Incertae

Van Vranken, Nathan E. & Boyd, Clint A., 2021, The first in situ collection of a mosasaurine from the marine Breien Member of the Hell Creek Formation in south-central North Dakota, USA, PaleoBios 38, pp. 1-11 : 5-7

INCERTAE SEDIS

FIGS. 3 , 4 .

Referred material— NDGS 2728: isolated vertebra; NDGS 2729: incomplete right articular-prearticular complex.

Locality— NDGS L301 : Morton County, North Dakota, USA; Township 135 North , Range 79 West , Section 17. Detailed locality information available to qualified re- searchers upon request .

Stratigraphic unit — Specimens collected from the Breien Member of the Hell Creek Formation.

Description— Both the surface-collected vertebra and the articular-prearticular complex collected in situ display poorly preserved surface texture. Three factors are proposed that may have either individually or jointly impacted the preservation of these specimens. First, subaerial erosion undoubtedly impacted these specimens given that the vertebra was completely exposed having fallen a few meters down the outcrop surface and that portions of the ventral and posterior margins of the articular-prearticular complex were lost to erosion prior to discovery. Second, the fossil-producing horizon is located several centimeters below the upper prairie surface and the rocks were extensively invaded by modern plant roots, including some that penetrated through fractures in the articular-prearticular complex. Finally, the rocks forming the fossil-bearing horizon are highly bioturbated, indicating the bones may have been negatively impacted by taphonomic processes either prior to or shortly after burial during the period where the bones were shallowly buried and remained within the bioturbation zone.

The articular-prearticular complex (NDGS 2729) is incomplete along the anteroventral margin, a short portion of the anterodorsal margin, and ventrolateral to the glenoid fossa and retroarticular process owing to subaerial erosion ( Fig. 3 ). It is preserved in two main pieces along with some associated fragments that cannot be placed at this time. There is some anteroposterior overlap between these two pieces, resulting in an estimated total length of at least 38.0 cm, though a portion of the posterior border of the retroarticular process is missing so the original length may have been slightly longer. Some distortion is present, mostly transverse crushing of the posterodorsal portion of the anterior piece, preventing the two pieces from being confidently reunited.

The anterior portion of the articular-prearticular complex consists of the transversely- narrow and dorsoventrally-tall blade that extended medial to the surangular and dorsal to the angular. The maximum preserved length of the anterior portion is 26.54 cm. The medial surface is nearly flat along much of its length, but transitions to slightly convex anteriorly. The dorsal margin is sinuous in medial view ( Fig. 3B ) with the posterior-most portion steeply dorsally inclined leading to the glenoid fossa. A similar steep incline in the dorsal margin of the prearticular-articular complex anterior to the glenoid fossa as seen in Mosasaurus hoffmannii ( Street and Caldwell 2017: fig. 14f). The dorsoventrally narrowest portion of the articular-prearticular complex is just anterior to that dorsally-inclined surface, where the minimum thickness is 4.46 cm. That narrow portion of the articular-prearticular complex is the only part that preserves the ventral margin, and at that location it was broadly rounded and thickened relative to the rest of the anterior portion of the articular-prearticular complex. Much of the lateral surface is deeply concave owing to the presence of a thickened ventral margin (where preserved) and a prominent anteroposteriorly-directed ridge situated near the dorsal margin. That ridge arises posteriorly on the lateral surface of the dorsal incline, though its posterior extent is unknown owing to damage, and continues anteriorly along approximately two-thirds the length of the anterior portion of the articular-prearticular complex. The dorsal surface of this ridge contacted a medially-directed wing on the surangular and formed the floor of the adductor fossa ( Street and Caldwell 2017).

