the anterior surface, in contrast to typical intercentra of A. gregorii. The pit does not appear to connect the anterior and posterior surfaces; this morphology may be influenced by both ontogeny and axial position. This possibility and the axial position assignments designated here are only tentative at present and require more fully articulated vertebral columns of North American metoposaurids to verify.

Table 1

Measurement data for the intercentra associated with a juvenile specimen of K. perfectus (Case, Reference Case1922) (PEFO 35392). Numerical assignments for each element were created solely for the basis of this paper and are ordered from largest to smallest to correspond with Figure 4 with the exception of element 9, the intercentrum on the dorsal surface of the skull that is not included in Figure 4. Diameter (D) was measured in the mediolateral axis, height (H) was measured in the dorsoventral axis, and length (L) was measured in the anteroposterior axis in the same manner as measurements made by Konietzko-Meier et al. (Reference Konietzko-Meier, Bodzioch and Sander2013). The length of element 9 could not be measured due to its position.

Because associated or articulated vertebral columns of A. gregorii are rare (Spielmann and Lucas, Reference Spielmann and Lucas2012), a separate histological survey of metoposaurid intercentra from the PEFO was conducted with the intention to utilize the findings of Konietzko-Meier et al. (Reference Konietzko-Meier, Bodzioch and Sander2013) to determine the ontogenetic maturity of various intercentra (Gee et al., Reference Gee, Parker and Marsh2017). Two of the intercentra of that study belong to PEFO 35392 and sections of this material are briefly described here. In sagittal profile, they are characterized by a relatively wide, ventral periosteal cortex, which is triangular with an apex that terminates at or below the mid-height of the intercentra. This region is made of a parallel-layered matrix that has been partially damaged by secondary precipitation of carbonate minerals. In transverse profile, the periosteal cortex extends up the lateral margins from the ventral margin. The remainder of the internal structure consists of more randomly oriented endochondral trabeculae that are thickest at the articular faces and thinnest in the dorsal portion of the element. The intertrabecular spacing increases in the dorsal region. Calcified cartilage is abundant within the intercentra, including in the interior endochondral region; in M. krasiejowensis, calcified cartilage decreases in relative abundance throughout ontogeny, particularly in the interior of the intercentra (Konietzko-Meier et al., Reference Konietzko-Meier, Bodzioch and Sander2013). The presence of calcified cartilage in the interior of both intercentra is thus indicative of their relative immaturity (Fig. 5.2, 5.4). A weathered external cortex is preserved in both of the specimens, but it appears to still be relatively well vascularized and shows no evidence of a compact lamellar annulus or LAGs that would indicate relative maturity (Fig. 5.1, 5.3). Using the histological ontogenetic stage (HOS) designations of Konietzko-Meier et al. (Reference Konietzko-Meier, Bodzioch and Sander2013), we designate these intercentra as HOS 3 based on the presence of a wide periosteal region (which originates in HOS 2) and an external cortex that is still relatively vascularized and lacking in LAGs (which originate in HOS 4).