The posterior portion consists of the articular contribution to the glenoid fossa and the dorsomedial portion of the retroarticular process and measures 12.74 cm in total length. The articular contribution to the glenoid fossa is anteroposteriorly longer than transversely wide. The glenoid surface is roughened and unfinished, indicating it was covered by cartilage in life. We do not think this roughened surface is the result of taphonomic processes as it is restricted to the glenoid surface and does not extend onto the surrounding bone surface. In dorsal view, the glenoid fossa is ovate, being broader anteriorly and pointed posteriorly, though the anterior-most portion is not completely preserved. The glenoid surface was gently dorsolaterally inclined and is anteroposteriorly concave. The medial surface immediately ventral to the glenoid slopes ventrolaterally, indicating a pronounced transverse thinning of the bone ventral to the glenoid. Lateral to the glenoid is a broad articulation surface for the surangular that is dorsomedially concave and appears to have connected to the medial ridge on the anterior portion of the articular-prearticular complex, but the connecting portion of the bone is missing. Posterior to the glenoid, the medial surface of the retroarticular process is dorsolaterally inclined, broadly concave both anteroposteriorly and dorsoventrally, and was penetrated by a prominent foramen that Russell (1967) identified as the pathway for the corda tympani. The dorsal portion of the retroarticular process curves strongly ventromedially so that the posterodorsal corner is nearly horizontal. The surface of that horizontal portion is roughened for muscle attachment, likely by the M. depressor mandibulae ( Lingham-Soliar 1995: fig. 24a). A small portion of the lateral surface of the retroarticular process posteroventral to the articulation surface for the surangular is preserved. That portion is relatively flat and indicates the transverse width of the retroarticular process was relatively thick and uniform throughout the preserved portion.

NDGS 2728 is a single vertebra that was surface collected a few meters away from the articular-prearticular complex ( Fig. 4 ). The centrum is strongly procoelous, has an anteroposterior length of 9.93 cm, a posterior height of 7.66 cm, a posterior width of 7.12 cm, an anterior height of 8.89 cm, and an anterior width of 8.91 cm. The breadth across the preserved transverse processes is approximately 15.7 cm, though the original measurement would have been larger given the eroded surface texture of the bone. The transverse processes arise near the anterior margin of the centrum at or slightly below mid-height and are relatively flattened ventrally and convex dorsally. There are slight ridges arising from the middle of the dorsal surface of each transverse process that run anterodorsally up the lateral surface of the centrum and connect to the anteroventral corner of the neural arch, seemingly at the base of the prezygapophysis. Loss of the surficial bone in this area makes it difficult to de- termine how pronounced those ridges were in life. On the preserved left lateral surface of the centrum a ridge is clearly present extending from the posterior margin of the transverse process posteriorly to the margin of the posterior condyle. The maximum preserved height of the vertebra is approximately 11.8 cm, but most of the neural arch is missing. The base of the neural arch is situated anteriorly on the centrum, with the anterior margin arising from the anterodorsal margin of the centrum and extends posteriorly 6.55 cm, ending well anterior of the posterior margin. There is a raised lip at the anterior end of the ventral surface of the neural canal. A pair of foramina are present on the ventral surface of the centrum near mid-length.

This specimen lacks either fused chevrons or articulation facets for the chevrons, indicating it is not from the caudal series. It also lacks any evidence of ventral peduncles for the hypapophyses or midline ventral tubercles that typify the cervical series. The position of the transverse processes at or slightly below mid-height, the raised, flattened border of the neural canal above the anterior articulation surface of the centrum, and the roughly equal heights and widths of the articulation surfaces best conform with a postieror trunk vertebrae (e.g., Holmes 1996), though given the quality of preservation in this specimen it is indistinguishable from vertebrae found in the pygal region of the caudal series.

Figures

Figure 3. Right articular-prearticular complex, NDGS 2729. A. Dorsal view. B. Medial view. Specimen oriented anterior to the left. Abbreviations: ac, adhered concretion; gf, glenoid fossa; rp, retroarticular process; sas, articulation surface for the surangular.

Figure 4. Pygal vertebra, NDGS 2728. A. Anterior view. B. Dorsal view. C. Posterior view.